[ no ] 



XIII. 



ON A CONVENIENT METHOD OF OBTAINING ANY RE- 

 QUIEED ELECTRICAL POTENTIAL FOR USE IN 

 LABORATORY TEACHING. By F. T. TROUTON, B.E. 



[Read March 21, 1888.] 



It would be an obvious advantage in the experimental teaching of 

 Electrostatics to be able conveniently to charge bodies to any re- 

 quired potential. Although it does not enable us to do this, yet 

 by the following method, two or more bodies can be charged to 

 potentials, the ratio of which to each other can be determined 

 and varied as required. 



To the poles of an electrical machine, say a Holtz, a length of 

 about five feet is connected of a poorly- conducting substance — an 

 ordinary cord, if smooth, does fairly well. It is, however, of con- 

 siderable importance, to prevent discharge into the air as much as 

 possible from the little points and roughnesses on the cord, by 

 having it smooth. Neglecting any discharge which may occur, 

 we can consider the potential as falling proportionately to the 

 distance along the cord, when the machine is working uniformly. 

 By touching the bodies to be charged to the proper points along 

 the cord, the desired potentials are obtained. The difference of 

 potential at each point touched from that at either pole is propor- 

 tional to the length measured along the cord. 



By this method the capacity of two bodies can be very simply 

 compared. First, connect one pole of the machine to earth ; then, 

 if the bodies be touched to the cord, at distances from the pole 

 to earth, that are in the inverse ratio of the capacities, equal 

 quantities of electricity are obtained on the bodies. This equality 

 is best tested by the null method described in Maxwell's Elemen- 

 tary Treatise on Electricity, Experiment vn. After a few trials 

 the correct distances are easily found ; then the inverse ratio of 

 the distances, measured along the cord, is the ratio of the capaci- 

 ties of the bodies. 



