SEPTEMBER 27, 1889.]

The corpuscular theory of light, which had shown such remarkable vitality, was now in the last stages of a fatal disease, due to indigestion and lack of assimilation. Foucault finished it off in 1865 with his crucial experiment to decide upon the relative velocity of light in air and water. The undulatory theory was thus fully established, and the doctrine of radiant energy in general began to be clearly apprehended. The grand generalisation of the conservation of energy was looming up all along the horizon of science, as the towers and spires of a great city appear to rise out of the sea to a traveller approaching the land. Victory was ready to perch on the banners of an army contending for the ether doctrine-not a decimated army, but one constantly augmenting in numbers by deserters from the enemy. At this period, sixteen years ago, appeared the epoch-making book of Maxwell on Electricity and Magnetism. Its author professes only to translate Faraday's ideas into mathematical language; but he did vastly more than this. He demonstrated mathematically that the properties of the medium required to transmit electro-magnetic action are identical with those of the luminiferous ether. It would be unphilosophical, he remarks, to fill all space with a new medium whenever any new phenomenon is to be explained; and since two branches of science had independently suggested a medium requiring the same properties to account for the same phenomena in each, the evidence for the existence of a single medium for both kinds of physical phenomena was thereby greatly strengthened. The step from identity of the medium to identity of phenomena, that is, that light itself is an electro-magnetic phenomenon, though it may now seem to be a short one, must nevertheless, upon careful consideration, always be accepted as evidence of the greatest genius. To walk in Maxwell's footsteps now and take the very steps he took is one thing, and a comparatively easy one; but to make original explorations into unknown regions of nature, and to tread where no human being has ever before set foot is quite another thing. The electro-magnetic theory of light must be regarded as a great generalisation, inferior only to that greatest one of all time-the conservation of energy.

The principal criteria upon which Maxwell relied for the confirmation of his theory may be briefly enumerated:

1. An electro-magnetic wave or undulation is propagated through the ether with a velocity equal to the ratio of the electro-magnetic to the electrostatic unit of quantity. If light is an electro-magnetic phenomenon its velocity must also be equal to this same ratio. The very close approximation of the one to the other, as determined by a variety of methods, has been known for some time.

2. The specific inductive capacity, K, of any transparent dielectric should equal the square of its index of refraction. The discrepancies at this point are so great that all one can say in the most favourable case is that K is the most important term in the expression for the refractive index, while in other cases no confirmation whatever can be drawn from this class of evidence.

3. The magnetic and electric disturbances are both at right angles to the direction of propagation of the wave and at right angles to each other. The mathematical form of the disturbance agrees with that which constitutes light in being transverse to the direction of propagation. Further, the electric disturbance should be perpendicular to the plane of polarisation of plane polarised light.

4. In non-conductors the disturbance should consist of electric displacements, but in conductors it should give rise both to electric displacements and electric currents by which the undulations are absorbed by the medium. Most transparent bodies, it is true, are good insulators, and all good conductors are opaque. The degree of opacity is, however, far from being proportional to the conductivity.

5. But, perhaps, the most important criterion of all is the one relating to the very existence itself of a medium. Such a test lies in the time element involved

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in transmission from point to point. Since energy is transmitted from a luminous body, as the source, to another body which may absorb it, then plainly if time is required for the transmission, the energy must reside in the medium by which the transmission is effected during the interval between the emission and the absorption. In the emission theory the light corpuscles are the receptacles of the energy and carry it with them in their flight. According to the undulatory theory the medium filling all space is the receptacle of the energy and passes it along from point to point by the action of contiguous parts.

Foucault's experimentum crucis proved the emission theory untenable. Roemer's observation of the retardation of the eclipses of Jupiter's satellites, when the earth is moving away from Jupiter, is, therefore, a confirmation of the undulatory theory of light and, in consequence, a demonstration of the existence of the luminiferous ether.

At this point the history of the nature of electrical action touches upon the third period.

