558 Prof. Richardson and Mr. Robertson on Contact 



the tube, exhausting with the mercury-vapour pump, etc., and 

 heating the filament for some hours, it was found to have 

 a fairly stable characteristic curve in the neighbourhood of 

 that to the left-hand side of fig. 1. Later on, the emission was 

 found to have risen to an enormously higher value,, and was 



Fig. 1. 



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right outside the scale of the available measuring instruments 

 with the same temperature as indicated by the resistance of 

 the filament. The emission was now lowered by reducing 

 the filament temperature to a value comparable with that 

 ruling before the change occurred, and the characteristic was 

 now found to be stable in the neighbourhood of the right- 

 ihand curve in fig. 1. This displacement of the characteristic 





