412 Mr Rutherford, The Discharge of [Feb. 21, 



In these experiments two sources of ultra-violet light have been 

 used, viz. an arc lamp and a spark between two zinc terminals. 

 For the spark two large Leyden jars were charged up by a 

 Rumkhorff coil and discharged through a spark gap of about 

 "8 cms. In order to get rid of electrostatic effects, the jars, coil, 

 and spark gap were all placed inside a metal tank connected to 

 earth. The opening above the sparking terminals was tightly 

 closed by a quartz plate, on both sides of which fine wire-gauze, 

 connected to earth, was stretched. This arrangement was found 

 to screen thoroughly from electrostatic disturbances. 



For experiments on the velocity of the carrier the arc light is 

 not nearly so satisfactory a source of ultra-violet rays as the 

 spark discharge. The arc as a rule gives out more intense 

 radiation but it has the drawback of rapidly raising the tempera- 

 ture of the plates and of the adjacent air. In consequence of 

 this the velocity of the carrier is changed, and the determination 

 of the least distance of no appreciable loss of charge cannot be 

 made with certainty. In this respect the spark light is far more 

 satisfactory and it is also a more constant source of rays, but it 

 has the disadvantage of being an intermittent source of light. 



A source of error in the determination of the velocity, which 

 is difficult to avoid, lies in the irregularity of the E.M.F. from the 

 town supply. During the daytime, the load on the machines was 

 light, and there were often rapid alterations in the E.M.F. and 

 period of the alternations. Another source of error also lies in 

 the assumption that the electromotive force curve of the alternator 

 is a sine curve. It is intended to continue these experiments 

 and to use instead of the transformer an alternating electromotive 

 force produced by reversing the sign of a steady e.m.f. by means 

 of a suitable revolving commutator. It is hoped in that way to 

 obtain a very accurate value of the velocity under varying condi- 

 tions of pressure and temperature. 



Effects of different metals. 



If the theory that the discharge of electrification is due to the 

 disintegration of negatively charged particles is true, we should 

 expect to obtain different velocities of the carrier according to 

 the metal on which the ultra-violet light falls. This point was 

 tested by replacing the zinc plates by similar sized plates of 

 aluminium, lead, copper, amalgamated zinc and a zinc plate 

 covered with mercury sodium amalgam. Although there were 

 differences in the rate of leak of these metals due to their different 

 degrees of sensitiveness to ultra-violet light, it was found that the 

 distance of no loss of charge was nearly the same in each case. 

 The arc light was used for this series of experiments, and allowing 



