of Varying Currents in Inductive Circuits. 261 



and in this way any number of points can be obtained on the 

 required current curve (indicated on the figure by a dotted 

 line) . The process is exceedingly simple, especially when, as 



would usually be the case, the ( -, t j curve is plotted on squared 

 paper. ' 



It is preferable to maintain the current in the slide- wire, 

 by frequent standardization with the aid of a Clark cell, at 



such a value that the quantities - (and not necessarily e, as in 



T 



ordinary potentiometer practice) can be read off in amperes 

 direct, making correction if needful for any lack of uniformity 

 in the slide-wire. 



The chief difficulty which must be met if the method is to 

 be successfully employed lies, as might be expected, in the 

 use of the stopped galvanometer ; and it was only after many 

 experiments that we succeeded in making one which answered 

 the required conditions satisfactorily. 



The points to be borne in mind when designing such a 

 galvanometer are the following : — 



(1) A sudden removal of the couple pressing the needle 

 against its stops should not produce a throw. 



This requires that both ends of the needle be provided with 

 stops ; and also that the needle shall be stiff to resist the 

 distorting action of couples acting in a horizontal plane ; and 

 if the arm is not formed of the magnetic needle itself, it 

 requires that the two be rigidly connected together. 



(2) The needle must not adhere to its stops even to the 

 slightest extent, since this would prevent the smaller 

 throws from taking place. 



This condition is a most important one. We at first tried a 

 pair of glass stops, but the needle always adhered slightly, 

 and we traced this with some difficulty to the presence of 

 moisture. We then carefully dried and replaced them in the 

 galvanometer together with a small vessel containing calcium 

 chloride. We found, however, that clean, dry, glass stops 

 almost invariably possessed a slight electrostatic charge, the 

 effects of which were sometimes as bad as those of moisture ; 

 and stops made of shellac or of sealing-wax — substances not 

 nearly so hygroscopic — were nevertheless unsatisfactory, 

 apparently from this cause. Metal stops were tried, but they 

 all seemed to be covered with a slight film of moisture which 

 even a drying agent could not be relied upon to entirely 

 remove. 



The material which we ultimately adopted was steatite or 