The period upon which we have just entered may not inappropriately be called the period of confirmation. Nothing further appears to be necessary for the complete demonstration and establishment of the electro-magnetic theory of light. The noteworthy experiments of Prof. Hertz, of Carlsruhe, are known to all. Rightly conceiving that the reality of electro-magnetic waves would be best established by the same experiments which would also establish the fundamental identity of such undulations with those of light, he had recourse to the principle of resonance or sympathetic vibrations for the detection of these long-period By a device no less remarkable for its simplicity than its effectiveness, he produced electrical oscillations of such rapidity that the waves in the surrounding region were short enough to be measured. This he accomplished by attaching to the secondary terminals of an induction coil two rectangular sheets of metal each supplied with a short, stout wire, ending in a small ball. The balls were brought near each other and the discharges of the coil took place between them. Under these conditions the discharge is oscillatory, and the period may be calculated by the formula of Sir Wm. Thomson, published in 1853.*

waves.

The receiving apparatus is also of the simplest design, consisting ordinarily of a circle of wire, interrupted at a point with an adjustable opening, and of such dimensions that the waves passing through the circle may set up electrical oscillations in it, synchronising with those of the transmitting apparatus. The passage of sparks across the narrow opening of the circle indicates an electrical flow; and the necessity of adjusting the size of the circle in order to obtain this flow proves that the forces acting are periodic. The receiving apparatus must in fact be tuned so that the period of an electrical oscillation in it shall correspond with the external impulses absorbed. The intensity of the electric and magnetic disturbances is indicated by the relative length of sparks obtainable.

Equipped with this apparatus, which was installed in a large lecture hall, Hertz found not only that his tuned receiver responded to the impulses of the transmitter in the precise manner pointed out by theory, but that the sparks showed a series of maximum and minimum values recurring in periodic order as the receiver was carried further away from the source of the disturbances. The astounding fact was thus brought out that these electro-magnetic waves were reflected from the thick wall of the room, and that the combination of the direct and reflected systems produced stationary waves with loops and nodes that could be traced out by the responsive circle of wire. In this manner wave-lengths were measured down to 60 cms., and the time element was experimentally detected in the propagation of electrostatic and electro-dynamic induction. It was demonstrated that the disturbances producing the waves are at right angles to the direction of propagation, as Maxwell predicted, and as interfe

* Math. and Phys. Papers, Vol. I., p. 540.

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rence phenomena show them to be in light. Hertz has also found an electro-dynamic shadow cast by an iron post; he has verified the laws of reflection from plane and concave metallic reflectors, and has shown that electric waves suffer polarisation and refraction in a manner exactly analogous to light. Prof. Fitzgerald, of Dublin, has added another confirmation of Maxwell's doctrine, demonstrating that the electric disturbance is perpendicular to the plane of polarisation as Maxwell's equations require. Finally, the velocity of propagation of these electro-dynamic waves is found to be the same as the velocity of light. Thus, not only have all of Maxwell's criteria except the second abundantly confirmed the judgment of the great physicist, but other proofs have been added. Electromagnetic waves are therefore not merely like light, but they are light. Or, perhaps, to speak more exactly, all radiant energy is transmitted as electro-magnetic waves in the luminiferous ether. Electricity has thus annexed the entire domain of light and radiant heat; and, as Prof. Lodge says, "has become a truly imperial realm." The difference of wave length in the three classes of phenomena is not a fundamental one. Increase the rate of the electrical oscillations a million fold in Hertz's experiments and the waves would not merely resemble light-they would be light. A wire through which such oscillations are surging back and forth would glow with light. Even the long heat waves would be absent, and only those producing the sensations of light and colour would remain.

