314 



A Standard Condenser of Small Capacity. 



By R. R. Ramsey, 



In radioactive measurements of substances which are very feebly radio- 

 active it is necessary to have an electroscope which is very sensitive. One 

 of the conditions to obtain this result is, the electroscope must have a very 

 small capacity. A capacity of one to ten centimeters. A sphere has a capac- 

 ity equal to its radius when far removed from other objects but when brougb.1 

 near to the electroscope its capacity changes to a value which depends upon 

 the position, size and shape of the electroscope. 



It is customary to use a cylindrical condenser. The capacity of a cylindri- 

 cal condenser is 



L 



C= — 



2 log e R,/R.> 

 where C is the capacity; L is the length; Ri is the inside radius of the out- 

 side cylinder; R 2 is the radius of the inside cylinder. This formula gives the 

 capacity if the effect of the ends can be neglected. This requires that the 

 length should be great compared to the difference of the two radii. When 

 these conditions are met the capacity will be 100 cm. or more. 



In order to correct for the end effects I have made a condenser in three 

 sections, the construction of which is illustrated in the cross sectional draw- 

 ing. The middle cydinder is made of a brass rod about 9 millimeters in diam- 

 eter. The outside cylinder is made of brass tubing whose inside diameter is 

 about 3.6 cm. The diameters are chosen large in order that the accuracy of 

 measurement may be great. The ratio of the diameters is made large in order 

 that the capacity per unit length may be small. 



The length of the end sections is 10 cm. The length of the middle sec- 

 tion is 20 cm. The middle rod is held in place in the end sections by means of 

 sulphur. This was accomplished by means of two Avooden discs which were 

 accurately turned to fit in the ends of the large cylinder and hold the middle 

 rod in the center. These discs were placed in the ends of the end sections. 

 The end section was stood upon the outside end and melted sulphur was 

 poured through a hole in the top disc until the cylinder was about one-third 

 filled. The discs were removed after the sulphur had hardened. Dowel pins 

 are placed on the middle rod to hold the middle section in place. 



