Mr. Norman Campbell on .Delta Rays. 281 



This apparatus appears to represent as near an approach 

 as can be made to the requisite condition. Any divergencies 

 from it will be noted in the discussion of the results. If the 

 active plate itself were used in place of the layer covering 

 the hole in the end of B, it would be impossible to prevent 

 the alpha rays emitted obliquely from striking I) and causing 

 the emission of delta rays therefrom. 



5. For the measurement of the current a compensation 

 method is necessary, since the potential of the insulated 

 electrode must not be allowed to diverge sensibly from zero 

 during the observation, lest uncertainty be introduced as to 

 the actual potential between the electrodes for which the 

 current is being measured. With a sensitive electrometer 

 measurements might be made by the ordinary method without 

 allowing the potential of the electrode to change by more 

 than 0*01 volt, but even such a small change is undesirable 

 when small differences of potential between the electrodes 

 are being investigated. The compensating current in these 

 experiments was produced by applying a known potential 

 by means of a potentiometer arrangement to one end of a 

 high resistance, the other end of which was connected to 

 the measuring instrument. The resistances employed are 

 described in the Appendix. All measurements of current 

 quoted in this paper are stated in terms of the potential in 

 volts which it was necessary to apply to the resistance in 

 order to compensate the current between the electrodes A 

 and B. 



It may be noted that there is some loss of sensitiveness 

 involved in the use of the resistance arrangement rather 

 than a saturated ionization current or a quartz piezo-eiectric 

 for the production of the compensation current. For the 

 difference of potential from earth which will be ultimately 

 attained as the result of a given residual current is propor- 

 tional to the resistance between the electrode and earth. In 

 the case of the liquid resistance this resistance is several 

 thousand times less than it would be if either of the other 

 arrangements were used. But it must be remembered 

 (1) that the unsaturated flow of electricity between the elec- 

 trodes in itself acts like a resistance to earth *, and (2) that the 

 time for which the current can be allowed to flow, before it 

 is decided whether compensation is complete, is limited by the 

 time during which the zero of the indicating instrument will 

 remain constant. With the arrangement used not more than 

 one half of the sensitiveness that might have been attained 



* Cf: Campbell. Tree. Camb. Phil. Soe. xv. p. 134 (1909). 