It will be observed that the oscillations of an electric discharge constitute the point of departure for the admirable researches of Hertz; and it is a matter in which we may modestly take a bit of national pride that the first case of electric oscillations was discovered by an American physicist. The oscillatory character of the Leyden jar discharge was demonstrated by Joseph Henry in 1832 by means of the magnetic effects produced in small steel needles. It was not until 21 years later that Sir Wm. Thomson published the_complete mathematical theory of such oscillations. They have since been observed directly by means of a rotating mirror. Dr. Oliver Lodge has lately shown that they rotate the plane of polarisation of light in one direction and then in the other as they surge back and forth. He has also reduced the number of oscillations from several millions per second to a few hundred by increasing the capacity and the self-induction. The discharge then vibrates within the limits of audibility and produces a musical note.

The well-known experiment of Henry, in which he observed an induction current in a wire stretched parallel to and distant 30 feet from one which served to discharge a Leyden jar is now seen to have been a case of resonance-that is, the absorption of electric waves by a conductor, producing currents therein. And it is an evidence of the great genius of Henry that be saw, somewhat dimly it may be, but still with a certain degree of rational apprehension, that the induction was transmitted across the intervening space with a velocity comparable only to that of light. He had perchance the divine touch of genius necessary for the great discovery of electro-magnetic waves coursing through the ether; but the way leading to this important physical fact had not then been sufficiently prepared, and its discovery was impossible.

Waves similar to those from a Leyden jar discharge, but of longer period, are sent out from a wire conveying alternating currents. We must conceive of such a wire, not simply as affected internally or even superficially by the electric energy surging through it, but as the source from which pulsate outward through the limitless ether, great waves of electro-magnetic disturbance. For 300 complete alternations per second these waves are a million metres, or over 600 miles in length. They present a marked contrast with the waves 'corresponding to the D lines of the spectrum, which are only about one five-millionth of a millimetre long,

These long waves from an alternating current represent energy. Through space it is conveyed with the velocity of light, and through other non-conductors or

[SEPTEMBER 27, 1889.

dielectrics with a smaller velocity precisely as in the case of the radiant energy of light or heat. Henceforth the complete equation for the distribution of energy by means of alternating currents must include a term to express the radiation from the circuit. It may, indeed, be found that this term represents no inconsiderable part of the energy communicated to the wire in the case of very rapid alternations.

Thus we see that the ether plays a magnificent role in what may be called its dynamic relation to electric displacements. In its capacity as a reservoir of static or potential energy, its agency has been better understood for a considerable period. When a continuous current begins to flow through a closed circuit, a single wave travels out from the conductor; and during its progress, while the current is approaching its constant value, the enclosing ether is assuming its condition of static repose under stress. The whole ether, extending indefinitely outward from the conductor, is profoundly modified. We know how to map out the circular lines of force about it by means of iron filings; but the iron serves only to show what has already taken place in the ether before the filings are brought into the field. Every little iron particle becomes a magnet, with all the north-seeking poles stretching in one direction round the wire, and all the south-seeking poles in the other. What the mechanism of the stress, or the motion in the ether to produce these effects may be, we do not know; but we do know that these lines of force are all subject to a tension tending to shorten them, and that they are mutually repellant laterally. When a current is sent through a conductor, the ether is expanded in concentric cylindrical layers about any straight portions of the circuit, and becomes the reservoir of potential energy. As soon as the current, which maintains this state of tension, ceases to flow, the stretched ether collapses upon the conductor, yielding up its energy in the form of self-induction. If a steady current is conceived as the setting up and breaking down of a static difference of potential energy at infinitesimal intervals of time, then the energy transmitted may depend upon a similar formation and decay of the static stress in the encompassing ether. The conductor is but the core of an electro-magnetic disturbance in the surrounding medium, and it may be that the enormous energy which a small copper wire can apparently convey is in reality transmitted by the invisible medium.

From this brief review of the theory of electric action, it will be quite evident that henceforth the language applied to electrical phenomena must always include the luminiferous ether as a prominent term. The experiments of Hertz have made it impossible to explain electrical facts without taking this invisible medium into account. There is no such thing as electric or magnetic action at a distance. The ether is always an essential part of. that complex system, the interactions of which manifest themselves as electric or magnetic phenomena.

