148 Prof. A. LI. Hughes on Characteristic 



carrying the cathode filament, together with the boron 

 target, were mounted in a glass tube which fitted into the 

 main apparatus by means of a ground-glass joint, sealed by 

 De Khotinsky cement. This enabled one easily to change 

 the filament or target. In the second type of apparatus the 

 ground-glass joint was replaced by a sealed glass joint, and 

 a wide tube containing specially prepared charcoal was 

 added, as shown. It w T as hoped that apparatus II. would 

 give better vacuum conditions. Both pieces of apparatus 

 were made of Corning G 702-P glass, through which 

 tungsten wires were sealed to make connexion with the 

 metallic parts inside. (Owing to the high melting-point of 

 this glass, it can be heated to from 500° to 600° C. during- 

 the outgassing process, and its mechanical strength and re- 

 sistance to sudden cooling are advantageous in a complicated 

 piece of apparatus.) 



The radiation was produced by the impact of electrons 

 from a hot tungsten filament F on a carbon or boron target 

 T, 1 mm. away. Part of the radiation fell on the plate Z 

 (nickel in apparatus I. and silver in apparatus II.), which 

 emitted photo- electrons under the action of the radiation. 

 The photo-electric current w r as measured by the rate of 

 charging-up of an electrometer. The electron current was 

 held constant at a selected value by slight adjustment of the 

 heating current when necessary. The potential accelerating 

 the electrons was measured by a Weston voltmeter (and 

 multiplier when beyond the range). 



As the photo-electric current rarely exceeded 10~ 13 amp., 

 and as the electron current from F to T was generally 

 between 10~ 5 and 10" 3 amp., it was necessary to make sure 

 that the effect measured was really due to the photo-electric 

 effect of the radiation, and not due in some way to electrons 

 or positive ions finding a path upwards. To ensure this, 

 fine gauzes (I -mm. mesh), 20 mm. apart, w r ere fitted into 

 the apparatus, as shown. The field accelerating the elec- 

 trons from F to T was produced by keeping F at zero 

 potential, while the potential of T was varied from -I- 5 to 

 + 500 volts. Thus, if the lower gauze R were kept at 

 a negative potential, no electrons could possibly pass up- 

 wards from the neighbourhood of the hot cathode and 

 target ; but, on the other hand, this arrangement would 

 help positive ions, if any were produced, to pass through the 

 gauze R. To turn these back, the gauze Q was kept at a 

 higher positive potential than any value of the potential ever 

 applied to F. Hence the positive ions, if any came through 

 R, would be turned back towards R. (The effect of the 



