REVIEW

We will assume that in every case the pressure at the terminals of the energy consuming device and the useful work consumed are the same, and likewise that the same total amount of copper has been put into the conductors, equal distances from generating to consuming station being understood. We then ask, which is the loss by heat generated in the conductors in the three cases?

In the first case the useful work is 3 I, E,, and making the resistance of one main R, the heat generated in the conductors is

Thus if you assume equal useful pressures the drop of pressure is three times as great when the first mode of distribution is adopted than when the second is adopted, and the ordinary two-wire current will work with even greater loss. In the first case the currents in the mains are larger than the useful currents, but the pressures between the terminalsdeducting the loss of pressure-are equal to the useful pressures, in the second case the reverse takes place, as is easily seen in the diagrams. The ratio between the two is always 1 : 2 cos 30° = 1 : 1,732,

a number which may be considered as specially characteristic of rotary current with three terminals.

Exactly in the same way when rotary currents with n mains are employed, assuming as before equal load in all branches, we obtain

as is easily seen in the diagram, fig. 3, by paying attention to the signs. Finally, we gather from the same diagram that C1 + lq + lz = 0 (15) because these pressures all occur within the closed line D, D D3 D1. If you wanted to measure the work-the value of which is given by the following expression—

according to formula (12) you would have to take six wattmeters the current coils of which were inserted in each of the six branches Ia, Ib, Ic, 11, 12, 13, and the pressure coils of which were inserted correspondingly between the end of each of these branches. The sum of their readings would then be the work A. Besides the great inconvenience of this mode of proceeding, it would be impossible in many cases to get the currents I, I, Ig into your current coils or to measure them at all. The necessity, therefore, ensues, to transform the expression (12) in such a way that it only contains the currents Ia Ib Ic and the pressures between P, and P, P, and P3, and P3 and P19 the values of which can easily be measured. These pressures we have called Ea, Eg, E, and obtain therefore the three further equations :

2 = 3 (ia ⋅ la + iv · ev + ic · ec) + ia (—la—ev — lc + &z — lz ! + ir (―ebec-la + €gly! +ic (-ec-la-lb + Cz―€ş)

In which substitute for the currents ia, ib, ie, the currents i, ig, is, according to equation (13), and you will obtain: Σ = 3 (ia ⋅ la + ir er + ic ec) +

2,

+ is (―ea-eb-Сc + е1−е3)—iz (—ea―er-ec + €1−(3) + i2 (-ev-ee-ea + e2-e1)—is (—eb-ec-la + lq—lq) +iz (-le-la-lb + еz-€2)—ig (-ee-la-eb + (3-(g) and therefrom by a slight transformation,

Σ = 3 (ia ⋅ la + iv · ep + ic ⋅ ec) + 3 (in · en + ig⋅ lg + iz Pg! (i2 + ig + iz) (l2 + €2 + €3)

and therefrom, finally, with regard to (15)

Σ = 3 (ia la + ir ⋅ eb + ic · ec) + 3 (in • e1 + ig • eg + iz ́ez). (19)

Consequently,

adt =

a d t = {} [ia (Ca−€,) + iv (€3−¤q) + ie (0,— €3)] d 1. +iò (©3−6a)+ic

3

(20)

This equation offers a more convenient method of measur ing the work performed by the rotary current. You wun' only three wattmeters, the current coils of which are inse in each one of the currents Ia, Ib, Ie, while one end of the pressure coil is connected to the corresponding terminal, P, the other end first to the second and then to the third termins In this way six works are found, three of which, according to

our rule about the positive direction, must be considered as negative. Therefore the simple addition of the works so found-provided that the dynamometer every time turns in the same sense gives the total amount of the work performed multiplied by three.

It is worth mentioning that in this deduction no assumption whatever has been made as to the shape of the single alternating currents that make up the rotary current. This method is, therefore, unconditionally correct. Nevertheless, it is far from simple and it will therefore be found well worth while in practice to investigate whether the loads in the branches, Ia, Ib, Ic, and at the same time in the branches, 11, 12, 13, are not equal, i.e., whether

ia (eae) = is (esea) = ic (es) (21) If this be the case equation (20) takes the following simple form:

a dt = ia (eae) dt

(22)

which allows of the employment of only one wattmeter, and of only one switching.

