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ELECTRICAL REVIEW.

mentative testimony, saying, among other things, that so far from his finding constant potential in Brush's specification, it had taken the Brush people five years to find it there, and he had not found it there yet. He had first told Brush of it.

Prof. Ewing offered didactic evidence; and gave more information about the storage of magnetism. He has, as is well-known, devoted great attention to magnetism, especially to the storage of magnetism. He stated in answer to a question of Lord Trayner, as to whether there was any manifest absurdity in the first three lines of page 7 of Varley's specification "On the contrary, they show that Varley's knowledge of the subject was very much in advance of his time."

The plaintiffs' case was concluded by the appearance of Mr. Brown himself, with an account of an exceedingly complicated accident that had spread like an epidemic over a system of batteries, dynamos and hydraulic rams, with which he had been performing some abstruse feat. The defendants did not cross-examine him as to this. Mr. King, who had looked after the technical part of the case did not appear.

The defendants opened their case with Prof. S. P Thompson, with his first name spelled wrong. His evidence was tuitional and ratiocinative. He hung the walls round with coloured charts and maps, and went to work with a long pointer. He understood and explained everything but Varley's specification; of that he could. make nothing. We think that if he went carefully through Varley's specification with an unbiassed mind he would find no difficulty in understanding all the connections and the working of the machine. Prof. Perry followed, supporting Prof. Thompson, and gave cautious but dogmatic evidence. We do not agree with him that a machine with wrought iron magnets sparks when unexcited.

Sir William Thomson then gave profuse and unbiassed evidence.

Mr. W. H. Preece then followed with much useful information, leading us to suppose compound-winding was invented by the Navy. He never gave any scientifically accurate definition of magnetic potential. His Lordship suddenly asked him whether he belonged to the post office in the next street, or came from somewhere else. He thought Varley's machine such a poor thing at the time it was invented, that it should have been chucked on the dust heap.

Mr. R. P. Sellon gave commercial evidence for his company, and the case was adjourned till Brush's own version could be obtained.

As to the first contention, the Judge held Brush's patent to be void. The whole of the question as to the clearness of the rest of Varley's specification is left untouched in the judgment, and naturally so, for it has little to do with the case; if the specification had been all nonsense except a clear description of series shunt winding, that would have invalidated Brush.

Similarly in the case of Varley's machine, it did not ter in the least how it might be connected up accord

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[JULY 5, 1889.

We are a little surprised at the decision as regards the detachable segments. We certainly think that the learned Judge confused Varley's commutator with Wilde's. In Wilde's the insulation did not form the body of the commutator, in Varley's and Brush's it did.

On reading over the evidence we cannot refrain from commenting on the strained views taken in many cases by the witnesses. This was commented on to some extent by the Judge. For instance, in the Brush ". Crompton case the Brush affidavits were practically unanimous in saying Varley described a double shunt machine. Brush, who probably had not time to wade through a sheaf of affidavits with a microscope-if mixed metaphors are allowed-made out Varley to mean separate armature excitation; and the other witnesses promptly find the same thing for the sake of uniformity.

Mr. Murray's cross-examination always had a saddening effect, and generally wrung an unwilling admission of what he wanted. He showed the keen interest of the sportsman. In one case the Judge said, "You've been playing this fish long enough, I think you'd better land him," which he promptly did.

We are glad, for the sake of the industry, that this patent is broken. Patents, one way or another, have been the greatest bar to progress in electric lighting, as in most new industries. One great defect of our system is that somehow or other the valuable inventions are covered by people who have not invented them. Generally something is described for some chance purpose, and covers subsequent developments as in this Jablochkoff's is similar. He saw that if he put his lamps in series on a circuit, and one went out, the others would go out, too; but he thought he would prevent this by putting them on transformer secondaries, so that the dynamo circuit would remain intact, and the lamps would be independent. He is then accidentally enabled to collect royalties on modern transformer distribution. Edison's lamp patent is a notorious example of a "fishing " patent.

case.

As we mentioned in our last, we hope Mr. Varley will get something substantial from the trade. A very few months of the royalties now saved might help him a good deal. The trade is also very deeply indebted to Messrs. King, Brown & Co. for fighting this case, for the validity of a doubtful patent is so much more important to the assignees than to any one alleged infringer that a large company has a very unfair advantage; but if a combination of all the alleged infringers defended, such patents would be properly fought out, and would then be done with.

