ELECTRON THEORY — KLOEFFLER 



247 



of our modern electronic application. DeForest added a third elec- 

 trode or grid to the Fleming valve and patented the three-electrode 

 tube in 1907. For many years it was known as the DeForest audion. 

 This device was primarily an amplifier of electric current, and it 

 constitutes one of the greatest inventions of this century. 



FREEING ELECTRONS FROM SOLIDS 



The operation of most electronic devices depends upon the removal 

 of electrons from solids in some manner. One method for removal is 

 accomplished by impact on the solid of an electron, an ion, or the 

 nucleus of an atom. A violent impact may splash electrons from the 

 outer orbits of atoms lying in the surface of the metal. Electrons 

 splashed out in this manner are known as delta rays. 



Electrons may also be "brushed out" or released from a few sensi- 

 tive substances by the impinging of electromagnetic radiations of 

 suitable frequency. The elec- 

 tromagnetic radiations are 

 light waves, either visible or 

 invisible. This phenomenon 

 of electron removal forms the 

 basis of action in photoelectric 

 cells. 



A third method for the re- 

 moval of electrons from solids 

 is through the application of 

 heat. This method is known 

 by the term thermionic emis- 

 sion and is used in millions of 

 electron tubes of today. The 

 nature of this emission is not difficult to visualize. The electrons are 

 bound to the atoms by their attraction for the nucleus. The electrons 

 are bound to the molecules and to the substance of which they are a 

 part by an attraction called electron affinity and the energy required 

 to remove an electron from a substance is termed the work function. 

 The molecules are in a constant state of agitation while the electrons, 

 in turn, are moving about in the orbits of the atoms. The kinetic 

 agitation of the molecules rises with the temperature and at a suffi- 

 ciently high temperature the velocity of motion of a few electrons 

 may become great enough to break through the surface of the solid. 

 This escape of the electron is called emission and it may be pictured 

 as in figure 3. Part a shows the emission at a relatively low tempera- 

 ture. At the higher temperature a cloud or atmosphere of electrons 

 emanates or evaporates from the hot filament or cathode as shown 

 in part b. When the electrons are hurled forth by their kinetic energy, 

 they give to the cathode an image positive charge which exerts a force 



(a) (b) 



Figure 3.— Emission of electrons from a heated body. 



