250 SIGNALING THROUGH SPACE WITHOUT WIRES. 



admirable lecture on "The work of Hertz" given in this hall by Prof. 

 Oliver Lodge three years ago. 1 



By the kindness of Prof. Silvanus Thompson I am able to illustrate 

 wave transmission by a very beautiful apparatus devised by him. At 

 one end we have the transmitter or oscillator, which is a heavy sus- 

 pended mass to which a blow or impulse is given, and which, in conse- 

 quence, vibrates a given number of times per minute. At the other 

 end is the receiver or resonator, timed to vibrate to the same period. 

 Connecting the two together is a row of leaden balls suspended so that 

 each ball gives a portion of its energy at each oscillation to the next in 

 the series. Each ball vibrates at right angles to or athwart the line of 

 propogation of the wave, and as they vibrate in different phases you 

 will see that a wave is transmitted from the transmitter to the receiver. 

 The receiver takes up these vibrations and responds in sympathy with 

 the transmitter. Here we have a visible illustration of that which is 

 absolutely invisible. The wave you see differs from a wave of light or 

 of electricity only in its length or in its frequency. Electric waves 

 vary from units per second in long submarine cables to millions per 

 second when excited by Hertz's method. Light waves vary per second 

 between 400 billions in the red to 800 billions in the violet, and electric 

 waves differ from them in no other respect. They are reflected, 

 refracted and polarized, they are subject to interference, and they move 

 through the ether in straight lines with the same velocity, viz, 186,400 

 miles per second — a number easily recalled when we remember that it 

 was in the year 1864 that Maxwell made his famous discovery of the 

 identity of light and electric waves. 



Electric waves, however, differ from light waves in this, that we have 

 also to regard the direction at right angles to the line of propagation 

 of the wave. The model gives an illustration of that which happens 

 along a line of eleetrie force; the other line of motion I speak of is a 

 circle around the point of disturbance, and these lines are called lines 

 of magnetic force. l The animal eye is tuned to one series of wave; the 

 "electric eye," as Lord Kelvin called Hertz's resonator, to another. If 

 electric waves could be reduced in length to the forty-thousandth of an 

 inch we should see them as colors. 



One more definition, and our ground is cleared. When electricity is 

 found stored up in a potential state in the molecules of a dielectric like 

 air, glass, or gutta-percha the molecules are strained, it is called a 

 charge, and it establishes in its neighborhood an electric field. When 

 it is active, or in its kinetic state in a circuit, it is called a current. It 

 is found in both states — kinetic and potential — when a current is main- 

 tained in a conductor. The surrounding neighborhood is then found 

 in a state of stress, forming what is called a magnetic field. 



'This is published in an enlarged and useful form by The Electrician Printing 

 and Publishing Company. — W. H. P. 

 1 Vide fig. 4, p. 256. 



