508 Prof. Fleming and Mr. Clinton on the 



o 



often arises for knowing their approximate absolute capacity, 

 and it is much to be desired that instrument-makers should 

 forsake the custom of denominating their different leyden- 

 jars in the present manner. 



Incidentally, the apparatus may be used for more or less 

 accurate determinations of the electromagnetic constant " v ". 

 One method by which a small capacity of known value can 

 be made with a fair amount of accuracy is as follows : — Two 

 cylindrical air condensers are constructed, each of which 

 consists of a cylindrical rod or thick-w 7 alled tube placed in the 

 interior of another concentric tube. Two condensers of this 

 kind are prepared, one about double the length of the other ; 

 one ma)' be, say, a foot long, and the other two feet long. The 

 tubes may be of the quality known as " triblet-drawn " brass 

 tube, one exceeding the other in diameter by about 2 milli- 

 metres. These tubes may then be cut up into the requisite 

 lengths. The ellipticity and variation of diameter in length 

 of such tubes are very small indeed. 



These pairs of tubes can then be formed into two air con- 

 densers by fixing the inner tube concentrically within the 

 outer tube by means of ebonite disks at the end. By the 

 adoption of known mechanical methods, the concentric 

 adjustment of the tubes can be made very accurately, and the 

 radial thickness of the air space determined. In this manner 

 two cylindrical air condensers can be constructed identical 

 in every wa}~, except that they differ in length. 



If c is the capacity per unit of length of the middle part of 

 the condenser, wmere the strain-lines are truly radial, and if 

 L is the length of the condenser, then the capacity C of the 

 condenser is expressed by a function of the form C = ^ + cL, 

 where x is an unknown quantity depending on the distribution 

 of the electric strain at the ends of the tubes. Hence if we 

 measure the capacity of the above tw T o condensers by means 

 of the rotating commutator, and take the difference of these 

 capacities, this is the value of the capacity of a length equal 

 to the difference of the lengths of the condensers, and there- 

 fore the capacity per unit of length of one of these condensers 

 is known, disregarding the uncertain distribution of the strain- 

 lines at the ends. If the device of employing a guard-ring 

 or guard-tube is adopted, then the capacity of the inner 

 portion of such a cylindrical guard-tube condenser cannot be 

 determined simply by charging and discharging that inner 

 portion through a galvanometer, without at the same time 

 charging and discharging the guard-tubes through a by- 

 path, or else redistributions of electricity take place at each 

 discharge, which vitiate the result. This source of error was 



