LIGHTNING CONDUCTORS.

By S. ALFRED VARLEY.

(Continued from page 10.)

THE writer has already stated he considered Mr. Preece to be in error in maintaining a lightning conductor could be touched with impunity at the time a lightning discharge was passing through; he has also given reasons for believing that Mr. Preece would be very ill-advised if he put into practice what he has publicly stated his willingness to do.

It seems necessary to say a few words on "surging" as bearing more or less on the views put forward by Prof. Lodge, and I quote from the Dr. Mann lectures, as the more recent paper discussed at the Institute of Electrical Engineers is to all intents and purposes a reiteration (in some cases expressed more forcibly) of the views developed in the Dr. Mann lectures.

Under the heading, Experiments on the by path, the following occurs :-" "No conductor is able to prevent side flash however thick it be made. It may be a foot or a yard thick and yet not stop it. Take a yard of stout brass or copper rod an inch thick, arrange it in the path of a Leyden jar discharge, and then arrange as a sort of by path or tapping circuit some very fine wire, such as Wollaston's platinum wire, fig. 1. It may

FIG. 1.

seem absurd for any portion of the discharge to leave the massive rod and take the hair-like wire by preference, especially if an air gap exists at A or B, or at both. Nevertheless a portion does choose the fine wire path, and you get a little spark at A or B about the sixteenth of an inch long.' It is clear from the passage quoted that Prof. Lodge considers self-induction has something to do with the spark seen at A or B, and also that the small spark observed arises from a diversion of a portion of the Leyden jar discharge passing through the brass or copper rod.

Fig. 2 represents a conductor with two conducting leads situated near to the extremities of the conductor, and with two leads in the neighbourhood of the centre of the conductor.

Now if the spark was caused by a diversion or surging of the discharge from the sides of the conductor, then it should pass as freely through the central leads, as through the leads at the extremities of the conductor. Fig.3 indicates a tubular conductor, shown in section, with a well insulated wire passing through the centre and over the outside, forming a single convolution. If the two ends of this insulated wire be approached so as to leave a small air space between them, the writer ventures confidently to assert a spark will be observed across the air space at the time a Leyden jar discharge is sent through the tubular conductor; and if the number of convolutions be increased by threading the insulated wire several times through the interior and over the outside, then the spark observed will be a heavier one than that produced by a single convolution; further, if the two ends of the convolutions be connected metallically, so as to form a closed circuit, and leads be taken from the extremities of the tubular conductor, and the ends of these be approached near to one another, the current developed in them at the time of a Leyden jar discharge through the conductor would be so small as to be imperceptible. Now it is evident, if what the writer has asserted be true, that what Prof. Lodge calls "side flash cannot be due to a surging or diversion of the Leyden jar discharge in this example, for there is no electric connection between the insulated convolutions and the tubular conductor.

Now if the spark observed at the small air space dividing the hair-like wire from the thick brass rod (fig. 1) arose simply from a portion of the Leyden jar discharge leaving the thick conductor, as Prof. Lodge would appear to believe, then if the ends of the central leads (fig. 2) were approached near to one another and a Leyden jar discharge be passed through the conductor, a spark should be observed at the air space separating them. The writer has not actually tried the experiment, but nevertheless he ventures to say no spark would be seen under the above suggested conditions; but if the two ends of the extreme leads were approached near to one another, a spark would then be seen at the air space dividing these leads when a Leyden jar discharge was sent through the conductor.

FIG. 4.

by Prof. Lodge to a surging of the Leyden jar discharge, in the following passage :

"I think there is a simple explanation of the phenomenon observed without assuming that any portion of the discharge from the Leyden jar leaves the massive conductor. For discharge to occur, the magnetic inertia of the rod has to be overcome, and directly as the sectional area brought under the influence of the electric current, so will be the amount of magnetism developed in concentric rings at right angles to the rod. When a second conductor is arranged parallel to the thick rod, as shown at fig. 10, a secondary current is developed in this wire at the commencement of the discharge, and another secondary current in the opposite direction to the first one is developed on the cessation of the discharge through the massive rod, and the magnetism developed in the thick rod is less directly as the amount of electricity set in motion in the wire Prof. Lodge terms the 'bye-path.'

"The action is precisely the same as that observed by me on a much larger scale, when a lightning discharge struck the suspended wires of the Constantinople and Varna circuit, which I have described in an early portion of this article."

