plicity the theory of Franklin) by the single transfer of an imponderable fluid from one side of the jar to the other; the phenomenon requires us to admit the existence of a principal discharge in one direction, and then several reflex actions backward and forward, each more feeble than the preceding, until the equilibrium is obtained. All the facts are shown to be in accordance with this hypothesis, and a ready explanation is afforded by it of a number of phenomena which are to be found in the older works on electricity, but which have until this time remained unexplained." *

* Scientific Writings of Joseph Henry, vol. i, p. 201, Smithsonian Institution, Washington.

CHAPTER XV.

STEP-UP TRANSFORMERS.

THE range of transformations of energy which the Ruhmkorff coil exhibits is by no means exhausted. When we succeed in producing from a battery a spark similar to that generated by an electrical machine, we have, by the use of a fine-wire coil wound upon a coarsewire coil, exalted the electro-motive force of three or four voltaic cells it may be eight volts-to perhaps twenty to thirty thousand. Starting from this great difference of potential, is it possible to treat the Ruhmkorff coil as a battery, and to still further exalt its difference of potential? This has been done by Prof. Elihu Thomson, and also by Tesla, and their experiments are most brilliant in the subject of the transformations of energy.

Prof. Thomson has succeeded in producing sparks five feet long, and states his belief that sparks twenty feet in length could be obtained by an extension of the method which he employed. This method was as follows: An open single-layer coil of coarse wire with about ten turns constituted the new primary. Upon this was wound about three hundred turns of fine wire. This latter coil constituted the new secondary. Both coils were immersed in oil. On passing through this new primary coil sparks from a Ruhmkorff coil very long

sparks can be obtained from the secondary. This ar rangement constitutes a species of double Ruhmkorff coil, or two step-up transformers. Instead of using a battery to excite the first Ruhmkorff coil, an alternatingcurrent dynamo is employed. Leyden jars are connected to the terminals of the first Ruhmkorff coil, and these are rapidly charged by the dynamo. A spark gap is interposed to the circuit between the first Ruhmkorff and the second, and the resulting spark is blown out by a jet of air under high pressure. The second Ruhmkorff gives sparks of the extraordinary length of five feet or more. This length of spark far exceeds that given by the most powerful electrical machine. If

mentioned, the spark produced in the secondary of the first Ruhmkorff at M N (Fig. 26) is constantly blown out by a strong blast of air. This blast serves the function of the break in the primary at P. A very high electromotive force can thus be obtained between C and D.

I have said that we have here two step-up transformers in the shape of two Ruhmkorff coils. A Leyden jar is interposed, as shown in the figure, in order to increase the quantity of electricity which is discharged through the primary of the second transformer, and also to act as a second alternating machine.

With this arrangement of the apparatus very strong insulation is needed, for the entire line is charged with electricity of high tension. The leading wires are luminous in the dark, from the brushlike discharges which are given off in every little break in the insulation of the wires. It must be remembered that the lines of force endeavour to leave the positive terminal of the Ruhmkorff coil at any point which offers a shorter passage to the negative terminal. In one sense, therefore, one should not be astonished to find that an exhausted globe will become luminous when it is attached to merely one terminal of the Ruhmkorff coil, for the walls of the room and the floor may constitute the other terminal of the coil, and the lines of force, stretching out and pervading the space in the room, converge on matter which affords in any way the easiest passage. Thus the forefinger glows when presented to either terminal of the coil. The lines of force find on the human body this short passage. When the electromotive force or electrical intensity is greatly enhanced the tendency of the lines of force to manifest themselves through the space inclosed in any ordinary room is greatly increased. At the same time the to-and-fro currents or electrical oscillations on the leading wires tend to confine themselves to the surface of these wires. This can be shown in a popular manner by connecting the terminals C and D (Fig. 26) by a thick copper rod, and, holding one terminal of an ordinary incandescent lamp in one hand (Fig. 27), touch the copper loop with the other terminal and also grasp the loop with the other hand. Only a slight shock is felt, and the currents passing over the surface of the human body raise the carbon filament of the lamp to a brilliant incandescence. The surface of the body is greater than that

of the copper loop, and the to-and-fro currents are not compelled to confine themselves to a thin film of copper constituting the surface of the copper. When steady currents are passed through the loop, they are not confined to the outer layer of the copper, and find an easier passage through the section of the copper loop than through the human body. With steady currents it is impossible to light the lamp in the above

manner through the human body. The effect of increasing the frequency of to-and-fro currents of electricity is thus to drive them to the surface of metallic conductors. When the frequency or rapidity of vibration is enormous, a rod of copper may not afford any better passage than a rod of glass. Hertz, in one of his papers, points out that our present nomenclature is limited, and only applies to the special cases of steady currents. With enormously rapid to-and-fro currents, a piece of copper acts like an insulator and prevents any to-and-fro currents from passing through it, whereas a piece of glass transmits them unimpaired. A thick disk of copper properly placed between the two coils