ELECTRIC UNITS OF CHARGE BY G. C. LAURENCE, B. SC. 187 



were straight. When we set dC/dh=0 to find the minimum 

 value of C we get h =L^/\2 and equation (2) becomes 



C=D/(U'^QM'R{R-r) log R/r)-[-C (3) 



In the case of the mechanically adjusted condenser the two 

 axes intersect only at the ends of the tube i. e. where the wedge 

 was inserted, hence x = {y^ — L^/4:)/M and integrating for the 

 whole length we get for the capacity 



C"=D/{2^QM'R{R-r) log R/r)^C (4) 



.-. C— C = (C"— C')/5 (5) 



The correction (C — C)/5 amounted to .03 m. m. f. making 

 C"=123.50 m. m. f.. The smallness of the correction justified 

 the assumption that the curve was parabolic. 



Results 



The most difficult quantity to measure in finding the electro- 

 static capacity of the condenser was the distance between the 

 inner and outer cylinders. This was determined to .05% by 

 weighing the quantity of water which this space could contain. 

 The result was checked, by two methods of direct measurement, 

 and differed from them by less than their probable errors (.1% 

 and .3%). The other dimensions were measured directly with 

 calipers. The probable error in the electrostatic capacity was 

 .07%. 



Eighteen separate electromagnetic measurements of the 

 difference in capacity of the two cylinders were made, using two 

 different fork frequencies — 90 and 126 cycles, two battery vol- 

 tages — 45 and 90 volts, and making similar changes in the re- 

 sistances of the arms of the bridge. Two independent adjust- 

 ments of the cylinders by the electrical method and two by the 