It will be in the memory also of some of the readers of this journal that in giving what was termed a constructive illustration of the inductive system of electrical research a few weeks since, the writer referring to certain observations in connection with the "Alternative Path Experiment," said: "Such result seems to

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me quite out of harmony with well-established laws, and therefore I suspect what was observed has arisen from causes other than those which seem to satisfy Prof. Lodge."

After the issue of the Journal containing the passage quoted, I received a letter from Mr. Wimshurst, agreeing with what I had said. In this letter (see fig. 4) attention was also called to what I was not previously aware of, viz., that Mr. Wimshurst had shown during the late discussion at the Institute of Electrical Engineers that the B spark of Prof. Lodge's alternative path experiment was in the opposite direction to that of the Leyden jar discharge through the metal conductor, L.

The significance of Mr. Wimshurst's experiment was by no means grasped at the time his letter came to hand; but after the writer's mind had been working unconsciously on it for several days, it broke in upon him as a sort of revelation, and he saw the explanation he had given last year of the by-path experiment (fig. 1) also explained the B spark of Prof. Lodge's alternative path experiment.

Faith in the Franklin-Faraday theory, and a belief in the superiority of the inductive system of research over that of the mathematical method for digging out electrical truths, has led me in the course of both articles to dissent strongly from certain views promulgated and assented to by leading mathematical physicists, and I have been seeking for an explanation of those phenomena observed and recorded by Prof. Lodge which I could not reconcile with what seemed to me well-established laws.

The important observation of Mr. Wimshurst has come to hand most opportunely, and I am naturally pleased to find it supports the views for which I have been contending all along."

Mr. Wimshurst is to be congratulated on having discovered the explanation of the "B spark," and every scientist should be pleased the discovery has been made by one who has devoted so many years to practical research, and who is also one of the most disinterested of electrical workers. At the same time, I cannot help feel a certain amount of regret that Prof. Lodge, the originator of the alternative path experiment, and who has recorded so large a number of painstaking observations in connection with it, should not himself have discovered the B spark was not a diversion or surging of the Leyden jar discharge.

I proceed to say a few words as to the bearing of Mr. Wimshurst's discovery, for facts fairly grasped almost necessarily lead to consequences. Mr. Wimshurst's observation seems to demonstrate in a very striking manner that it is possible to have two electric impulses temporarily moving in opposite directions at one and the same time in a conductor. Faraday's generalisation that there is no hard and fast line between insulation and conduction, has been already referred to in the course of this article. Wherever there is insulation (and the best of conductors insulate) there induction is possible. As far back as 1858 it was pointed out by the writer that in a submarine circuit regarded as a Leyden arrangement, the resistance of the metallic conductor was the only insulation. Mr. Wimshurst's observation goes to show that to sudden discharges of high potential, the magnetic inertia of the molecules of the conductor causes them to act as insulators, through which induction takes place, without immediately breaking them down; and instead of the inertia being overcome by the sudden discharge, the electricity of

* I have received a second letter from Mr. Wimshurst, dated July 14th, in which he says: "And in order that you may quite see my view, I may give you a few more words. First, then, I find when the by pass' and the balls for the 'B' spark are arranged, the sudden discharge produces a positive side flash from one end of the by pass,' and a negative side at its other end, the middle of the by pass' gives no side flash. Another feature, so far as I can test it, is the charge from the outside coatings passes silently through the 'by pass,' but the magnetic induction (?) of the path produces a в spark in an opposite direction to the first or primary discharge, that is, the charge of the outside coatings is passing in direction, and the 'B'spark in direction. It may, of course, be an oscillation, but clearly it is a very much weaker spark than is due to the primary discharge, and is in the least a measure of the resistance of the by pass."

[JULY 19, 1889.

the Leyden jars makes room for itself to enter the conductor by driving out the electricity associated with the matter of the conductor, and the elec tricity makes room for itself to enter and meet the + electricity of the Leyden jar discharge by driving out the electricity, combined with and forming part of the conductor in common with all matter.

2. It emphasises what is already known, that electricity cannot be taken from or added to matter, and it goes to show that when energy is transmitted through a metallic conductor, the quantity of electricity forming the vehicle of such transmission is determined by the mass of conducting matter in the conductor, and this energy is transmitted from molecule to molecule, conduction proper involving a continuous breaking down and reformation of their atomic constitution in succession, the rate at which the molecules are broken down and reconstituted being determined by the potential; in other words, the amount of energy which comes into play.

