210 
MR. A. Ll. hughes ON THE 
The maximum emission velocity is given by the potential to which the disc rises 
when illuminated. As this frequently exceeds 2 volts there is danger of insulation 
leak. This is avoided by charging the case of the apparatus to a negative potential 
slightly less than the maximum emission velocity. The potential of N will then only 
rise to a fraction of a volt. Hence the ebonite insulation e and the sulphur insulation 
/have only to insulate for potential differences of less than T volt. The quartz rod, 
Q, however, has to insulate the full difference of potential. Fortunately its insulating 
properties were found to he practically perfect. The potential to which the system 
rises is always measured by comparison with a potentiometer. 
In order to produce a very good vacuum, the apparatus is always exhausted by a 
Toepler pump to a pressure of '01 mm. before communication with the cooled charcoal 
tube is made. 
G. Velocity Distribution Curves .—A curve giving the relation between the photo- 
