166 Mr. G. W. Todd : Further Experiments on the 



electroscope plate is d, then for an ion just to go the whole 

 distance we must have 



d 



where it is the mobility and E the value of the alternating- 

 potential. The town mains gave very approximately a sine 

 curve for the value of E, so that the mobility was given by 



w "E n T" 



In the present apparatus d = 21'0 cm., T = l/90 sec, and 

 putting E = ^/5 . e, where e is the " effective " value of the 

 alternating potential, we find the mobility 



88200 

 u= - 



was 



T 



In the case of the commutator the value of I E . dt 



Jo 



determined from the form of the curve, which was known 

 from the construction of the commutator. Putting in this 

 value and the proper constants, the mobility was given by 



1764 



u— -FT-™, 



where n is the frequency of alternation. 



Experiments. — As in previous experiments, it was found 

 that the presence of traces of water-vapour seriously affected 

 the mobilities at low pressures. When water-vapour was 

 present, diminution of the pressure beyond a certain point 

 apparently depending on the amount present resulted in a 

 diminution of the mobility. By the prolonged application 

 of liquid air to a charcoal tube in connexion with the appa- 

 ratus, and by introducing carefully dried gas, this diminution 

 disappeared. Mobilities could easily be measured down to 

 pressures of about 1/20 mm. At high pressures, whether 

 the town mains or the commutator were used as the source 

 of alternating potential, approximately the same mobilities 

 were obtained as those in previous experiments, but as soon 

 as the pressure went down below a millimetre the mobilities 

 varied with the rate of alternation of the field if this were 

 high, and, moreover, they showed nothing like the enormous 

 increases which were observed with the alternating chamber 

 of small dimensions. The tables and curves below give some 