WE are assured that an Electric Lighting Company in London is placing the conductors inside some houses directly into the damp wall plaster without any protection whatsoever. We most of us know how electric bell circuits act in a short time when cheap and nasty work has necessitated similar methods of running leads. Mr. Heaphy's attention might well be called to this little incident,

ELECTRICAL REVIEW.

Sir WILLIAM THOMSON, in consequence of the recent fatal accident at the works of Messrs. Siemens Bros., where a labourer was killed by an electric shock, is going to issue a memorandum of instructions to users of his electrostatic voltmeters. A copy of these rules may be seen in our other pages.

WE learn upon very good authority that it is highly probable that the compound-winding case will not, after all, be carried to a higher court. While this, however, remains uncertain, the Anglo-American Brush Corporation has sent notices to the trade stating that it is not the Corporation's intention to collect royalties. pending further consideration. Probably this was a wise decision to arrive at, for it is not likely they would have been paid under the present circumstances, and we have always felt that the claim for royalty would never have been generally put in force had it not been for the ill-advised action of Messrs. Siemens in the first place passively submitting to the demands of the Corporation.

IN the opening number of a new journal, the Steamship, Mr. G. J. Burns, B.Sc., assistant-surveyor to the R.E. Department, commences a series of papers on electric lighting.

IT behoves Mr. Preece to push forward his arrangements for the barrel of gunpowder experiment in which he is to take the active part. Everybody is now on the tiptoe of expectation, and the experimental details should certainly be in readiness for the next thunderstorm.

Perhaps, however, Mr. Preece had something up his sleeve when making his now famous boast, and that his gunpowder after all is only tea. In any case, the matter is assuming dimensions which threaten to become serious if we may judge from an article in our own columns and the cartoon in this month's Electrical Plant.

THE fourth Edition of "Everybody's Pocket Encyclopædia" has been revised and enlarged. This compilation of things worth knowing; things difficult to remember; and tables of reference is about the cheapest and best sixpennyworth of which we know.

MUCH activity is manifest just now in electric lighting circles. It is evident that a great desire exists to introduce the most formidable competitor of gas into a number of towns in the United Kingdom, and this will doubtless be satisfactorily accomplished within a short period. We do not anticipate, however, that gas shareholders need be alarmed, for we believe that the progress of electric lighting will lead to a still greater development of gas consumption in many ways.

A GLANCE at our columns devoted to the registration of new companies will show that, as we have observed above, extraordinary activity reigns supreme in electric lighting circles. There were registered between Tuesday of last week and the same day of the present week, no fewer than 63 new companies, just onethird of which are connected with electricity. Twelve of these, for different districts, are promoted by the House-to-House Electric Supply Company, Limited, and the remainder are miscellaneous. During the electric craze of some years ago, we think that the greatest number of companies registered in any one week fell considerably short of those we chronicle elsewhere, but it is now necessary to register companies within a given period in order to obtain the Board of Trade license for this year. It will be noticed that amongst the objects of some of these companies is one

to train and supply consulting electricians, engineers, and skilled workinen. Surely these are not altogether marketable commodities?

THE following incident reminds one of the old saw of "The better the day the better the deed," for on Sunday last a river trip was organised by General Brine for the purpose of taking General Boulanger up to Hampton Court in the electric launch Volta. France's only General, unfortunately, or fortunately as the sequel shows, did not put in an appearance, but about twelve British generals and officers of rank were on board. On the return journey, after a sumptuous dinner, everything went well until about 9.30 in the evening, when the launch was run aground, below Hammersmith, on a sand bank in the middle of the river. The united strength and stratagetic manœuvres of our gallant defenders failed to release the Volta from her fearful position, and it was past three o'clock the next morning before the rise of tide enabled the brave voyagers to wend their way homewards. This record of a day's trip reads like a huge joke, though we are assured that the facts are precisely as we state them, but we cannot understand even now why this cockle shell of a vessel should have been left for five minutes hard and fast with such a capable crew on board. The steering at all events must have been erratic, but what can be expected with a "longshoreman" at the helm and pleasure at the prow?