As the ear responds to the slow oscillations of an electric discharge through the intermediate agency of heat, so the eye of the mind responds to those more rapid oscillations, the existence of which has been demonstrated by experiment. No less clearly does the magnetic field appear as a system of lines of stress in the ambient ether. Definiteness has taken the place of the metaphysical speculations of earlier times. Complete ignorance has, at least, been superseded by half knowledge. We may not yet affirm with Edlund that the ether is electricity, but we are doubtless nearer a solution of this old problem than ever before.

The discord is vanishing slowly,

And melts in the dominant tone. And they that have heard it can never Return to confusion again, Their voices are music for ever,

And join in the mystical strain.

The Westinghouse Company.-This company has decided to build a branch works in England, which will probably be situate in the neighbourhood of York Road adjoining the Westinghouse Air Brake Works.

SEPTEMBER 27, 1889.]

ELECTRICAL SURGINGS-AN EXPERIMENT *

By Prof. JOHN E. DAVIES.

IN the discussion of John B. Verity's paper on "Underderground Conduits," read before the Institution of Electrical Engineers on the 11th of April, the following remarks of the Chairman, Prof. Ayrton, attracted my attention, in consequence of a peculiar experience of mine some time ago while testing the potential of a 50 arc light dynamo carrying its full load. Among other things, Prof. Ayrton said :-"The question of the puncturing of insulated cables used for electric lighting is an extremely important one, and one which I think has not had sufficient attention given to it. It has been assumed that the electromotive force that the electric light cable will work at, is the electromotive force produced by the dynamo, and no more; but, as Mr. Verity points out quite rightly in his paper, there is the probability of a very much higher electromotive force being brought to bear on cables used for electric lighting, in consequence of self-induction-a subject which will come before us in another way at our next meeting, when Prof. Oliver Lodge is to give us a paper on lightning conductors. The surging backward and forward that he has so ably drawn attention to, that you have in certain cases in electrical conductors, produces an electromotive force infinitely greater than you would expect. A case was brought to my notice the other day of a somewhat extraordinary character. Two people were walking together in the Inventions Exhibition, along a court where there were a good many electric wires. They were several feet away from the wires, which were overhead. They were walking on wood, and they both say that they simultaneously got a sharp shock. Of course, from our old point of view, we should have said that it was quite impossible; they could not have received a spark of several feet with a potential difference of even a few thousand volts. At the same time, you have their evidence that they independently felt a shock at a certain moment, and said immediately to one another, I felt a shock.' They applied to the people in authority, but of course could get no information. Now, with the illustrations that we have had recently shown us by Dr. Lodge at the Royal Institution, it does not seem improbable that you may have potential differences set up in wires far greater than the potential difference that the dynamo can produce, due to the sudden stopping of the current; for example, whether you may be working at 100 or 1,000 volts, you may have 10,000 volts, or even more, produced. I think that may explain the breakdown of some of these cables."

For certain reasons, though with some misgivings, I determined some time ago to apply one of Sir William Thomson's graded voltmeters to the measurement of the potential difference of a 50 arc light dynamo under full load. Being apprehensive, I examined the instrument with great care, even going so far as to take off the rounded end block and examine minutely the small block of hard rubber to which the copper spring clips are screwed.

Everything was apparently all right; but for precaution I discarded the twisted leads sent with the instrument, which showed some signs of wear at one point, and used instead gutta-percha coated wires widely separated. On closing the key there was a loud snap, a tremendous flash, and the experiment was over. The last mark on the graduated scale of the voltmeter indicated th of a division per volt, in a dyne field; and I had on an accessory magnet which brought the field up to more than 10 dynes, thus making each division of the divided arc of the voltmeter equal to about 160 volts; and as the instrument could be read to 40 and 50 divisions, and the scale was even longer than theth mark, I ought to have been able to measure to at least 6,400 volts, and much more, unless the graduations of the instrument were meaningless. Moreover, the instrument had in the lower parts

*Western Electrician.

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of its scale been calibrated by me and found correct as marked, and the field of the controlling magnet was essentially what it was marked. This I had previously determined with some care.