In the experiments carried on at the works of Messrs. Siemens and Halske this condition could be considered as fulfilled, since the differences found amounted to no more than an admissibly small percentage. The simplified method was therefore exclusively applied. For this purpose wattmeters are at present being wound in the shops of that firm, the pressure coils of which contain two windings. Such an instrument will measure directly the work expressed by (22), if the one winding is excited by the pressure, ea, and the other by the-pressure, y, and the same end of both windings is connected to the pole, A. Thus you can obtain the total work with one single reading.

JOEL'S PATENT SWITCHES FOR ELECTRIC LIGHT.

THESE Switches are used for controlling groups of electric light lamps in departments, i.e., main switches, and are made in various sizes by Messrs. Henry F. Joel & Co.

The base is glazed porcelain, and fireproof; the contact bar is made of hard rolled brass, and is slotted and cut into separate and springy combs at each end. Each tooth of the metal combs presses independently on the terminal block, the object being to make as many points of contact as possible. These contact bars pass under sparking shields fixed on the terminal blocks. The switch, shown in fig. 1, is a

FIG. 3.

The switch shown in fig. 3 has a four-way action, and is of the same type as the preceding with detachable handle. This switch is used for changing the current to the lamps, say, from a dynamo installation alternatively with the current from a supply company's mains, and separate clamping screws are provided on the ends of the curved arms used for bridging across the terminals, so that when the changes are not often required the contacts can be clamped together.

Messrs. Joel & Co. have made many hundreds of these switches during the last few years, and have proved them to be very reliable and most useful. They are the only switches made specially for use in departments of warehouses with glass covers and detachable handles, and are exceedingly neat and workmanlike.

double break spring off switch; the spring action is obtained by two flat steel springs pressing upon an elongated stem in the centre, flat one way when the springs are off, and pointed the other way, and thereby distending the springs when the contacts are on. The switch bar is put "on" or "off" by means of a detachable key, fitting on to a square end on the centre stem. The whole of the switch action and terminals is enclosed in a brass cover with a bevelled plate glass front.

WIMSHURST'S ALTERNATING INFLUENCE

MACHINE."

AMONG the various machines which have been constructed for the generation of statical, or so-called frictional electricity, none can be compared with that of Mr. James Wimshurst, of the consultative staff of the Board of Trade, London, either for simplicity, reliability, or efficiency. These merits of the Wimshurst influence machine are known to a very wide circle of electricians and physicists; consequently the news that the inventor has brought out another,

Engineering.

D

520

having very curious and unlooked-for qualities, will be received with interest and expectation in many laboratories, schools, and homes, and will secure for it an attentive reception, which its remarkable features fully justify.

A remarkable feature of the new machine is that while it gives an abundance of sparks, it is impossible to charge a Leyden jar, or similar contrivance, from it. After the electricity has been apparently flowing for minutes into the jar in rapid sparks, the latter is found to be free from all charge. Neither spark nor shock can be obtained from it, and delicate tests fail to reveal the presence of electricity in it. It is as if one had been pumping water into a bucket with no bottom; the stream is plainly to be seen, but nothing accumulates. It is, however, easy to demonstrate the integrity of the jar, and its capacity to hold a charge when filled from an ordinary machine. The conclusion is therefore forced upon us that the current of sparks is really the oscillation of a small charge, which flows backwards and forwards with great rapidity between the machine and the jar. The electricity that is developed is alternately positive and negativé, and the normal condition of the jar is not upset.

[APRIL 24, 1891.

The appearance of the machine is clearly shown in the engraving. A central spindle carries a glass disc 16 inches in diameter. This is varnished, and has affixed to it a number of sectors of tinfoil. The sectors measure 4 inches radially, while in breadth they may vary from inch to 5 inches. Usually the sectors on one side of the disc are arranged to break joint with those on the other side, but this is not essential. Indeed in many respects the construction of the machine is very elastic, and many modifications ar introduced. For instance, the number of tinfoil sectors may vary from 2 to 16 or more, on each side, and as we have said their relative positions on the two surfaces of the discs are not important. The results of different combinations are given in the table. The disc itself is mounted in the centre of, and in the same plane as, a wooden frame 20 inches square. This frame carries four inductors of the shape show in the detached view; they are 9 inches square, with one corner cut away to admit the spindle and boss of the disc. Two of the inductors are mounted on each side of the frame, at opposite diagonal corners; those at the side of the machine nearest the spectator are, say, at the upper left hand and the

Is self-exciting.

