ing poles. Instead of a disk an armature consisting of copper rods was made and the new alternating-current motor sprung into existence. Its most striking peculiarity consists in its absence of brushes, such as are used on ordinary dynamos. Its armature, or revolving part, is practically a copper disk revolving under exactly the same conditions as in Arago's experiment. To this or to a similar alternating-current motor we must look for aid in transmitting power great distances over a wire.

It is interesting to note that the conditions for the transmission of power by electrical means over long distances are closely analogous to those which are employed in long-distance telephony. The transmitter is an alternating-current machine operated by the human voice, and the telephone at the receiving end is an alternating-current motor which impresses its motion upon the air and thus reproduces the speaker's voice a thousand miles away. In long-distance telephony, too, the step-up transformer is used.

By the method of step-up and step-down transformers, which we have described, one hundred horse power has been transmitted one hundred miles from Lauffen to Frankfort over a wire which resembles in size an ordinary telegraph wire. Although the expense of providing copper conductors of great size (for instance, three inches in diameter) has been obviated, another difficulty now remains-that of properly insulating the line which is conveying a pressure of 12,000 to 20,000 volts. The higher the pressure the more tendency there is for electricity to escape from the line. In fact, a spark could be obtained by presenting one's knuckles to the wire charged to a pressure of 20,000 volts, and therefore the tendency of the current to

leave the wire at each support and escape to the ground is very great. The wire between Lauffen and Frankfort was carefully insulated at the poles, carrying it by various devices, in one of which insulating oil was used. The pressure of 12,000 volts which was actually used between Lauffen and Frankfort was dangerous to human life. A skull and crossbones was painted on the poles which carried the wires, and no one cared to touch the wires. It is evident that if 12,000 to 20,000 volts are to be used on overhead wires in the future they must be as much respected as the way along which an express train travels.

It seems to me undoubtedly true that a diminution in our coal supply would result in making the transmission of power from Niagara Falls to New York a success. Mankind has often cast uneasy glances at these unemployed giants-waterfalls and tides-as if envying them their freedom. If we could make them store up their energy in a practical form, we should no longer be compelled to delve in mines a mile under the earth for coal, and we could fold our hands while the water ran on and did our work.

When the storage battery was invented it was thought that Niagara Falls had lost its freedom and would be immediately set to work. There was a period when the elation of mankind grew less and faith in storage batteries declined; for they were far from perfect and could not be relied upon. It was much as if the swift trotting horse had appeared before long experience had been obtained in training him and in properly nourishing him. The early experimenters endeavoured to drive the storage batteries too hard, and the battery broke down under severe usage. To-day the storage batteries are becoming a commercial suc

cess, and people understand better how to keep them in condition. There is therefore a possibility of employing them to convey a portion of the power of Niagara to Chicago. Let us see what this possibility is.

Roughly speaking, six horse power can be stored in a ton of material which constitutes the storage battery. The equivalent of fifty horse power could be certainly carried in one freight car, and it would therefore require one hundred freight cars to transport 5,000 horse power from Niagara Falls to Chicago. The cost of the batteries would be in the neighbourhood of $2,000,000, and in order to maintain 5,000 horse power in Chicago relays of batteries would have to be employed. Against the expense of this method we must place the cost of the high insulation of a line of four hundred miles under a pressure of 20,000 volts. In both cases, neglecting the factor of sublimity, it would be more economical to generate the electricity at Chicago from coal by the ordinary method of employing a steam engine to drive a dynamo. Improvements in constructing high insulation wires and in transforming high electrical pressures to low ones and, vice versa, may, however, make a greater revolution in the subject than has been made since the first experiments of Depretz,

in 1882.

It is interesting to note in connection with the plan of utilizing the power of Niagara Falls, that some of our most thriving manufacturing centres are not located on water courses, and depend upon coal and not upon water power. The cities of Fall River and New Bedford in Massachusetts are rivalling Lawrence and Lowell in the number of their spindles. Both can obtain their supply of coal by cheap transportation in vessels. The cost of regulating water power is a large item in the

use of it, and steam is found to be more reliable and cheaper in many instances. There is no doubt that it would be more economical to generate 5,000 horse power in Chicago by means of coal than to attempt to transmit it from Niagara Falls. A great change, however, in our coal supply would speedily turn attention to the immense waste of energy which is going on at Niagara Falls, and might convert New York State into a beehive of industries.

CHAPTER XIII.

SELF-INDUCTION.

WITH the use of alternating or periodic currents of electricity the phenomenon of self-induction becomes of great importance, and it is desirable to obtain a clear conception of it. The quantity we call self-induction of a wire or coil is generally small compared with its resistance. The larger the self-induction, the more slowly will a continuous electric current rise to its stationary value.

The self-induction is measured by the number of lines of force established by the current. To send out these lines of force requires an expenditure of energy which is measured by the product of the lines of force due to a unit current multiplied by the square of the current, and this multiplied by one half. This energy produces a stress in the medium surrounding the wire or coil. The greater the self-induction, the longer the time it takes to bring a current up to a certain value. When this energy which is stored in the medium is allowed to return to a wire which is coiled around a bar of iron, the magnet thus formed can be demagnetized. This can be accomplished by suddenly placing a wire of small resistance across the poles of the battery which is exciting the electro-magnet. The lines of force are thus withdrawn from the field.