186 THE REALITIES OF MODERN SCIENCE 



reaches the melting point of the substance itself. 

 This phenomenon is made of practical use in the so- 

 called " audions, " thermionic devices which have proved 

 of great value in radio-communication. 



As a metal is heated we may obtain through our 

 physical senses an indication of its increased electronic 

 activity, for we may feel the heat radiated to us through 

 space. As the temperature rises still further we ob- 

 tain a visual indication in the dull red color of the 

 metal. The kinetic energy of the electrons has on the 

 average been increased and hence their velocity. The 

 frequency with which an electron, although bound 

 within an atom, may move back and forth or once 

 round its restricted path is thus increased. We thus 

 associate the radiation of heat with slower vibrations 

 than correspond to the radiation of light. Not all the 

 electrons have the same frequency, since not all would 

 have just the average value of kinetic energy. The 

 result is that as the temperature rises some of the elec- 

 trons reach the frequency at which they radiate red 

 light before the others. As the temperature is still 

 further increased the average frequency rises and more 

 electrons emit red light. Those which are faster than 

 the average now emit a yellow light while the slower 

 ones still radiate heat. As the temperature rises the 

 other colors of the spectrum are emitted and the metal 

 becomes incandescent. 1 



The radiations of the fastest electrons are beyond the 

 range of visibility, but they affect photographic plates 

 and may otherwise be detected. We are accustomed 

 to divide up this range of frequencies and to call those 



1 A more exact statement is beyond the scope of this chapter. 



