1890.] Ratio of Electromagnetic Unit to Electrostatic Unit 377 



As the use of tlie auxiliary condenser introduces additional sources 

 of error, we endeavoured to determine the electromagnetic measure 

 of the capacity of the guard ring directly, using a complicated 

 commutator, which worked both the guard ring and condenser. The 

 first commutator we used was one where the contacts were made by 

 platinum styles attached to a tuning fork ; the results obtained with 

 this were not so regular as we desired, so we replaced the tuning 

 fork commutator by a rotating one driven by a water motor. A 

 stroboscopic arrangement was attached to the commutator, which 

 enabled its speed to be measured and kept constant. With this 

 arrangement, which worked perfectly, we got values for the electro- 

 magnetic measure of the capacity of the condenser distinctly less 

 than those obtained by the old method. We then endeavoured to 

 find out the reason for this difference, and after a good deal of 

 trouble discovered that in the experiments by which the equality of 

 the capacities of the guard ring and auxiliary condensers were tested 

 the guard ring did not produce its full effect. When the guard ring 

 of the standard - condenser was removed and the capacity of the 

 auxiliary condenser made the same, the two methods gave identical 

 results, but the effect produced by adding the guard ring was less in 

 the old method than in the new. We found by calculation that the 

 effect produced by the addition of the guard ring in the old method 

 was distinctly too small, while in the new the observed and calculated 

 effects agreed well together. As the new method was working 

 perfectly satisfactorily, and as it possesses great advantages over the 

 old one, inasmuch as we get rid entirely of the auxiliary condenser, 

 and, since the commutator is a rotating one, its speed can be altered 

 with much greater ease and accuracy than can be done with a tuning 

 fork, we discarded the old method and adopted the new one. 



The following are the results obtained by this method : — 



Electrostatic measure of the capacity, 397*991. 



Electromagnetic Measure. 



Eirst Set of Experiments. 



Number of times the 

 condenser is charged 



per second. Capacity x 10 21 . 



64 443-427 



32 443571 



48 443-523 



80 443-459 



64 443-298 



55 443-478 



42 443-443 



Mean, 443 457. 