WE are afraid that English manufacturers find it exceedingly difficult to compete with foreigners for the supply of goods to the Continent, especially with Antwerp firms. We are informed that Edison dynamos and lamps can be delivered in Oporto, without any trouble to the purchaser, at a considerably less cost than English lamps and machines (for a similar output), can be bought here.

THE British Association will hold its meeting this year at Newcastle-upon-Tyne, under the presidency of Prof. William Henry Flower, F.R.S., commencing on the 11th, and ending on the 19th September. Every effort is being made to render the gathering successful, both in point of numbers attending and attractions offered to visitors.

THE Surrey Comet of June 29th contains several columns of correspondence relative to the proposed electric lighting of Richmond, Wimbledon, Hampton Wick, Surbiton, and the neighbouring districts. The most importment letter is from Mr. Daniel De Castro, who foreshadows the operations of the " South Western District and Thames Valley Electricity Supply Company, Limited." Mr. De Castro estimates that if the whole district can be secured current can be delivered in any part at 4 d. per unit, about equivalent to gas at 3s. 6d. per 1,000 cubic feet, and that the company can easily pay a dividend of 5 to 6 per cent., besides annually setting aside substantial sums as a sinking fund.

THE report of Crompton and Company, Limited, reads very well when we take into consideration that the company has only been just 12 months in existence, and doubtless the second year's result will be still more to the satisfaction of the shareholders. Certainly the outlook is to-day very much more encouraging to manufacturing companies than it was last summer, and everything points to a considerable strain upon the resources of electrical factories generally in supplying demands,

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ELECTRICAL REVIEW.

THE INVERSIONS OF POLARITY IN SERIES DYNAMOS.

Note by M. A. WITZ.

DYNAMO machines, when excited in series, are subject to reversals of polarity, which limit their usefulness, as this causes at times great inconvenience. Happening to come across a machine of extreme instability, we were led to study this curious phenomenon and investigate the causes of it.

The most simple case is when the series dynamos are applied to electro-chemica! operations, especially to the charging of accumulators. When one of these generators supplies a certain number of secondary batteries, the magnetising is effected without difficulty, because the current, supplied by the incompletely discharged apparatus, is sufficient in general to determine the polarity of the inductors; the direction in which the armature must be put in motion is the direction in which it would turn under the action of this residual charge, if it was movable. The operation goes on very smoothly until the counter-electromotive force of the accumulators attains a certain value; then the slightest variation in the proceedings is sufficient to change the poles of the generator and reverse the nature of the current. This inconvenience is avoided by the employment of special coupling and disconnecting gear, and better still by exciting the dynamos in derivation.

These facts are known to all, but I have referred to them in order to compare them to analogous phenomena that I have observed in experiments on the transport of energy. For this purpose I employ two little dynamos of the Edison type, which are identical, and only differ in the mode of excitation: the generator is a series dynamo, while the receiver is excited separately. The machines having attained the rate desired, the difference of the electromotive forces E — e remains constant, the resisting couple being itself constant. Now, let us diminish the effort to be overcome; the receiver accelerates its pace, and, the inductive felds being equal, it tends to take the same speed as the generator: at the same time the counter-electromotive force, e, increases and the intensity of the current decreases in consequence. At this moment the slightest variation in speed is sufficient to reverse the poles of the generator, and I then see the receiver take a periodic alternating movement, performing indefinitely some turns to the right and some to the left with astonishing

This alternation in the rotations will be the better strained the more rapidly the receiver is discharged, The greater the speed that it takes, and the more intense the field that is excited between the poles. It can be avuded by reducing the field so as to prevent all angmentation of speed. The same results are obtained by empurring a magneto as receiver: with a series ernans I only observed one demagnetisation of the generator, and therefore one stoppage of the receiver; * was easy to foresee.

