832 The Outline of Science . 



of electrons. They leap back and forth several times before com- 

 ing to rest. The spark, therefore, really consists of a number of 

 sparks, each one the result of a leap of electrons. These separate 

 sparks have actually been made visible by employing a rapidly 

 revolving mirror. A single spark would give just one streak of 

 light in the mirror. A long steady spark would give a band of 

 light, but the separate sparks of the Leyden jar give a series of 

 bright images separated by dark intervals (see Fig. 3 facing 

 p. 828). 



Now we have seen that the oscillations produced by the 

 Leyden jar gradually die away. Oscillations which grow smaller 

 and smaller are said to be damped. There are also oscillations 

 which do not die away, but which remain of the same strength the 

 whole time. Such oscillations are said to be undamped, and they 

 travel far. These oscillations are dealt with later. The difference 

 between these sorts of oscillations is of great importance in wireless 

 telegraphy (see Figs. 4 and 5 facing p. 828). 



2 



We are now in a position to understand the principles of the 

 transmission of wireless telegraph waves. In the first place, a 

 Leyden jar is continuously charged by suitable apparatus, and 

 allowed to discharge itself through a circuit, consisting of a coil 

 of wire and a spark gap. As we have seen, oscillations will be set 

 up in the circuit. It was one of the most celebrated discoveries of 

 Faraday that whenever a current of electricity varies in one of two 

 adjacent coils of wire, a varying electric current is set up in the 

 other coil, due to electromagnetic lines of force. Therefore, if 

 we bring a second coil near to the first coil, oscillations will be set 

 up in the second coil. One end of the second coil is connected 

 to earth, and the other end is connected to what is called the 

 antenna or aerial, thus completing the transmitting circuit. At 

 one end the current is led to the earth, at the other it is radiated 

 into space. 