On examining the instrument carefully after the experiment I could see nothing wrong; the minute terminals of the coil, where visible, were intact (the whole coil has a resistance of nearly 9,000 ohms); even the delicate solderings of the ends of these wires to the copper clips were apparently the same as ever. An ordinary current went freely through the coil along the very circuit where the current of large potential had just refused to go. A slight discoloration of the

6

wood near the end of the hard rubber block led me to very carefully take out all the little screws running into it. And here I found the trouble. The ends of these screws came to within about th of an inch of each other in the solid hard rubber; but in the case of one pair, either the hole bored for the reception of one of the screws had been carried clear through the hard rubber block, thus leaving an air space of th of an inch between the screw ends, or else some other defect of the material, or excess of length of the screws, had enabled the electricity to work its way through this way and form a temporary arc. Luckily I had used for one of the gutta-percha coated leads a quite small wire, which fused at the dynamo, or the consequences might have been more serious. The rubber had either been perforated at this place, or what is more likely, a spark had leaped across air. Now, taking the voltage of the dynamo at 2,800, or even 3,000, it is not likely that this would have occurred, save for the sudden diversion of the electricity from its regular channel into this side path-the "electrical surging," as Dr. Oliver Lodge so happily terms it. Another noteworthy point is the evident "throttling" effects of the selfinduction of the voltmeter coil. Rather than overcome this self-induction, the sudden rush of the current, like a huge volume of water under tremendous pressure approaching a long, tortuous, and narrow channel, preferred a side leap rather than enter the confined spaces of the wire coil.

I have for years felt that a sudden charging or discharging of a piece of apparatus, or circuit, was a very different thing from the gradual doing of the same; just as one can bear without injury a very heavy load placed gradually upon him, while a far lighter one suddenly applied may be destructive. The beautiful experiments of Dr. Lodge upon the differences between the striking effects of charges suddenly or gradually applied are a demonstration of the reality of this difference in so subtle a medium as the ether, and his calculations show us what differences to expect. As to the practical bearings of the above experiment, it is pretty evident that no matter what the graduations of the instruments might lead one to anticipate, it is not safe to apply them to dynamos carrying heavy currents under these high voltages. A previous test upon the same dynamo with Sir W. Thomson's electrostatic voltmeter had given good results. The only drawbacks to this beautiful instrument are the long time required for the needle to settle to its final reading, the uncertainty of its division values especially in the lower part of the scale, and the difficulty of accurately calibrating it. If one reads the description of it in Gray's "Absolute Electrical Measurements," or Rankin Kennedy's "Alternating Current Transformers," or Swinburne's "Practical Electrical Measurements," and then compare the figures given for the values of the weights required for 50, 100 and 200 volts per division, as given in Sir W. Thomson's latest circulars, he will be puzzled by strange discrepancies, as the figures are entirely different. My own weights are different from either or all of these-the instrument being one of the earlier ones sent out from the Glasgow laboratory— being in fact kindly selected and calibrated for use by Sir William himself. The only way to make reliable measurements with one of these instruments which one may purchase at random is to calibrate it for one's self; and this I have found it no easy matter to do for want of other standards of high potential whose value

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is absolutely and independently known, which one can compare it with.

It is a great pity also that, even though the cost would have been slightly enhanced thereby, the bearings on which the steel knife edge rests are not agate mounted. It seems ridiculous almost to make the axle a knife edge of apparently hardened steel while the surface on which it rides is merely of soft brass. An agate mounting would surely increase the sensitiveness of the instrument as well as make its indications more reliable.

DEPREZ-D'ARSONVAL DIFFERENTIAL

GALVANOMETER.

THE value of the Deprez-d'Arsonval galvanometer is beyond dispute. Thanks to its dead-beat action which allows of rapid readings, and to the slight influence which neighbouring bodies exercise on it, it has rapidly taken a first place in industrial laboratories, and it tends to supplant the ordinary needle galvanometer for measurements of precision.

