250 Mr. Norman Shaw on Charged Surface Layers in 



depends on the speed with which C is insulated after the 

 brass has been charged. It ranged from 0*030 to 0*040 volt 

 for lead-brass. The effect usually lasted as long as 66 seconds, 

 but was almost completed in about 40. 



These variations could be greatly increased or altered in- 

 direction by charging up the brass and its layer to several 

 volts immediately before bringing it to the potential P. In 

 this case changes as large as 0*100 volt could be produced, 

 growing rapidly and then slowly vanishing as before, but if 

 voltages over three volts were used disturbances due to the 

 insulation were obtained which slightly altered the variation- 

 By keeping both plates earthed for several hours (either in 

 normal or ionized air) it is possible to make the first few 

 creeps slower than the normal ones of type (a). This 

 appears merely to indicate that the layers become more 

 compact or thicker if held for a considerable time. It 

 requires at least an hour to produce a perceptible difference 

 of this kind, but two or three minutes is sufficient to bring 

 it back to the normal state. The layers themselves probably 

 do not reach the same potential in ionized air as under 

 ordinary conditions. It is also possible temporarily to alter 

 the final value or contact potential by bringing the plates 

 into close proximity without contact. If the plate C is left 

 insulated in ionized air at a distance from the plate B 

 greater than 1 mm., this preliminary retardation is not 

 produced. If left for several days, however, there may 

 occur small permanent changes in any circumstances due to 

 the slow oxidation or deterioration of the polished surface , 

 It is, indeed, usually necessary to wait several days before a 

 polished surface is sufficiently constant in contact potential 

 to render such comparative experiments as those mentioned 

 above possible. 



§ 4. Some Experiments on the Influence of the Gaseous Medium 

 and of the State of the Metals on the Formation of the 

 Layers. 



(1) The well-known effects of polishing the plates dis- 

 cussed in detail by Kelvin *, Erskine-Murray t, and others 

 were repeated and examined by these ionization methods* 

 Results similar to those obtained by the electrostatic 

 methods were presented, and no new point was apparent. 

 [The type of the curves lor the various chargings varied 

 proportionally with the magnitude of the contact potential 



* Kelvin, Phil. Mag. vol. lvi. p. 82 (1898). 



t Erskine-Murray, Proc. Roy. Soc. vol. lxiii. p. 113 (1898). 