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bm the ♬ the most singular point about this periodialtered rotation. After the receiver has proTugas UT it reaction the reversal of the poles of the poterator. i fire exhausts the energy of its flywheel, S1 M* FLÆBc changes its sign: it then turns in the anana se ion trail its speed is sufficient to reproNow this speed is relatively 121+ seered & by means of a vibrating VIEL scribed its vibrations on a disc It is much less than that which art for the concier-electromotive force as that of the generator, as the followThe generator making 1.71 revolu$ou Pra ega see took a maximum speed of 252 POVINUR Que eggy var being held, the 1781 revojutor generator gave 193 volte, and the generator being held, the 201 revolutions of the receiver could only deep darbe vole, that is to say the onetenth at the out the following

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[JULY 5, 1889.

manner, which also applies to the charging of the accumulators. Let us construct the external characteristic of the generator. We will take through the origin of the co-ordinates a straight line, O, A, making with the axis of the r an angle, a, so that its tangent may be equal to, R, the total resistance of the circuit. This being done, let us trace a tangent to the characteristic parallel to the straight line, O, A ; it cuts the axis of the y's at a point, B. The point of contact of the tangent to the characteristic gives the value of the electromotive force, E, of the generator at the moment of the inversion, and the ordinate, 0, B, marks the counter-electromotive force, e, of the receiver capable of producing this inversion. The value of e will be less as the total resistance, R, is more considerable; on the other hand, we see that diminution of speed in the generator will lessen the characteristic and hasten the moment of the inversion; this is what we observe in reality. We may remark, also, that an increase of speed in the receiver increases the apparent resistance of this machine, and, consequently, also the total resistance, R. Lastly, let us prolong the tangent to the characteristic as far as its meeting with the symmetrical branch of this curve. The ordinate of the point of intersection gives the value of the electromotive force developed instantaneously after the inversion of the rotation and the reversal of the poles. We then sum up the facts observed, and the agreement between the values thus obtained and the figures arrived at experimentally is such as to enable us to think this explanation is perfectly justified.-Comptes Rendus.

PRECAUTIONS FOR SAFETY IN THE USE OF SIR WILLIAM THOMSON'S ELECTROSTATIC VOLTMETERS IN CONNECTION WITH DYNAMOS, WHETHER FOR DIRECT OR ALTERNATING CURRENTS.

SEC. 1. In all applications in which one of the two conductors connected with the voltmeter is kept permanently connected with the earth, this conductor should be connected with the outer case of the voltmeter. The other conductor is to be connected with the insulated terminals and must be carefully guarded against accidental contacts.

Sec. 2. To provide for use in any application not fulfilling the condition of sec. 1. All the electrostatic voltmeters are supplied with thoroughly insulating feet and the precautions stated below in sec. 3, 4 must be observed.

Sec. 3. The vertical scale voltmeter for from 400 to 12,000 volts, when set up for permanent use, should be enclosed in a case (which may be of wood with a glass front), preventing any person from accidentally touching the metal case or the terminals of the instrument. The vibration checker is worked with perfect safety by a silk cord passing through a hole in the wood or glass of the protecting case to the front outside.

For temporary or experimental applications the user must take his own precautions: an outer enclosing case might be found too cumbrous.

Sec. 4. For ordinary domestic electric lighting, or other applications to less than 200 volts, the multicellular voltmeter may be left unprotected, so far as personal danger is concerned; but to avoid chances of damage to instruments or wires, or of melting a fuse, its outer case as well as its terminal insulated from the outer case, ought to be perfectly guarded against accidental contacts, when the instrument is set up for permanent use. Glass and vulcanite sheaths are provided for this purpose by the instrument maker when desired.

Sec. 5. General Warning.-Never open the case of the vertical-scale voltmeter to change the weights, nor touch its terminals to connect or disconnect (or to secure either connection if imperfectly made), without being sure either that the dynamo is not running, or that both the conductors leading to the voltmeter are safely discconnected from its circuit.

JULY 5, 1889.]

ELECTRICAL REVIEW.

Sec. 6. It may be asked, with reference to the vertical-scale voltmeter, Why is the inner case made of metal ? The answer is, that the electric conditions for definiteness of measurement, require the wave to be protected from sensibly disturbing influence of any substance other than the air around it, differing in potential from itself, unless at the same potential as the quadrants. Why, then, not coat the metal inner case with wood or vulcanite, or other non-conducting material? Ans. The protection thus imagined might be delusive when 10,000 volts is dealt with. Safety is most surely secured by an outer case an inch or so from the inner metal; unless, which is always best when it can be arranged for, one of the conductors is kept connected with the earth, and with the metal case of the electrometer also connected with the earth.

