214 



COMPTON— ARC SPECTRA AND 



16.3 volts, is approached. Ionization is first detected a couple of 

 volts below this because the electrons are emitted from the filament 

 with small initial speeds. When the current has increased to a certain 

 value, the arc strikes, with a sudden large increase in current, at a 

 voltage higher than the minimum ionizing potential. As the voltage 

 is then diminished the arc persists down to the minimum ionizing 

 potential, but breaks at this voltage. If the filament is hotter, so as 

 to emit more electrons, as in curve (b), the arc strikes at a voltage 



50 

 45 

 40 

 55 

 '50 



CO 

 Oi 



i■25^ 



20 

 15 

 10 

 5 



a 



5 / 



15 10 Z5^,^_^ 5 10 15 10 15 

 Volts 



Fig. 2. 



nearer the ionizing potential and still breaks at this voltage. In no 

 case was it possible to produce or maintain the arc at a voltage less 

 than the minimum ionizing potential, about 16.3, except for a very- 

 small amount depending on the temperature of the filament, due to 

 the initial energy of emission, and easily allowed for. 



When the outer tube was very hot, the conditions were quite dif- 

 ferent, as seen by Fig. 3. The initial current is larger, there is ioniza- 

 tion beginning at about 10 volts, and stronger ionization at about 14 

 volts. In curve (a) the arc struck and was maintained at about 13.8 



