Fig. 90 represents an arrangement of 400 elements, divided into ten batteries of forty couples. Two Bunsen couples are sufficient to charge each group for quantity; then, by means of a special commutator, they are grouped for tension, to effect the discharge. In a few seconds the work of several hours of the two Bunsen elements is expended, and this will explain the intensity of the effect.

These experiments, where the current possesses both tension and quantity, are very dangerous, and the apparatus must be handled with the greatest care. Planté received one day the discharge of 600 couples, and he describes the sensation as if a burning iron had been passing through his whole body. Fortunately, this accident had no fatal consequences; but, adds Planté, "it might have been otherwise if the 800 secondary couples had been in action at that moment."

Planté's Rheostatic Machine.-It has been mentioned that Planté had obtained tensions of more than 2000 volts with 800 secondary elements. These results even have been surpassed, and the rheostatic machine enables us more completely to transform the force of the voltaic battery, and to obtain a tension equivalent to that of apparatus of statical electricity. We again give his own account of the invention and construction of this interesting electric transformer

"

Having observed how easy it was to rapidly charge, with a battery of 800 couples, a condenser with a sufficiently thin insulating plate of glass, mica, indiarubber, paraffin, etc., I joined a certain number of condensers made of mica covered with tinfoil, and arranged them like the couples of the secondary battery itself, so as easily to charge them in quantity and discharge them in tension.

"All the pieces of the apparatus must of course be carefully insulated. The commutator consists of a long cylinder

QT (Fig. 91) of hardened indiarubber, and is provided with longitudinal metallic bands, which unite the con

densers for surface; copper wires, bent at right angles at their extremities, at the same time traverse the commutator to associate the condensers for tension. Thin plates or metallic wires in the shape of springs are in contact with the two armatures of each condenser, and fixed to an ebonite plate on each side of the cylinder to which a rotatory motion can be imparted.

"If the two terminals of the apparatus T and T' are connected with a battery of 800 couples, by means of the conductors P and P', even several days after having been charged with two Bunsen elements, and the commutator set in motion, there appear, between the branches of the charging machine which are coterminous to the armatures of the last condensers, a number of sparks, similar to those of an electric machine which is provided with condensers.

"By using an apparatus consisting of thirty condensers only, each of which has a surface of three decimetres square, I have obtained sparks of four centimetres in length.

"This apparatus gives favourable results with less than 800 secondary couples; 200 couples gave a spark of eight millimetres, and, by diminishing the thickness of the insulating plates, and increasing the number of condensers, results might be obtained with an electric source of even smaller tension.

"It ought to be understood that the discharges of statical electricity in the apparatus are not alternately positive and negative, but always take place in the same sense, and that the loss of power resulting from the transformation must be less than in an induction coil, for the voltaic circuit does not close upon itself for a single instant and there is consequently no conversion of a part of the

current into caloric effect. The apparatus can be kept in motion, and can produce a considerable number of discharges, without sensibly weakening the secondary battery.

"This is explained by the fact that each discharge only uses up a very small quantity of electricity, and that the circuit of the battery is not closed by a conductor. The electricity of the source simply spreads over the polar surfaces of the condensers while they are discharged. But this emission, which is constantly repeated, must finally carry off a certain quantity of electricity, and when the instrument is charged with a secondary battery, it seems quite possible at length to exhaust, in the form of statical effects, the limited quantity of electricity furnished by the current of the battery.

"The transformation of dynamical electricity is thus effected by another method than induction properly speaking, simply by a protracted statical effect, and this apparatus is properly designated by the name of 'rheostatic machine."

After having enumerated the different methods by which electric currents are produced and transformed, we now pass to the description of some of the interesting applications of this mysterious force.

PART II.

ELECTRIC LIGHTING.

ELECTRIC lighting, both as regards its past history as also its future, ranks first among the applications of electricity, although several of these applications have been developed to a much higher degree.

The voltaic arc was first produced by Sir Humphry Davy, in 1813, with a battery of 2000 zinc and copper elements; but it was not till 1842 that Deleuil's and Archereau's experiments foreshadowed the possibility of its use as a source of lighting.

At that time the only batteries known were hydroelectric batteries; electro-dynamic machines were still in their infancy, and the means for regulating the light more than primitive.

In 1857, the Alliance machines, perfected by Van Malderen, showed that electric light could be advantageously produced in certain special cases, and the application of these machines for lighting the lighthouses of La Hève, as early as 1863, brought the electric light for the first time within the reach of practical questions.

The invention of the Gramme machine in 1870 gave a new impetus to the electric light, which up to that time had