•88 Professor J. A. Fleming [March 27, 



but it also dissipates it because no dielectric is altogether free from 

 conductance or leakance. When a current flows through the copper 

 conductor it creates around it a magnetic field which represents 

 stored energy, but at the same time some energy is being dissipated 

 as heat because the conductor has electrical resistance. 



From this double storage of energy it follows that if the electro- 

 motive force applied to the end of a cable is alternating or periodic 

 in nature, the current is propagated along the cable as a wave 

 motion. This means that the current at various points in the cable 

 is not flowing in the same direction equally at the same instant, but 

 ebbs and flows from point to point so that it is not high tide, so to 

 speak, at all points at the same moment, but the high tide or maximum 

 current takes place progressively along the cable. 



A wave of current is therefore propagated along the cable, and 

 the nearest distance between two places at which the current is a 

 maximum or zero at the same instant is called the wave length. 



The waves die away in amplitude or attenuate as they run along 

 the cable because their energy is gradually frittered away by the 

 heat they produce in the wire, and also by the similar action in the 

 insulator. 



These statements can be best illustrated by making use of the 

 visible vibrations of flexible strings to represent the invisible oscilla- 

 tion of currents. 



We have on the shaft of a small electric motor a pulley which 

 has a crank pin in it driving a crank shaft, the other end of this 

 crank shaft being attached to a rocking lever. A long cotton cord is 

 fastened to a loop on the crank shaft, so that when the motor revolves 

 one end of the cord has a circular but irrotational motion given to it. 

 If the string is stretched horizontally by a weight passing over a 

 pulley we have a medium along which we can propagate waves. The 

 string possesses elasticity and also density or mass, and in virtue of 

 this can store up energy in two forms, potential and kinetic, and have 

 waves of displacement propagated along it. When one end of it is 

 revolved uniformly by the motor, the rotation given to the end of 

 the stretched string, therefore, travels along it as a wave of displace- 

 ment. The speed with which this wave travels is directly propor- 

 tional to the square root of the tension or pull on the string, and 

 inversely as the square root of the weight of the string per unit of 

 length. 



Tyndall describes in his " Lectures on Sound " (Lect. III.) an 

 experiment in which a cord fixed at one end by a swivel had its 

 other end rotated by a whirling table so as to produce on it station- 

 ary waves. The use of an electric motor is, however, a considerable 

 improvement on any method depending on hand rotation. 



If the length of the string is properly adjusted, the repeated 

 waves produced by the revolution of the motor run up the string and 

 are reflected at the fixed end, thus meeting the outgoing waves and 



