PKEFACE 



AFTEB studying the changes which take place in the 

 conductivity of gases through which ions are passing 

 under various conditions, I was led to propose the 

 theory of ionization by collision to explain the develop- 

 ment of currents in gases. The accuracy of the theory 

 has been established by a large number of experiments 

 specially arranged to measure conductivities which could 

 be compared with the values obtained from theoretical 

 considerations. The researches have brought to light 

 the fact that ionization by collision takes place when 

 comparatively small potential differences are established 

 between electrodes in a gas at a suitable pressure, so 

 that considerable multiplication of the ions may be 

 obtained by this process with forty or fifty volts, and 

 consequently with comparatively small velocities of the 

 ions. 



This is in marked contrast with the previously known 

 cases of ionization produced by the motion of ions 

 through a gas, where the velocity is always very high, 

 sometimes approaching that of light. Such, for instance, 

 is the case for the negative particles emitted by radio- 

 active substances, or for the particles composing the 

 cathode rays and the Lenard rays when the ions move 

 with velocities that are acquired under the action of 

 some thousands of volts. 



The collision theory of the genesis of ions might have 

 been framed from a consideration of some of Stoletow's 



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