WILLIAM THOMSON.

THE PRESENT POSITION OF THE ELECTRIC LIGHT IN PARIS.

[A COMMUNICATION.]

THE question of the electric lighting of great cities has now become such a prominent one, and especially in London, since the holding of the Board of Trade inquiry by Major Marindin, that we offer no apologies to your readers for placing before them the following latest and exhaustive details on the electric light as it is at present installed in Paris.

Three companies have installed electricity on the boulevards. The companies which share this lighting are-The Popp Company, the Edison Continental Company, and the Transmission of Power by Electricity Company. The Popp Company has charge of the section comprised between the Place de la Concorde and de l'Opéra. It possesses at Saint Fargeau a large central motive power works (4,500 horse) for compressed air, which is canalised in Paris. It has also a large number of small provisional electro-motor stations, most of them for theatre and private lighting, among which are the following:-1. Place de la Madelene; 2. Rue Meyerbeer; 3. At the Montagnes Russes (switchback railway); 4. Boulevard des Capucines, No. 5; 5. At the Eden Theatre; 6. At the Variétés; 7. At the Bourse du Commerce; 8. At the Château d'Eau; 9. At Bataclan; 10. At the Magasins du Travailleur. Three large works are in view to replace them: the first, in the Rue Boissy d'Anglas, near the Madeleine, is expected to be ready in two months; another is projected near the Bourse du Commerce; and the last, of 2,000 horsepower, in the Rue Royale, which will be more specially destined for arc lamp lighting. Each will be capable of feeding 20,000 lamps. These will be joined, and the current will pass through accumulator batteries which are intended to prevent variations and extinction in case of accident. The compressed air receivers generally have a force of 50 horse-power, sometimes 100. One receiver works a Thomson-Houston dynamo which feeds 30 to 50 Thomson-Houston arc lamps by continuous high tension currents (30 ampères, about 2,500 volts) with Gramme dynamos as exciters. This company employs Faure-Sellon-Volckmar accumulators. The canalisation is made in metal tubes, with Indiarubber joints placed in trenches at the necessary depth. The cables, of small diameter, are composed of copper wires covered with India-rubber canvas in a layer of India-rubber, and enclosed in leaden pipes. They are laid freely in the conduits without supports, and may be drawn out by traction. The section of the cable is calculated per 10 ampères, and it is insulated for 2,500 volts. Draft-holes, placed at about every 50 mètres, have permitted the introduction of the cables into the conduits. A commutator in each draft-hole gives the facility of cutting off the current which feeds a corresponding candelabra. The shape of the ThomsonHouston lamps is elegant: they are composed of a sort of lyre, which bears a spherical globe and a reflector, and a cylindrical muff, which encloses the regulating apparatus. The candelabra, of a model only adopted by the

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City of Paris, are 6 mètres high, slim in shape, moulded and bronzed. They are installed either on rounded refuges, 4 mètres long by 1 m. 70 wide, or on the edge of the footways. In the Rue Royale they have been disposed in quincunx. Their distance varies from 30 to 50 mètres. Since May 24th the boulevards have been lighted by the electric light by the Popp Company, from the Place de la Concorde to the Place de l'Opéra, by means of 44 arc lamps.