Prof. Lodge has directed attention to the fact that in the alternative path experiment, the noise made by the discharge is greatest of all, when there exists no alternative metallic path, L, so that discharge occurs through the two air spaces, A and B. He refers also to the fact that the discharge makes the least noise when it occurs through the metallic path, L; and the knobs, B, are so far apart, that no spark occur at the air space dividing the knobs, B ; and he mentions that when the B knobs are close enough for spark to occur between them at the time the Leyden jar discharge occurs, although not so heavy as when the path, L, is removed altogether, it is louder than when no spark at B is produced.

I think it has been perhaps overlooked that when the metal path, L, is removed altogether, and the noise of discharge is loudest of all, the statical charge has to be raised to a higher potential to bring about discharge, than when the outsides of the Leyden jars are connected by the conductor, L. Discharge under such conditions is a louder and a heavier one, from two causes ; there is, in the first place, a greater accumulation or storage of energy on the surfaces of the Leyden jars, and this greater amount of energy is dissipated in a shorter interval of time, owing to relatively smaller amount of magnetic inertia of the dielectric air which forms the path through which the energy is transmitted.

The observation of Mr. Wimshurst's shows that the side flash experiment is really only another form of the alternative path experiment. I would direct attention to the quotation from my previous article; when suggesting an explanation of the phenomena there observed, I say "the magnetism developed in the thick rod is less directly as the amount of electricity set in motion in the wire Prof. Lodge terms the by-path :" what 1 desire to emphasise is, that the act of the duration of discharge is shorter, and consequently the noise is somewhat greater when a spark occurs at B than when the B knobs are separated so far apart that no spark is produced at the time the Leyden jar discharges through the metal path, L.

I pass on to the bearing of what Mr. Wimshurst has demonstrated on the recorded observations of Prof. Lodge. First, when the outsides of the Leyden jars in the alternative path experiments are connected by the path, L, the potential of the statical charge at the time discharge occurs is practically the same in all cases, for the resistance of the conductors, L, in these experiments may be regarded as almost, but perhaps not quite, a negligible quantity; and therefore the amount of energy dissipated is a definite quantity, and the potential of the induced discharge which occurs at air space, B, can only approach, and never exceed the potential of the statical charge of the Leyden jars; it will be seen that the effect of increasing the sectional area, say of copper conductors forming the path, L, is to increase the magnetic inertia and to reduce the resistance proper; but this latter may here be overlooked, and therefore increasing the sectional area of the conductor, L, would be expected to increase both the quantity and the potential of the electricity de

JULY 19, 1889.]

veloped at the air space, B, but the potential would not be increased so much as the quantity; the spark produced at B would therefore be brighter when conductors of large sectional formed the L path, but the air space through which it was capable of passing would not be so much affected, and this harmonises with what was actually observed.

It must not be overlooked that the electricity passing at B is by no means a measure of the quantity of electricity accumulated on the surfaces of the Leyden jars; but the amount passing at B increases as the sectional area of the conductor, L, is increased, and the greater the sectional area, the more nearly it approaches to the energy stored up in the Leyden jar; and what I desire to lay the greatest emphasis upon, and which has been referred to more than once both in this and my previous article, is that the duration of the act of the Leyden jar discharge becomes reduced directly as the ratio of the electricity which passes across the air space, B, increases relatively to the energy stored up in the Leyden jars. I think in what has been above said. we have an explanation of how it was, varying the sectional areas of the copper wires, L, in the alternative path experiments was observed to affect in a small degree only the length of air space the B spark was capable of passing.

With respect to what was observed in the experiments where iron wires of varying sectional area formed the path, L, and which fairly agreed among themselves, the only suggestion the writer can now make is the specifically greater electrical resistance of iron over that of copper. Should such be the explanation, and it is not put forward with confidence, then one would expect that were German silver wire substituted for iron a greater reduction in the length of the B spark would be observed.

an

I think the true explanation of what was observed in the tinfoil spiral experiments of Prof. Lodge can now be attempted. The writer, perhaps, owes apology to Prof. Lodge for suggesting that the insulation of the paraffined paper had given way, but, as he said, it was impossible for an electric discharge to pass round a glass tube enclosing a bundle of iron wires without doing work of some kind, and he felt the suggestion of Lord Rayleigh was not sufficient to account for what was observed by Prof. Lodge; the only way he could then see out of the dilemma was to suggest, what practical men too often have experience of, viz., the breaking down of insulation. Mr. Wimshurst's observation affords the explanation-work is done in the tinfoil spiral itself, and the result is the B spark.

In the continuation of this article 1 purpose to show how a link which was wanting, and which has now been supplied by Mr. Wimshurst, confirms my contention as to the basis of the dynamo principle.