Is not self-exciting but requires little assistance.

Constant current while not overtaxed, gives large current.

Is self-exciting.

Alternations occur each revolution.

Is more freely self-exciting. Alternations occur each revolution.

Is self-exciting.

Is self-exciting.

revolu

Is freely self-exciting.

Constant, but gives less current.

Is freely self-exciting with 3 revolutions. Alternations occur each tion.

Is self-exciting.

Sixteen sectors on one side.

Is self-exciting when sectors are the rod.

Is not self-exciting when sectors an next inductors.

If we investigate the action of the machine more minutely by the aid of an electroscope of an exceedingly delicate construction, we find confirmation of the hypothesis that positive and negative electricity is produced alternately. Commencing to turn the disc exceedingly slowly, say, at the rate of three or four revolutions per minute, the leaves of the electroscope suddenly diverge, being repelled from each other. They remain apart quite steadily until the disc has made rather more than three-quarters of a revolution, when they suddenly approach and cling together, showing that the sign of their charge has changed. They remain thus for an instant, and during another three-quarters of a revolution they again separate, and so on. But if the disc be turned rapidly the leaves are no longer able to respond in this way to the changes of charge. Such light appliances, with their large surfaces exposed to the air, are incapable of rapidly approaching and receding through a considerable distance under the small stresses brought to bear upon them. They consequently take up a mean position between the extreme ends of the range, and show the alternate attractions and repulsions to which they are subject by a quick fluttering or tremor. It is abundantly evident that they are subject to varying stresses, which the retarding effect of the atmosphere will not allow them to completely obey.

lower right hand corners. At the opposite side the inductors are at the upper right hand and the lower left hand corners. Upon each inductor plate is a tinfoil patch of the shape show measuring 4 inches radially and 7 inches circumferentially. Other inductors are provided, having upon them the smalles tinfoil patch practicable. Cemented over each patch is s wooden disc which on its face carries a bearing in whit there rides a bent brass rod, having a plume of fine wires its end, touching the glass discs at a point 90° remote frez the inductor, and opposite one of the inductors at the oth side of the plate. The disc is thus touched on both sides two places. It is driven by a cord from a small handw? below, and revolves in the direction of the hands of a chri To further vary the capabilities of the machine a bar of sulating material is made to carry two metal wire plumes. its ends, these having metallic connection with two termina this device takes the place of two of the inductors, and t its use a constant flow of electricity may be maintained.

The machine which we illustrate has been constructed = the view of being most readily used for the purpose of r monstration and experiment. It is provided with s discs, having different arrangements of sectors, and al parts are made interchangeable. By withdrawing the bearing the nut on the spindle can be screwed off at

when half the boss comes away and the disc can be withrawn and be replaced by another in a few moments. The nductors lie on pegs on the square frame, and are held by ight steel clips; provision is made by a number of holes in he frame for the use of inductors of different sizes. To couple one inductor to another, if such an arrangement should be desired, holes are bored in the wooden discs carrying the rush-holders, and into these holders the ends of wires can be inserted, the wires being bent in such a way to join any air of inductors. The parts of the machine are beautifully inished, and all the ends are attained by the simplest deices.

The method of action of this instrument is not very easy o follow, and indeed no theory has yet been put forward in

521

be able to see why this special method has been adopted, it being one which is suited to all the requirements of this particular case.

As it was possible to utilise, without great expense, a waterfall that had already been turned to account for a paper manufactory which had been destroyed by fire, the employment of alternating currents of high tension with transformers was necessary. The distance of this waterfall, which is situated at Ponts-Neufs, at about 13 kilometres from Saint Brieux, is sufficiently great to entail a loss along the line amounting, at full charge, to 20 per cent. of the total power, in order to realise a saving by the establishment of this line. This loss, which at first seems considerable, is justified by the fact that the available motive force is amply sufficient to

THE ELECTRIC LIGHTING OF SAINT BRIEUX.

