THE GROWTH OF PHYSICAL IDEAS 109 



different masses are known as isotopic elements. As a result 

 of Thomson's work, the general nature of electricity became 

 clear, and it was realized that a current of electricity was a 

 current of electrons, which are the atoms of electricity in the 

 sense that they are the smallest unit of electricity known. 

 Each electron carries its unit charge, while its mass is approxi- 

 mately one two-thousandth of that of the atom of hydrogen. 



The elucidation of the nature of electricity had two results 

 of the utmost importance. It made possible a new field of 

 electrical engineering, ^vhich has become generally known as 

 electronics. It also made possible the understanding of the 

 structure of the chemical atoms and of the nature of radio- 

 activity, and this we shall deal with later. 



The engineering applications of electronics depend upon 

 the use of streams of electrons to control electric circuits. 

 The first observation which led to this was made by Edison, 

 who observed that when he sealed two elements into a lamp 

 and heated one of them by a current, the second filament in 

 the vacuum received electricity across the space from the 

 heated filament. This was before the work of Thomson, but 

 we now see that what Edison observed was the passage of 

 electrons across the vacuum from the heated filament to the 

 cold one. Edison did not follow up the observation, but it 

 was studied by others, notably by J. A. Fleming and by Lee de 

 Forest, who had the idea of introducing into the space be- 

 tween the two filaments a grid of wires, by charging which 

 he could control the flow of electrons across the space. 



The electronic tubes, now so widely used, are essentially 

 valves which control the flow of electric current through a 

 circuit as a valve controls the flow of water through a pipe. 

 When a valve tube is put into an electric circuit, the circuit 

 is broken because, in the tube, there is an open space across 

 which the electrons must pass in order to maintain the flow 

 of current through the circuit. At this point the current can 

 be controlled. For example, if an alternating current is ap- 

 plied to the tube, the anode at w^hich the electrons are re- 

 cei\ed becomes alternately positively and negatively charged. 