The principle which I maintain forms the basis of the mode of action of the dynamo, and for which I have contended ever since I discovered the dynamo principle and constructed the first self-exciting machine in 1866, does not yet seem to have been grasped by our mathematical physicists, and it was characterised not long ago at the Court of Sessions, at Edinburgh, as "simply absurd."

(To be continued.)

THE BIRMINGHAM ELECTRICAL

EXHIBITION.

A VERY important event as regards the electrical engineering and allied industries will take place on the 1st proximo, when the Electrical and Industrial Exhibition at Bingley Hall, Birmingham, will be inaugurated by Lady Randolph Churchill. Before stating anything further about this exhibition, it may be as well to say a few words regarding the theories generally held by manufacturers concerning the utility of exhibitions, with special reference to the bearing of such theories upon electrical exhibitions. Let us first see whether the engineers, manufacturers, and others who have displayed their goods at the various exhibitions held during the last 20 years have derived any benefit there

79

from. To answer this question we had better see what the manufacturers have to say. On inquiring of different manufacturers in various parts of the country as to what extent they have been benefited by exhibiting their goods at exhibitions, we are almost invariably met by an answer something like this :-"Benefit! Why, none whatever. We found a slight improvement in our business for a certain time after the Exhibition,

but since that period there has been a decline, especially from the foreign markets where they are making our goods themselves. This is because we have shown to foreigners for a shilling, inventions which have cost us considerable sums of money to bring out and many hours of serious consideration. We have thus been taught a lesson which we shall not easily forget, and we shall in future be rather reserved in taking part in exhibitions."

It must be confessed that on the face of it there seems a little likelihood of the answer being correct, but on looking more closely into the matter, the reply will be found to be almost entirely at variance with the truth. Years ago before the patent laws were so stringent such a state of affairs might have obtained, but at present it would be almost impossible considering the facilities existing for patenting inventions in different countries, although piracy could of course take place sometimes where patents have not been taken up. As, however, the latter is very rarely the case, the manufacturers' theory may be considered as having no solid foundation, and may therefore be dismissed from the question. The crux of the whole matter lies in the fact that the secret of the invention has been revealed either by workmen employed by the firm, or it has been discovered by those persons who have been permitted to look over the works or factories as the case may be. It has frequently been cases of the firstmentioned order that have been overlooked, and manufacturers have consequently come to wrong conclusions regarding the utility or otherwise of exhibitions. In the second order, however, they may be said to be fully alive to the michief caused by showing strangers over their establishments, even with excellent recommendations from soi disant friends.

Turning now to the electrical part of the subject, we find that hitherto only a few electrical exhibitions have been held. That in Paris in 1881 was on a fairly large scale, as was also that at the Crystal Palace in 1882; but since the latter took place no purely electrical exhibition has been held. These were exceedingly well representative of the science of electrical engineering in those days, but since then new apparatus have been brought out, and very great improvements have been made in the construction of all kinds of electrical machinery and accessories, and especially as regards dynamos, alternators, accumulators, lamps, &c. At the Paris Exhibition this year there is a large electrical section, but it is from the Birmingham Exhibition that we expect great things. Almost all, if not the whole, of the electrical firms in the Kingdom will therein participate, and the exhibition will be the largest and the most representative of electrical engineering ever held. Before proceeding further it will, perhaps, not be out of place to mention the proposal for the holding of an electrical exhibition next year in Edinburgh, and we also learn that a similar display will be made at no very distant date in Canada.

Now as far as we have been able to form an opinion from the nature of the exhibits and from the names of the firms who intend making a display at Birmingham, we find that electrical engineers, and those indirectly connected with the trade, have not fallen into the same error as that previously referred to as having been committed by general manufacturers, consequently the theory usually held by the latter does not obtain with electrical engineers, and for very good reasons.

In the first place, only the principals in electrical engineering firms carry on experiments with a view to bringing out something new. These experiments are conducted in such a manner that the workmen cannot possibly become acquainted with the details, and can, therefore, not disclose the secret of the invention, a

80

patent for which the firm speedily obtains. This is a great point which many of the manufacturers already referred to do not consider until it is too late. In the second place, electrical engineers also very wisely exclude obtrusive visitors from their works. When these two points are taken into consideration everybody will naturally expect that a very large display of electrical machinery and apparatus will be made. It may here be mentioned that Monday next is the last day for receiving exhibits. The secret of success is generally considered to be due to advertising, and we hope that our electrical firms will in this instance make such an exhibition as will bring more prominently before the notice of the commercial world the advantages to be derived from the use of electric light and power.

