196 



Mr. S. Ratner on Polarizatic 



diminishes, and the beam of X-rays from the aluminium 

 window becomes more penetrating in spite of the main- 

 tenance of a constant pressure in the bulb. The potential 

 difference across the bulb must then be gradually increased, 

 and finally a time arrives where the highest voltage supplied 

 by the coil is insufficient to break down the bulb's resis- 

 tance, and the discharge ceases. In order to maintain the 

 discharge, it is now necessary to increase the pressure, but 

 after a further long run the resistance of the bulb becomes 

 again too high, and the pressure must be raised still 

 further. By repeating this operation several times, a stage 

 is ultimately reached where a p.d. of more than 50,000 

 volts was not sufficient to produce a discharge, the pressure 

 in the bulb being as high as '065 mm. 



Fig. 1. 



When the bulb is approaching this stage of polarization, 

 the discharge usually shows an increasing tendency to 

 choose an alternative path through the bulb, starting not 

 from the cathode itself, but from a point on the rod support- 

 ing the cathode. This lowers the sparking potential and 

 introduces some other complications in the progress of the 

 experiments. This tendency is especially well marked when 

 the exhaust-tube leading to the pump ends in the neck of the 

 bulb opposite the rod. If the rod be enclosed in a glass 

 tube, this tube usually cracks and the discharge passes 

 through it. It was found, however, in the course of the 

 experiments that this troublesome effect could be completely 

 eliminated by covering the rod with a stout glass tube and 



