MODERN VIEWS WITH RESPECT TO ELECTRIC CURRENTS 657 



The fact of the oscillatory character of the Leyden jar discharge was 

 first demonstrated by our own countryman, Henry, in 1832, but he pur- 

 sued the subject only a short distance, and it remained for Sir William 

 Thomson to give the mathematical theory and prove the laws according 

 to which the phenomenon takes place. 



Thus, in the case of a charged Leyden jar whose inner and outer coat- 

 ings have been suddenly joined by a wire, the electricity flows back and 

 forth along the wire until all the energy originally stored up in the jar 

 has expended itself in heating the wire or the air where the spark takes 

 place and in generating waves of disturbance in the ether which move 

 outward into space with the velocity of light. These ethereal waves we 

 have demonstrated by letting them fall on this coil of wire and causing 

 the electrical disturbance to manifest itself by electric sparks. 



I have here another more powerful arrangement for producing electro- 

 magnetic waves of very long wave-length, each one being about 500 

 miles long. It consists of a coil, within which is a bundle of iron wires. 

 On passing a powerful alternating current through the coil, the iron 

 wires are rapidly magnetized and demagnetized, and send forth into 

 space a system of electro-magnetic waves at the rate of 360 in a second. 



Here, also, I have another piece of apparatus [a lamp] for sending 

 out the same kind of electro-magnetic waves; on applying a match, we 

 start it into action. But the last apparatus is tuned to so high a pitch 

 that the waves are only so ^ 00 inch long, and 55,000,000,000,000 are 

 given out in one second. These short waves are known by the name of 

 light and radiant heat, though the name radiation is more exact. Plac- 

 ing any body near the lamp so that the radiation can fall on it, we ob- 

 serve that when the body absorbs the rays it is heated by them; the 

 well-known property of so-called radiant heat and light. Is it not pos- 

 sible for us to get some substance to absorb the long waves of disturb- 

 ance, and so obtain a heating effect? I have here such a substance in 

 the shape of a sheet of copper, which I fasten on the face of a thermo- 

 pile, and I hold it where the waves are the strongest [near the coil while 

 the alternating current is passing through it]. As I have anticipated, 

 great heat is generated by their absorption, and soon the plate of copper 

 becomes very warm, as we see by this thermometer, by feeling it with 

 the hand, or even by the steam from water thrown upon it. In this ex- 

 periment the copper has not touched the coil or the iron wire core, 

 although if it did they are very much cooler than itself. The heat has 

 been produced by the absorption of the waves in the same way as a 

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