The great number of improvements of detail which have been made in the instrument since it was first brought out have very considerably extended the range of the apparatus. M. Ledeboer has himself described an arrangement carried out by M. Carpentier with the view of rendering the instrument differential and useful for the purpose of numerous methods of measuring resistances based upon the compensation of the magnetic effects of two currents. In this arrangement two bobbins are wound parallel on the movable frame of the galvanometer, the ends of the coils being connected to four terminals through a double bifilar suspension.

The construction of such an instrument is found to be difficult, for though it is easy to give to the circuit of the two bobbins equal resistances and the same number of turns, it is extremely difficult to produce exact equilibrium by the differential effect of two currents acting in a magnetic field the lines of force of which are very irregularly distributed.

It is true that the two bobbins can be wound with a double wire as is often done in the case of differential electromagnets, but the necessity of reducing as much as possible the size of the movable frame prevents the employment of a thick covering to the wire, and there is a consequent liability of contacts between the two wires.

[SEPTEMBER 27, 1889,

In order to facilitate the construction and the adjustment of the instrument, M. Eric Gérard arms the poles of the magnet with two armatures which, instead of being fixed as in the ordinary apparatus, are movable about vertical hinges, and can be approached or withdrawn obliquely to a central bar of soft iron by means of two screws of very low pitch.

By this means the distribution of the lines of force of the field can be regulated at will and it is easy to produce magnetic equilibrium of the currents in the two bobbins.

In the apparatus (shown by the figure) which has been constructed by MM. Emile Gerard et Cie., the two regulating screws are represented on the back face of the magnet. The double bobbin is supported by a cocoon fibre which is axial to a bronze spiral spring forming a connection common to the two circuits. The other connections are made below by two wires. It must be evident that if the two ends of each bobbin require to be available, the upper spiral spring must be replaced by a two wire suspension, though at the cost of diminished sensitiveness.

BRADFORD CORPORATION ELECTRICITY SUPPLY.

THE electric lighting station was formally opened by the Mayor (Alderman W. Moulson) on Friday last. The installation is the continuous current system; and is intended for the supply of lighting, motive power. storage, and for other purposes.

It is proposed, in the first instance, to supply that portion of the centre of the town which comprises Market Street, Kirkgate, and the streets in the immediate vicinity thereof.

The mains, lead encased, and with suitable outer protection, are laid underground in all the above streets. The cables are placed directly in the earth, generally under the causeways or pavement, and where crossing the streets a flag-topped brick culvert protects the cables from the heavy traffic.

The central station is situated at Bolton Road, a short distance outside the district of supply. The buildings, when fully completed, will be capable of containing considerably more generating plant than is at present placed there.

The present generating plant consists of three Lancashire mild-steel boilers (28' x 7') of 180 H.P. each: three steam engines, of the inverted vertical type, of 150 H.P. each, and working at 120 lbs. steam pressure. Two of these engines being made by Willans and Robinson, Limited, of Thames Ditton; the third one being by Marshall, Sons and Co., Limited, of Gainsboro. Each of these engines is coupled to and drives direct a Siemens's dynamo, capable of developing 120 electrical H.P. (90 kilo-watts). Each of these dynamos delivers its current to a conductor common to the three; whence it is supplied to the various feeding mains.

These feeding mains, at present four in number, deliver the current at their outer extremity to four feeding centres situated at different points in the town; suitable controlling apparatus being placed at the central station to ensure a uniform electrical pressure of delivery at the feeding centres.

From these various feeding centres extends a network of distributing cables connecting together the various streets, and the several feeding centres. To these distributing mains the houses are directly connected.

About ten miles of cable are comprised in these feeding and distributing mains, exclusive of the connections to the various houses.

A number of street boxes and other forms of junctions assist in forming the branches in the several streets and help to complete this system of distribution.

The branch service line to each house ends in a terminal box and an electricity meter to which are connected the wires of the private lighting of each premises. The meters at present in use are those known

as the " Aron," the "Hookham," and the "Edison," the particular meter supplied to each premises being in accordance with the quantity of current required.

