of the magnet, and a copper plate is placed partly beWhen the magnet is excited the disk is

neath the disk.

FIG. 20.

set in rapid rotation.

An interesting modification of the latter experiment is, to substitute a hollow light brass ball, such as are used as ornaments on certain curtain fixtures. The ball rapidly spins about its axis of suspension.

The copper ring in this experiment gets very hot if strong alternating currents are used. If it is suitably placed in a vessel of water, the water can be made to boil. We have here a transformation of electric energy into motion and also into heat. The heating effect can also

be shown by the employment of very simple means within the reach of almost any experimenter. To the

FIG. 21.

copper wire which we have used to show the effects of repulsion, solder an iron wire at one end of a diameter

and a copper wire at the other end of this diameter; connect these wires to a suitable galvanometer (Fig. 21), slip the copper ring on the pole of the magnet, insulating it from it by a roll of paper; when the circuit is made and broken through the electro-magnet the galvanometer speedily shows that the copper ring is heating. We have in this case a thermal juncture of iron and copper on the ring and the other juncture outside the ring. Two cells of a battery will readily show this phenomenon. The ring is a step-down transformer of very small resistance, combined with a very small electro-motive force, and with consequently a comparatively large current.

CHAPTER XII.

TRANSMISSION OF POWER BY ELECTRICITY.

WE have pointed out that the force of gravitation affords us our practical measures of electricity. It can also produce electricity by means of the weight of water.

In 1877 Sir William Siemens, in his presidential address before the Iron and Steel Institute of Great Britain, spoke of the possibility of utilizing the power wasted in the falls of Niagara, and said: "Time will probably reveal to us effectual means of carrying power to great distances, but I can not refrain from alluding to one which is, in my opinion, worthy of consideration—namely, the electrical conductor. A copper rod three inches in diameter would be capable of transmitting 1,000 horse power to a distance, say, of 30 miles." Again, in 1878, he states that there would be sixty per cent lost in transmitting this amount of power by electrical means over a distance of 30 miles.

In the year 1882 M. Depretz attempted to transmit power from Weissbach to Munich over 35 miles of iron telegraph wire 0.18 inch in diameter. He used a dynamo such as is commonly employed to-day on arclight circuits, and the pressure which forced the electricity, in common parlance, along the wires was 1,500 units, or volts, as they are termed. It is important to notice the amount of this pressure, for in modifying this

factor greater success has been reached. The increase in the pressure seems to be the key to the entire situation. The first experiment of Depretz was not entirely satisfactory, and it was repeated in 1883; but the second experiment was far from being successful. In 1883 still another experiment was made, which was far in advance of previous experiments. Power was transmitted from Vizille to Grenoble, in France, a distance of 8 miles, with a silicium bronze wire of 0.079 inch in diameter. Seven horse power was obtained at the receiving end, the loss being only sixty-two per cent. The improvement resulted from employing 3,000 volts instead of 1,500. Here it was clearly indicated that the direction in which to work was in employing high electrical pressure or voltage.

It was soon discovered that advance was barred in this direction of increasing the pressure by the impossibility of making a dynamo which would furnish the high pressure with a continuous current, such as is commonly employed to-day on our street arc lights. In a subsequent experiment M. Depretz endeavoured to construct a dynamo which would furnish a higher voltage; and although the experimental dynamo could not furnish the high voltage of 6,000 units which was desired and was burned out in the experiments, nevertheless M. Depretz showed that 52 horse power could be transmitted 35 miles over a copper wire 0-2 inch thick. Although this latter experiment was a failure, it clearly showed how greater success could be obtained. A higher pressure or voltage must be used, and instead of a directcurrent dynamo a new type must be employed-namely, an alternating-current dynamo, one in which the electrical current pulsates to and fro, now in one direction and now in the opposite.

It will be noticed that during the years 1877-'83 men's minds had changed greatly in considering the subject of the transmission of power from Niagara Falls to even the distance of 30 miles. The early objectors to the scheme calculated the expense of the enormous copper conductor three inches in diameter, and showed the practical impossibility of the plan. The later objectors pointed out that, although power from the falls could be transmitted 30 miles over a wire only 0.2 of an inch in diameter instead of three inches in diameter, no dynamo could be constructed which could give the large pressure of 6,000 units and maintain its life. It would be burned out by the excess of its emotion.

The remarkable new developments, therefore, in the subject of the transmission of power by electricity come from the employment of a high electrical pressure or voltage generated by a to-and-fro or alternating current instead of a direct current; and, strange to say, an apparatus which has long been used on professors' tables to illustrate the conversion of a low-pressure current of electricity into a high-pressure current has now come to have a great commercial value; this apparatus is the Ruhmkorff coil. In its elements we have seen it consists merely of two coils of wire entirely separate from each other, which are slipped on a bundle of iron wire which forms a core. If an alternating or rapidly interrupted current of electricity from a battery or a dynamo is sent through one of the coils, a current is generated by induction in the neighbouring coil. The pressure in this latter coil depends largely upon the number of windings in it. A pressure of only two units in the coil which is connected with the battery or dynamo can be exalted to a pressure of 12,000 to 100,000 units in the independent coil by properly increas