THE INHERENT DEFECTS OF LEAD SECONDARY BATTERIES.*

By DR. LOUIS DUNCAN and H. WIEGAND. SINCE the year 1881, when the inventions of Faure gave such an impetus to the industrial development of lead secondary batteries, their commercial history has been marked by numerous and disastrous failures, while at the same time there has been a steady improvement in their construction and performance until at present they have reached a stage which makes them for some purposes a commercial success. At the same time there remain in the best batteries a number of defects which prevent their taking the place in the practical development of electricity which rightfully belongs to them. The action of the secondary battery is quite well understood. In the Faure type a support plate, usually made of lead or an alloy of lead, has mechanically applied to it some salt of lead (minium or litharge). A number of such plates are placed in dilute sulphuric acid, the alternate plates being connected respectively to the + and poles of some source of electricity, and a current is sent between them. The result is a reduction on the positive plates (the plates through which the current enters) to Pb. O, on the negative plate to spongy lead. After this formation" the action, roughly speaking, consists in a reduction of both peroxide and spongy lead to sulphate of lead on discharge, while on charging they are reduced again to their original composition. We find that in the process of charge and discharge there is a loss of energy varying from 15 to 40 per cent. within the practical limits of discharge rate. If we calculate the theoretical storage capacity of a given weight of lead and peroxide, we shall find that the plates of even the best modern batteries weigh for the same capacity ten times as much as would those of a theoretically perfect cell. We shall find, too, that there is a constant depreciation especially of the peroxide plates, the rate of depreciation increasing with the rate of discharge, and in genera! depending partly on the way in which the cell is treated, partly on its construction. The principal defects of the modern lead secondary battery are (1) the comparatively small storage capacity, (2) the loss of energy, (3) the depreciation, (4) the low discharge rate necessitated by considerations of efficiency and depreciation.

It was especially the question of the loss of energy in the battery that we wished first to investigate. There are two factors which determine the extent of this loss: In the first place, the number of ampère-hours obtained on the discharge of the cell is less than the number put in; and, in the second place, the P.D. at the terminals is greater during charge than during discharge. This loss of energy exhibits itself in two ways-in a generation of heat and in chemical actions which are not reversed on discharge. It is well known that after a cell has been in use for some time, especially if it be submitted to rapid charge and discharge, there will be found in the bottom of the containing vessel a white, powdery deposit, a sulphate of lead which has been formed from the active material of the plates, and which has not been afterwards reversed. Again, when the cell is charged we find bubbles of gas escaping from the plates during almost the whole of the charge, the escape becoming quite violent towards the last. The escape is at first principally from the positive plate, but afterwards it is from both plates. This escape represents, of course, a loss of energy due to the electrolysis of the dilute acid in the cell, the products being free hydrogen and oxygen.

Let us consider what takes place during the charge and discharge of a cell. Supposing the cell to have been discharged until its P.D. has dropped to 18 volts, on beginning to charge, the P.D. increases until it reaches a value of about 21 volts-at a normal charge rate-then increases very slowly during a considerable portion of the charge, then increases faster until it reaches a value of from 2.4 to 2.5 volts, when the cell is "boiling." The chemical action results principally in the reduction of the sulphate of lead on the two plates to peroxide and spongy lead respectively. The greater the charge rate the higher will be the P.D., and the sooner will the cell begin to boil and the greater will be the loss.

before the American Institute of Electrical Engineers, May 22nd, 1889.

[JULY 19, 1889.

On discharge, the P.D. drops to from 2 to 195 volts for normal discharge rate, where it remains during the greater part of the discharge, there being a gradual fall during the latter part to 1's volts, when the discharge should cease. If a high discharge rate be employed, there is a decrease in the capacity and efficiency, and a more rapid depreciation. If the discharge be continued after the P.D. has dropped below 1.8 volts, there will be a formation of white sulphate on the plates, there will be a loss of energy, as will be shown, and there will be a rapid depreciation of the cell. The result of the discharge is a formation of sulphate of lead on both positive and negative plates. If we test the specific gravity of the solution at different times we shall find that the solution has a maximum strength-say 1.200-when fully charged, with a minimum on discharge-say 1·150-the sulphating of the lead decreasing the strength of the solution. The number of ampère-hours obtained on discharge is less than the number put in by an amount depending on the construction of the cell and the conditions of charge and discharge. There is a further apparent loss of energy in the fact that the electromotive force on discharge is less than that during charge.