The prices at which the Corporation propose to supply the electric current is 5d. per Board of Trade unit; or about double the present price of gas in the borough.

In consequence of the proximity of the district of supply to the generating station the current is supplied at present, direct at the low pressure at which the houses are required to be supplied by the Board of Trade. But later on, as more distant districts have to be supplied, the "feeding mains," or carriers, will be put upon high pressure; arrangements being made in the laying of the mains to enable this to be done.

The buildings have been constructed by Mr. Wm. Johnson, of Bradford; the boilers by Messrs. Holdsworth and Sons, also of Bradford; while the entire of the electric plant, including dynamos, cables, electric instruments and appliances of all kinds for the regulation of the current, have been constructed and laid by Messrs. Siemens Bros. and Co., Limited, of London. The installation has been designed for the Corporation, and superintended during erection by their electrical engineer, Mr. James N. Shoolbred, Mem. Inst. C.E., of Westminster.

THE BRITISH ASSOCIATION.

SERIES ELECTRICAL TRACTION.

(Northfleet Tramways.)

(Read in Section G, September 16th, 1889.)

By EDWARD MANVILLE, M.I.E.E.

THE expenses of horsing a tramway form such a serious proportion of the total working expense, that any cheaper effective method of propulsion will naturally be gladly received by the various companies.

Steam and compressed air have been extensively tried, and although in some cases their expense has compared favourably with horsing, the machinery used is so cumbersome, and the nuisance so great, that even companies who have incurred the expense of abandoning horses in favour of steam are seriously meditating the adoption of electricity.

The cable systems have met with considerable success in the United States, and have achieved some measure of success in the United Kingdom; but the conditions under which a cable line can be operated economically are tremendously heavy traffics, such as do not exist in the United Kingdom, but do in the United States, where, as is well known, tram cars are practically the only available street conveyances.

The

Many leading authorities in tramway administration who have tested and considered the various methods of tram car propulsion, have of late publicly expressed their confidence in the great suitability and economy of electricity for the purpose. subject is therefore one of peculiar significance, not only to shareholders in the direction of increased dividends, but also to the large body of the public using this popular method of conveyance from the point of view of reduced fares.

It is obvious that in systems of mechanical propulsion requiring the distribution of power over many miles of line, that the efficiency of the distribution will be better as the number of generating stations is diminished, and that the maximum economy is attained when all the power for a complete system is developed at one only.

To distribute electrical power economically from one generating station over distances such as are covered by tramway systems of any extent, it is essential that high tension currents should be employed, so as to minimise the loss of power in the conductors. Now, if the full tension of such currents were applied to the driving motor on each car, grave difficulties would present themselves in manipulation and in preserving the motor from injury. By the series system, however, it is possible by running the motors in series-as the name indicates-to attain the maximum ease of manipulation together with perfect safety to the motor. This was recognised at an early stage by the late Prof. Fleeming Jenkin and Profs. Ayrton and Perry, and the first electric locomotives ever run in series were used by them in connection with telpherage.

The subject of electrical traction reached such a degree of prominence in this country that early last year steps were taken to demonstrate the practicability of running electric motors for tramcars in series with each other, and an electrical tramway on the series system was equipped and put into operation at Northfleet.

The essential parts of an electrical tramway line in which the motors are run in series are: an electric generator producing a current of constant quantity and an electrical pressure varying continually, according to the total work being done by the whole

of the motors together; the conductor taking the current from the generator to the motors, provided with automatic switches of such a character as to enable the continuity of the conductor to be broken as long as the motor is in electrical contact with the open ends of the conductor, and to be closed as soon as the motor, in passing away, ceases to be in contact with the ends of the conductor.

This operation must be performed without disturbing the metallic continuity of the circuit, and without short circuiting the motor.

