468 Mr. W. H. Preece [June 4, 



" The Work of Hertz " given in this hall by Prof. Oliver Lodge 

 three years ago.* 



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

 trate wave transmission by a very beautiful apparatus devised by 

 him. At one end we have the transmitter or oscillator, which is a 

 heavy suspended mass to which a blow or impulse is given, and 

 which, in consequence, 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 propagation 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. laght- 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 

 polarised, 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 Maxw^ell 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 pro- 

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

 happens along a line of electric 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.\ The animal eye is tuned to one 

 series of waves, 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 

 colours. 



One more definition, and our ground is cleared. When elec- 

 tricity 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 neighbourhood 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 maintained in a conductor. The surrounding 



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

 Printing and Publishing Company. — W. H. P. 

 t Vide Fig. 4, p. 474. 



