Temperature upon Photo-electric Effects in High Vacuum. 195 



entered through the quartz plate Q, traversed the metal tube 

 A, and finally fell upon the disk d under examination. The 

 tube A was coated with a dull black finish on the inside so as 

 to avoid reflexions and preA'ent, to as large a measure as 

 possible, photo-electric effects from its surface. Its left end 

 was closed with a metal stop having a circular opening 7 mm. 

 in diameter, and its right end with a stop having an opening 

 8 mm. in diameter, the size of the openings being chosen so 

 that the whole beam which entered the tube might fall upon 

 the disk d and none of it strike the aluminium wheel TT". 

 Care was taken also so that this beam might not fall upon 

 the inner walls of the tube A. The metal tube A was con- 

 nected to the wire gauze g, which completely covered the 

 inside of the glass tube. It was also connected to the elec- 

 trode E", so that tube and gauze might be connected to earth 

 either through FJ or E" . The electrode E was connected 

 directly to the aluminium frame, /, which supported the 

 wheel, and f was kept in conducting contact with the wheel 

 by means of a platinum spring p. The bulb was enclosed in 

 an electric furnace of dimensions 30 cm. x 30 cm. x 20 cm. 

 This furnace was provided with an electric fan for maintain- 

 ing constancy of temperature. 



Observations upon the effect of illuminating any particular 

 disk with ultra-violet light were made in both of the follow- 

 ing ways : — 1. The electrometer was connected to E and the 

 system charged to a potential of about — 20 volts. The gauze 

 and tube being in this case connected to earth, observations 

 were made upon the rate at which the electrometer system 

 lost its charge under the influence of the light. 2. The 

 electrometer was connected to the gauze and tube by means 

 of the electrode E' or E'\ and the wheel was charged to a 

 potential of from —20 to —300 volts. In. this case the 

 charge which was lost by the wheel because of the influence 

 of the light was caught upon the gauze and measured by the 

 amount of the electrometer deflexion. 



It will be observed that with either arrangement the electro- 

 meter was connected to an electrode which was sealed into 

 glass. As the following observations were taken in mid- 

 summer 1906, this contact of. the charged system with glass 

 made the elimination of the natural leak due to the conduc- 

 tivity of the glass surface of the tube a matter of exceeding 

 difficulty. The elimination of this leak was, how r ever, con- 

 sidered of the greatest importance, since there was reason to 

 believe that the irregularities in the readings given in the 

 preceding table were due to uncertainty in the corrections 

 due to the natural leak. After some months of experimenting 



02 