The generator used at Northfleet is one of Statter's patent constant current dynamos, in which type of machine the E.M.F. is varied by the alteration of the position of the brushes on the commutator; this is effected by an electrical regulator. The current, after being generated in the dynamo, before going to line, passes through a solenoid containing a moving soft iron core. The position of this core regulates the position of the brushes by means of a double ratchet working on a wheel with two sets of teeth on its periphery, which wheel moves the brushes through a gear. The double ratchet is kept rapidly moving too-and-fro by an eccentric working off the dynamo shaft through a reducing gear. So long as the current is of the proper quantity, the position of the core is such that the ratchet does not engage with either of the two sets of teeth on the wheel, but when the current rises or falls the position of the core is altered, so as to cause the ratchet to engage with one or other of the sets of teeth, when the position of the brushes is rapidly altered to adjust the current quantity. The pole pieces of the dynamo are so shaped as to reduce the "sparking" that might be supposed to result from the variation of the lead of the brushes so much as to make it practically negligible. The E.M.F. is varied from a few volts to upwards of 400. The current, after passing through the regulator, goes to a highly-insulated cable, reaching the whole length of the line. The cable is cut at distances of 21 feet along the line, and the ends of it are led into terminals connected with the opposite faces of a spring-jack," which is at the same time the automatic switch and contact point from which the current is collected. From the last "spring-jack" at the end of the line, a return cable, with its insulation unbroken, is brought back to the other terminal of the generator, owing to the line at Northfleet being a single track.

The "spring-jack" just referred to consists of a pair of glazed eathenware blocks, 14 inches by 3 inches by 4 inches. To each block is attached, by means of a double spiral spring, a gunmetal casting, curved at the ends, but flat in the centre. The springs are of sufficient strength to press the two castings together with a force of 6 lbs. Under the car, and for its entire length, is the collector, or "arrow," which consists of two thicknesses of India-rubber belting, each having a broad brass strip rivetted to it for nearly its entire length. The nose of the arrow at each end is shod with wrought iron, brought to a knife edge, so as to easily force its way between the two faces of the "spring-jack," which automatically close after its passage. The maximum thickness of the "arrow is 1 inch, which is consequently the extent to which the gun-metal cheeks are separated. The conductor on each side of the arrow is lapped round one end, and an insulated space is left, slightly greater than the surface of contact of the "spring-jack near the extreme ends on opposite sides. By these gaps, the passage of the collector, from one "spring-jack" to another, is effected without short-circuiting the motor. It will be readily seen from this that at no moment is the current cut off from the motor, or at no moment is the motor short-circuited. In fact, the ammeter carried on each car shows the constant value of the current, without any variation.

The conductor on each side of the arrow is connected by an insulated cable with the motor carried on the car. On each platform of the car are two strong switches. One of these serves to reverse the direction of the rotation of the armature by altering the connections between the field magnets and the armature, the other regulating the power of the motor by shunting more or less current from the field magnets. Three speeds are provided for, and the stopping of the car is brought about by completely short-circuiting the field magnet, still allowing the current to pass through the armature.

The motors are run at 400 revolutions per minute, delivering at this speed, when the field magnets are fully excited, 15 H.P. on the brake. This slow speed was adopted to do away with intermediate gearing between the motor shaft and the car shaft, which has up to this time been the usual practice.

At Northfleet the motor pinion gears direct with a spur wheel on the car shaft in the ratio of 1-4. The gearing is double helical, with a view to noiseless running, and the practical working of this gear is most satisfactory.

The motors are mounted in a way now generally adopted in the United States, one end being supported by two half bearings on the driving shaft of the car, the other end being suspended by a spiral spring from a stout beam across the car body.

The motors are of the Elwell-Parker type, and the field magnets are connected in series to the armature. One of the cars is lighted electrically by means of low resistance Bernstein lamps. Three accumulator cells, of the tramway type of the Electrical Power Storage Company, are inserted in the main circuit, and five lamps, of 10 ampères each and 6·5 volts, are run in parallel off the accumulators. The accumulators themselves, however, are not discharged; they only serve as potential reducers, the whole of the main current going through the lamps. Should one of the lamps break, the remainder are slightly more brightly incandesced,