The Edison Company, to which the Palais Royal station belongs, possesses another factory situated in the Rue du Faubourg Montmartre, known as the Drouot station. This works is to serve the Grand Boulevards from the Place de l'Opéra to the Porte St. Denis. It already supplies 4,200 incandescent lamps for private service by an aerial canalisation, in which a current of 1,600 ampères is furnished by two Edison dynamos commanded by two Weyher and Richemond horizontal engines of 150 horse-power each. Since May 24th it has lighted by electricity the salle of the Folies Bergères by means of 520 incandescent lamps and 34 arc lamps, through a subterranean canalisation which is the beginning of an intended circuit in the Faubourg Montmartre. In this connection a Corliss engine of 800 horse-power works two Edison dynamos of 800 ampères. Its boulevard canalisation, which is in course of execution, will comprise branches in the principal neighbouring thoroughfares the Rue Richelieu, Rue Vivienne, Rue Taitbout, Rue du Helder, Faubourg Poissonnière, Rue Halévy, Rue de la Chaussée d'Antin, &c. 6,500 ampères at 120 volts will be produced by the Edison dynamos, worked by 1,600 horsepower-300 of which are held in reserve-engines which are being installed in the same central station of the Faubourg Montmartre. 10,500 kilos, of steam will be furnished by four Babcock and Wilcox boilers, and 8,500 by four Belleville boilers. The Edison Company's cables are formed by 30 to 40 silicious copper wires, supplied by the Lazare Weiller firm, which are twisted in one or several strands. They have no covering; they are supported by porcelain insulators and pressed on their stays by pressure screws. The vertical insulators are made fast in a conduit of mortar and cement, established at a small depth under the pavements. In passing the transversal streets, the cables descend into a shaft about 0 m. 75 in diameter, several mètres deep, furnished with steps, pass under the sewers, and remount on the other side of the street in a similar shaft. The secondary wires are laid in an insulant and in a leaden tube when they leave the collector conduit and when they no longer repose on insulators. The circuit comprises 38 arc lamps; each lamp takes 10 ampères. lamps are placed on 100 volts; they are of the Pilsen system. The interior aspect is that of a large lantern formed of two cone trunks, the lower cone of rough glass, the upper of metal. The candelabra are placed on refuges, in the middle of the road. The canalisation works are nearly finished, but the central station is not yet quite ready to furnish the current.

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The Transmission of Power by Electricity Company (Marcel-Deprez system) has its central works at Saint Ouen. Besides this it has set up stations at the Rue Filles Dieu and Rue de Bondy. The Rue de Bondy station at present supplies the following theatres :-The Ambigu, the Folies Dramatiques, the Porte St. Martin, and the Renaissance. For this lighting it has two semi-fixed horizontal Weyher and Richemond engines, each of 70 horse-power, which work four Breguet dynamos and 250 ampères and 120 volts; and two Lecouteux engines, of 70 horse-power each, working two Thury (Switzerland) dynamos, of 500 ampères and 120 volts. In order to assure the lighting of the section of the Grand Boulevards, from the Porte St. Denis to the Place de la République, this company is going to place in this same station four engines of 200 horse-power, which would feed 25 arc lamps of the "French Company's homogeneous lamp" system. It uses cables analogous to those of the Edison Company, but of stronger section, and formed of a larger number of strands, 50 to 60 wires. The insulators

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are a little different, and the conduits are 0 m. 60 by 0 m. 50 in size. This company's work also is not yet completely finished.

Let us add, as another detail as to the electric light in Paris, that the express boats on the Seine, which are employed in the Exhibition service, have for some months now been lighted by electricity. A special steam engine of four to five horse-power works a continuous current Breguet dynamo. Both are placed symmetrically on each wall of the keel. The current produced feeds 17 incandescent lamps, which give light to the engine room, to the interior saloons, and to the fore and aft lamps.

Lastly, much has been said of the recent installations at the Elysée. The installation of the electric light carried out by the Continental Edison Company is the first in Paris in which secondary transformers have been used. In preceding years the grand official balls given at the Elysée were alone lighted by electricity. It was then necessary to have a temporary installation, to use accumulators, and then to remove the whole to begin again afresh the next time. M. Carnot, having taken into consideration the inconveniences of this arrangement, decided to make a definite installation which would serve for the whole of the receptions. It being deemed unadvisable to introduce steam engines into the palace on account of the trouble it would cause, and also because it would involve the employment of a special staff, recourse was had to secondary transformers, which permit of electric lamps being fed from a distance. The Edison Company, to whom the contract was given, therefore installed at its Palais Royal works a Zipernowsky dynamo of alternating currents, with which it supplied the transformers placed at the Elysée. The canalisation between the Palais Royal and the palace was composed of two copper cables, perfectly insulated, and fixed in the vault of the sewers. These cables were made at Bezons, in the workshops of the General Telephone Company, whose facilities allowed them to establish them in less than 15 days. The transformers of the Zipernowsky type, 12 in number, are connected in parallel on these two cables; the tension on the primary circuit is about 1,800 volts. Each transformer feeds a special circuit of distribution, on which the incandescent lamps, of 44 volts each, are joined according to the three-wire system of Edison. The electric light, which numbers about 2,000 incandescent lamps, is installed in the salles de fête and in all the rooms on the ground floor; the lamps being fixed in the lustres, &c. The light, which is of remarkable steadiness, shows off the richness of the furniture, the tapestry, and the decoration of the salles splendidly.