Our first experiment was made to determine, if possible, whether part of this difference of E.M.F. was not due to the fact that the strength of the solution in the plugs varied, it being stronger during charge than during discharge. During discharge, the sulphuric acid in the plugs has its strength decreased by the sulphating of the lead or peroxide. This weakening continues until the diffusion of the stronger acid in the cell produces a condition of equilibrium. It is known that the electromotive force of a cell varies with the strength of the solution, being higher as the strength increases. Gladstone and Tribe have found that when the acid is very weak the chemical action is changed, the result on a positive plate of sheet lead being the formation of streaks of a mixture of yellow and puce-coloured oxides, while on other parts a white substance is formed, which is easily detached, falling in clouds into the liquid. This white substance is probably a basic sulphate of lead. When this action takes place the corrosion of the plate is more than doubled. So if the diffusion in the plug is slow, it may very well happen that there will be a great difference of density during charge and discharge, causing a difference in electromotive force and a formation on discharge of chemical compounds which are not afterwards reduced. A rapid discharge rate would tend to greatly weaken the acid, and therefore to decrease the efficiency and hasten corrosion of the positive plate.

To find the rate of diffusion we soaked the plates or single plugs to be experimented on in acid of a specific gravity of 1175, and then placed them in vessels of distilled water, letting them remain for different intervals of time, and determining the amount of acid diffused out into the water. To give some idea of the magnitude of the result, I select the following figures from a number of experiments. The plates used weighed about a pound and a half (7 kilos), and were of the grid type:

JULY 19, 1889.]

The curve shown in fig. 1, gives the diffusion from a charged positive plate.

The total amount of acid in the charged positive plates was (25 grms. of 1.175) about 5 grms. ; so it will be seen that the rate of diffusion of the acid in the interior of the plugs is slow, for after 30 minutes, when half of the acid remains in the plug, the rate of diffusion has decreased from about 7 grammes for the first minute, to about 025 grammes per minute. While this is hardly the condition of affairs in actual practice where the rate at which the acid is being added or abstracted varies in different parts of the plug, yet it gives us some idea of the magnitude of the quantity. It should be noted that the rate of diffusion is materially the same for positive and negative plates, but that the rate for a discharged is considerably less than that for a charged plate. Keeping these facts in mind, let us pass to the phenomena of charge and discharge.

To investigate the loss of energy from heating, we placed the cell to be experimented on in a wooden box lined with a layer of felt about an inch thick. There was a top for the box, also lined with felt, and through it passed the rod of a stirring paddle and the stem of a thermometer. Experiment showed that the loss of temperature in this arrangement was for a

In the experiments we tried as nearly as possible to keep the air and cell at the same temperature, and the correction for radiation could usually be neglected. The cell was charged and discharged under a number of conditions, and the rise of temperature and other data were observed. The losses of energy that occur must, as has been stated, exhibit themselves in heat or in chemical changes which are not reversed. The cell was of the grid type, with four positive and five negative plates. The weights were :

12

FIG. 3.

The next charge was at 20 ampères. The rate of rise of temperature is given by the curve, fig. 4. The total rise was 30.7°. The rise for 100 ampère-hours was 11.5°. The corresponding values of c2 R were 34 watts and 20 watts.

The effect of a rest is shown on the portion of the curve between 105 and 120 ampères, where a 16-hour rest gives a considerable reduction in the rate of temperature increase. The maximum rate is about 3° per ampère-hour.

The cell thus charged was discharged at a rate of 30 ampères. The discharge was divided into periods of 20 minutes, with periods of equal length between, the object being to determine, if possible, whether there is a local action in the mass of the plug due to the different chemical conditions of the different parts of the plug, which would especially be the case if the charge or discharge were rapid. As in the previous case of discharge, the rise of temperature was slow, there being a gain of but 1-3° for a discharge of 40 ampère-hours, the heating effect of the current in that time being 12 watts, equivalent to almost 4°. For the first three periods there was little if any rise during the time of repose; for the fourth period there was a rise of about 1°, or 3 watt hours. During the fifth period the E.M.F. began to fall, and during repose after it the temperature rose 15°. The sixth discharge period was with the same current, but at a greatly reduced P.D. and resistance. It lasted 12 minutes; after it the temperature rose 5°, corresponding to 15 watt hours. The output of this discharge was 44 ampère hours at a normal P.D.; 11 at a low P.D. The total rise of temperature was 3.5°; the value of c2 R was 16 watts.

Rate of rise of temperature in degrees per ampère hours.

FIG. 4.