24 PROCEEDINGS OF THE AMERICAN ACADEMY 



and utilized. On this latter principle depends the dynamo ma- 

 chine, which furnishes electricity to be used for light or power. 

 On the former depends the electric motor that propels our street 

 cars, and furnishes power to many of our industries. 



Again, the electric current flows through wires with enormous 

 velocity, and consequently it offers an excellent vehicle for the 

 rapid communication of intelligence, — for telegraphy. A tele- 

 egraph wire between two cities is merely a path through which the 

 current may readily flow. At one end is a reservoir of electricity. 

 The "key" is a device by which electricity may be allowed to 

 flow into the line. By allowing small quantities of electricity to 

 flow into the line at intervals of time in accordance with a pre- 

 arranged code, and noting at their distant end their arrival, we 

 may communicate any ideas that are capable of being expressed in 

 such a code. 



The arrival of these little currents of ether may be manifested 

 to the eye in various ways. In the so-called needle telegraph, the 

 current passing by a magnetic needle causes it to point in a given 

 direction, just as the wind passing a weather-vane causes it to poiut 

 in a given direction. 



If it were possible to have on one hill-top a reservoir of air un- 

 der pressure, or wind, and a key by which small puffs of air could 

 be let out at intervals in accordance with a pre-arranged code, and 

 on a neighboring hill a weather-vane, we might thus have a wind 

 telegraph operated exactly as is the electric telegraph. But such 

 a telegraph would be slow and cumbersome in operation, and is not 

 likely to come into general use. 



There is one other phenomenon on which I desire to dwell a 

 moment, — a phenomenon of great importance, as it furnishes the 

 first proof of the identity of electricity and the ether. 



Suppose we have an ordinary telegraph wire in which there is a 

 steady current flowing. Suppose ten feet away from this wire, and 

 entirely disconnected from it, we arrange a short wire with any 

 suitable means of detecting an electric current in it, should such 

 a current exist. So long as the current in the first wire flows 

 steadily on, there will be no current in the secondary wire. If, 

 however, the current in the first wire be broken uj) into rapid 

 pulsations, say 100 per second, by rapidly opening and closing the 

 circuit we shall find in the secondary wire short pulsating cur- 

 rents occurring with the same frequency. How does this action 

 take place ? How are the impulses transmitted across apparently 



