192 Prof. Millikan and Mr. Winchester on Influence of 



although the combination of elements may be of sufficient 

 interest to justify the diagram and a brief description. 



The pump is composed entirely of glass, no stop-cocks 

 or wax-sealed joints being used anywhere in connexion with 

 the vacuum-chamber. The pressure within the forty-litre 

 bottle. j5, is maintained at from one to five centimetres of 

 mercury by means of an aspirator-pump, p. The mercury is 

 forced into the bulb, b\ by connecting b with the outside ait- 

 through the two-way stop-cock, c, and the drying-tube, t. 

 After the bubble of air has been forced out at o, the cock c is 

 turned so as to establish connexion between B and b, when 

 the mercury falls again in V . With such a pump the mer- 

 cury remains clean for an indefinite period, and the vacuum 

 can easily be maintained for months, in which a MacLeod 

 gauge will register '00001 mm. of mercury or less. This 

 means, of course, that practically all of the gas save mercury 

 vapour (pressure about *001 mm.) has been removed. 



4. Early Experiments upon Aluminium. — The first series of 

 observations upon the effect of temperature upon the dis- 

 charge was made in May 1905, upon an aluminium electrodec 

 The source of the ultra-violet light was a spark from the zinc 

 electrodes e (fig. 1), which w T ere made the terminals of the 

 secondary of an induction-coil as shown, a small condenser 

 k being introduced for the sake of increasing the intensity of 

 the light. The coil and electrodes were enclosed, in the 

 usual way, in a metallic box, so as to screen the charged 

 system from the disturbances produced by the spark. The 

 ultra-violet light passed first through a hole, 5 mm. in 

 diameter, in this box, then, when the shutter s w T as lifted, 

 through the quartz plate Q, then through a hole 7 mm. in 

 diameter in the metal washer w, then through a wire gauze 

 screen, of one and one half mm. mesh, and finally fell upon 

 the charged plate a 10 mm. in diameter. This piate was 

 connected to one pair of quadrants of an electrometer, which 

 was charged at the beginning of each observation to the 

 potential of — 20 volts. For the sake of reducing the natural 

 rate of leak, the capacity of the electrometer system was 

 increased by means of the air-condenser ti ' . The electrometer 

 was an Edelmann instrument furnished with an oil-damping 

 device. 



The deflexion of the electrometer for the charge of —20 

 volts was about 200 cm. on a scale 2 m. distant. In these 

 first experiments the leak when the electrode was not illu- 

 minated amounted to about 2 mm. in 30 seconds. When 

 the light was turned on for a period of 15 seconds by lifting 

 the shutter s, the discharge was sufficient to cause a change 

 in deflexion of about 20 mm. The method employed for 



