352 



Mr. W. H. Preece. 



[May 21, 



a most reliable, simple, and valuable instrument, and my current 

 meter a Siemens' electrodynamometer. 



7. I was at first much troubled with electrical leakage. The 

 current escaped over the edges of the box through creeping by capil- 

 lary action, and the formation of moisture from the spray of the 

 solution when bubbles of gas arose in the liquid and burst on the 

 surface. This was quite cured by standing each box on three white 

 porcelain supports of the form shown in the following figure, the 



cups being half filled with resin oil on Messrs. Johnson and Phillips' 

 plan. It is now quite impossible to measure the leakage except with 

 a delicate galvanometer, and the insulation may be said to be prac- 

 tically perfect. 



8. The E.M.¥. of the battery at its terminals :— 



When charging 2'25 per cell. 



When idle 2'05 



When discharging 1*90 ,, 



The internal resistance* is, per cell : — 



When charging "0060 w 



When discharging '001 7* 



9. But the latter varies very markedly with the strength of current 

 of discharge. This is shown by the following experiment made 

 with 23 cells of a smaller type than those described above, which are 

 used in the Post Office. 



Current of discharge Internal resistance 

 in amperes. in ohms. 



4-39 0-7608 



7-25 0-4607 



15-84 0-2816 



25-07 0-1969 



* The term "internal resistance" means the effect of counter-electromotive 

 force as well as of resistance to conduction. 



