554 Dr. H. Lester on the Variation of 



The second general effect was a peculiar over-shooting in 

 the current *. For a field of 40 volts between the electrodes- 

 the currents became steady. Upon changing the potential 

 to 400 volts there was a relatively large current that changed 

 in a few moments to a smaller steady value. The magnitude 

 o£ the initial current was often 150 per cent, of the steady 

 value. The change occurred mostly in the first 20 seconds, and 

 the total change took about ten minutes in most cases. Upon 

 changing potentials from 400 to 40 A r olts there was an 

 initially small current that rose to a steady value. In this 

 case the greater part of the change occurred in the first five 

 minutes, but the steady value was often not established after 

 two hours. 



In addition to the above general effects some special inves- 

 tigations established the following facts : — (1) Wires cleaned 

 by boiling for a long time in HN0 3 , wires not so treated, 

 and wires purposely contaminated with dust from the floor 

 of the room behaved essentially the same as regards the 

 phenomenon of lack of saturation. Wires known to be very 

 pare in most cases showed currents of smaller magnitude 



Ai 



and smaller values of — , and smaller over-shooting effects. 



than wires known to be impure. However, there were some 



Az 

 exceptions. For instance, the largest value of -t- obtained 



was from a very pure wire. Its magnitude was 33 for 



M 

 negative and 27 for positive currents, The value of — 



ranged from *1 to 25 for most wires, the most common 

 values being between 10 and 15 for impure wires and 

 between 2 and 7 for pure wires. For impure wires the over- 

 shooting was generally large and the time decay small ; for 

 pure wires the over-shooting was generally small and the 

 time decay fairly rapid. Wires sprayed with NaOH and 

 KOH. behaved essentially the same as unsprayed wires, except 

 for the magnitude of the currents and the temperatures at 

 which emission became noticeable. A wire coated with 

 CaCl 2 showed the remarkable property of perfect saturation 

 when first heated, but after an hour the current had decayed 



Ai 

 20 per cent,, and there was a value of — of magnitude *3. 



Az l 



• — was still growing when observations were discontinued. 



I o o 



* Effects similar to this have been recorded by Richardson, Phil. Trans- 

 A. vol. ccvii. p. 29(1906). 