LECTRIC lighting is slowly but surely gaining ground, and e towns which at first seemed least suited to its developent, now enter freely into the movement adopting this ode of illumination, appealing, in order to solve the esh problems which present themselves in each particular se, to the most improved methods, and the most elegant lutions now known in electrical industry, an industry of eat richness and fertility, notwithstanding its youth. The ghting of the town of Saint Brieux supplies us with a typical ample, and we think it will be interesting to give a general scription of it, for we see in this instance, for the first time combination, the utilisation of a waterfall, the transport of ergy to a great distance, the automatic compounding, and e coupling in parallel of the alternators by means of a active coil. As we proceed with our examination, we shall

provide for all requirements at all seasons. At the time of the year when the water is low the days are longer, and the duration of the lighting shorter, and the pond supplying the waterfall contains sufficient water in reserve to ensure the working of the lighting system. The quantity of water supplied by the little tributary streams is always sufficient to make up for the lowering of level produced during the few hours that the system works during the dry season.

The fall at the Ponts-Neufs is about 11 metres. It works two vertical turbines of the "Hercules type, constructed by Messrs. Singrün Bros., of Epinal. The total power of these two turbines exceeds 300 H.P. They turn at a normal angular speed of 250 revolutions per minute, which speed is kept constant by a centrifugal force regulator acting on the sluice gates, until the speed becomes normal again, and thus the quantity of water brought to the turbine is regulated according to the power to be produced.

These turbines drive two Thomson alternators of 35 ampères, the electromotive force of which varies, according to the charge of the line, between 2,100 ond 2,600 volts, with a view to maintaining at the extremity of the line, at Saint Brieux, an effective difference of potential of about 2,100 volts.

Dynamos.-Each alternating current dynamo turns at the normal angular speed of 1,050 revolutions per minute, and

522

bears 14 inductive poles, and 14 inductive coils, thus corresponding to a frequency of 70 periods per second. The mode of excitation of these dynamos is worthy of special mention. The inductors are furnished with two distinct windings, the action of which is united.

The first winding is actuated by a special continuous current shunt dynamo which serves as an exciter. Each alternator has its dynamo and special exciter, but one single exciter is sufficient, in case of need, to excite both the alternators. The excitation of the alternator is so regulated that with an open circuit it produces exactly 2,100 volts at its normal speed, by acting upon two rheostats introduced, the first, into the excitation in a shunt circuit of the exciter, the second, into the circuit of excitation itself.

The second winding of the alternator is fed by the principal current of the line. For this purpose the circuit of

[APRIL 24, 1891.

exciter introduced into the circuit of excitation of the alternator, two for the rheostat shunting the excitation by redressed currents; and lastly, two from the collecting brushes bringing the alternating current to the centre of distribution.

The Centre of Distribution.-The distributing station contains, in addition to apparatus for regulating and controlling the excitation, which apparatus we have shown in their theoretical place in our diagram in order to simplify it, all the apparatus necessary for the introduction into the circuit of either alternator, for controlling the coincidence of its phases, for the parallel coupling, for the protection of the line and the station, for the testing of the state of insulation of the line, and, lastly, for the indication of the currents and the differences of potential.

We see by the diagram that the principal wires of each alternator arrive at a double interrupter after having passed

the 14 armatures is cut in the middle by a current redresser consisting of two rings forming a commutator, each provided with seven teeth on which two brushes rub, these being placed accordingly, and communicating with the second excitation winding of the alternator. It is easy to see that the alternating current is thus redressed and passes through the inductors in such a manner as to increase the excitation by a quantity in proportion to the principal current supplied along the line. The winding of the inductors is calculated so that the compounding thus obtained should be greater than is required. It is reduced to the proper value by mounting a shunt in derivation on the brushes of the excitation. The diagram accompanying this article shows clearly the principle of the arrangements we have described.

In short, then, for each dynamo there are eight wires leading to the centre of distribution; two for the rheostat of itation of the exciter, two for the rheostat coming from the

through fusible circuit-breakers. The object of this don interrupter is to insulate completely from the station the machine which is not in use. At the terminals of the alternators is mounted the phase indicator, consisting of tv little transformers, the primaries of which are coupled r derivation on the terminals of each alternator; the secondaries are coupled together in tension, and two lamps of 100 rob are introduced into these circuits. The connections are s arranged that the electromotive forces are united when the alternators are in the same phase, the lamps then burn their full brilliancy; they are extinguished when the phe are contrary, for the electromotive forces induced in th secondaries are then in opposition.

In order to place an alternator on the line, it is necess to close the corresponding double interrupter and alse single interrupter. We will suppose that, machine A, be in circuit, it is required to connect the alternator, B, in pa