236 Mr. P. J. Edmund? on the 



then the insulated system slowly charged till a discharge was 

 detected by the galvanometer. If this discharge was con- 

 tinuous, there was usually no difficulty in determining the 

 potential necessaiw to produce a discharge. Occasionally 

 the potential dropped slightly on the commencement o£ a 

 discharge, but if the system was discharged and then charged 

 anew to the lower potential and maintained at that potential 

 for a short time, a current would commence, showing that 

 this potential was sufficient to start the discharge as well as 

 maintain it. 



On the other hand, if the discharge was discontinuous and 

 consisted of a single spark, the procedure w^as as follows : — 

 the potential at which the first spark occurred was noted and 

 the system was charged to a somewhat lower potential and 

 maintained at that potential. It w r as usually found that a 

 discharge would occur at this lower potential after waiting 

 for a short time. The process was then repeated until a 

 point was found at which no discharge could be obtained. 

 From test experiments it was found that it might be assumed 

 the potential was below the sparking potential if no discharge 

 occurred within about five minutes. The lowest potential at 

 which a discharge was detected was considered as the true 

 sparking potential. 



This phenomenon of "lag" or retardation is well known. 

 In the present investigation it was most marked in the 

 case of the smallest wire used, diameter 0*5 millimetre. 

 With this wire the potential could sometimes be maintained 

 for some minutes, without any discharge passing, at a poten- 

 tial fif tj per cent, higher than that ultimately determined as 

 the sparking potential. The discharge in such abnormal 

 cases was usually rather violent (as judged by the drop of 

 potential, throw of galvanometer, and appearance of dis- 

 charge). The fact of a discharge having occurred a short 

 time previous by no means made it certain that the following 

 discharge would be free from lag. To illustrate the above 

 remarks the following set of figures are given. They were 

 obtained wdth a wire 0*5 mm. diameter, the point being 

 distant 0*25 cm. from the plane, with fresh air at 761 mm. 

 pressure. The w r ire was slowly charged up positively till a 

 discharge occurred. This was repeated as rapidly as possible. 

 The potentials, in order, at which sparks passed were : 



4950, 4750, 3950, 3950, 4000, 3750, 4500, 4050, 4150, 

 4250 volts. 



In order to lessen the difficulties caused by the lag it was 

 decided to increase the initial ionization by an external 