STUDY OF THE DURATION OF LIGHTNING.*

Note by M. E. L. TROUVELOT.

IN an article published last year, I asserted that lightning has not the instantaneity generally attributed to it, and, in support of this assertion, I reproduced in my note a photograph obtained during the storm of the 22nd July, 1888. This photograph showed the lightning in the form of a wide vertical band with numerous parallel strokes, and formed of numberless horizontal striæ. This singular appearance of the lightning I attributed to the horizontal movement that I imparted to the apparatus during the time of pose.

This simple manner of explaining the phenomenon, however, gave rise to many objections on the part of several distinguished savants. It would have been easy for me to reply to these objections and show their weak points, but as I had in hand palpable facts, I preferred to let these facts speak for themselves, feeling convinced that they are unanswerable.

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[JULY 5, 1889.

I will now give these facts: On the 22nd of July last, I saw on the horizon at Paris a brilliant flash of lightning, which seemed to me to last several seconds, during which it showed several fluctuations in brilliancy, and seemed as if it were the product of an oscillating luminous electric current of very variable intensity. Convinced by this observation that the lightning could not always be instantaneous, and wishing to obtain a palpable proof that it is not, I immediately directed my photographic apparatus to the same part of the heavens, taking care to impart to it a slight horizontal backwards and forwards motion during the pose. I had not to wait long. A flash appeared, and the development revealed the manifold forms already described, which I have the honour to submit to the Academy.

On the 7th of June last, a far-off storm gave flashes weakened by the rain and mist. The apparatus was directed to that part of the sky where the flashes appeared most frequently, and during the pose I imparted to it a somewhat rapid horizontal swinging movement on its axis. From rather a high point above the horizon appeared a flash, which spread out on each side, forming several horizontal branches. The development showed it with its wide horizontal striæ parallel to the direction of the movement imparted to the apparatus, having the appearance of a light streamer waving in the breeze.

We will not dwell more at length on the meaning of this representation, for, in our opinion, it indicates sufficiently that lightning has an appreciable duration, if not in all cases, at least in some; and consequently the Wheatstone experiments, which only give to a flash the duration of ro of a second, are somewhat in

error.

If this last photograph of the flash still left any doubts as to the appreciable duration of a flash we might add that a peculiarity of detail in our plate of the 22nd July, 1888, which at first escaped notice, shows clearly that the multiplication of the fulminating stroke results from the displacement of the apparatus during the duration of the flash. In fact the little cloud which, towards the bottom of our photograph, traverses the multiplied flash is doubled on the plate in consequence of the displacement undergone by the apparatus during the pose; and the displacement, measured on the inequalities of the edge of each image, is absolutely the same as that which the flash has undergone.

As we showed above, it is not only by means of the photographic plate that we can prove that lightning has an appreciable duration; the eye itself is capable of appreciating this phenomenon, as we were able to prove on the 7th of June last. In fact, a flash coming from a part of the sky hidden by a building soon shows several branches in the part of the sky that is visible. Now, it could be plainly seen that these branches advanced progressively in the sky; they could be followed by the eye as easily as the progressive expansion of an electric spark on a plate of glass or ebonite. Moreover, the numerous plates that we now possess enable us to assert that electrical discharges are not made instantaneously, but progressively, on photographic plates.

GANZ'S STANDARD SCREW THREADS.

MESSRS. GANZ & Co., of Budapest, hand us the following particulars of their new standard screw threads, remarking that of the different systems of screw threads which have been proposed for large machines, only those of Whitworth and of Sellers have been accepted in practice, the former being used chiefly in Europe and the latter in America. Though a very remarkable unanimity has taken place in the use of these systems, it is quite different with the thread scales for instrument makers and mechanics. Every manufacturer has his own thread, and, besides, there are very often several different screw plates in use in the same workshop. The natural consequence of this is, that gene

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