252 



Hull — Initial Velocities of the Electrons. 



Fig. 1. 



author was unable to get results comparable with theirs. The 

 current of electrons increased continually and rapidly with the 

 potential difference between the illuminated plate and gauze, 

 until the latter reached about 40 volts. From this point on 

 the current remained practically saturated, showing that the 

 Jack of saturation at lower potentials was not due to an insuf- 

 ficient vacuum. It might be explained by the assumption that 

 the photo-electric radiation of electrons, even from a polished 

 plate, is diffuse, as Lenard showed it to be from a plate covered 

 with lampblack. But this assumption would not explain the 

 saturation observed by Ladenburg and Markau. 



The method finally adopted was the following : Light from 

 an internal capillary discharge tube passes through a screen 



cell C (fig. 1) l ctn in length, 

 closed by flnorite windows, 

 through the limiting dia- 

 phragms a and b, and falls 

 on the insulated electrode A. 

 A is supported by a stem 

 which passes through amber 

 plugs in the brass tube c y 

 is sealed into the glass tube 

 by amber and sealing wax, 

 and connected to a Dole- 

 zalek electrometer. The 

 diaphragms a and b are con- 

 nected to the brass cylinder 

 B, which is insulated and can 

 be charged to known poten- 

 tials. All parts of B and A 

 are covered with lampblack 

 from an acetylene flame. In 

 the lower end of the glass 

 tube containing the elec- 

 trodes is 10 grams of cocoa- 

 nut charcoal, which is heated 

 while the tube is being 

 exhausted, so that after cool- 

 ing the pressure is *001 mm . The tube is then sealed off from 

 the pump, and immersed in liquid air above the level of the 

 cylinder B. Diffusion of gas out of the lampblack on the 

 electrodes is thus prevented, and a high vacuum assured. 



To determine the initial velocities of the electrons, B is 

 charged to small negative potentials, and the current between 

 A and B measured by the electrometer. Only those electrons 

 whose initial kinetic energy, divided by their charge, exceeds 

 the potential difference between A and B will be able to escape 



