268 Dr. Draper on the Use of a Secondary Wire as a 



abled to pass a resisting medium ; the term intensity will be 

 strictly confined to the acceptation in which writers on analy- 

 tical mechanics use it. " By the intensity of a force, we under- 

 stand its greater or lesser capacity to produce motion," 

 (Boucharlat) and in the case before us, the intensity will be 

 regarded as a function of the quantity and tension conjoindy. 

 Thus, the deviation of a magnetic needle does not indicate 

 the tension, but the intensity, of a current. 



Suppose now we had a current of electricity passing under 

 a certain tension, along a channel of conduction, as a bar of 

 large dimensions, and were suddenly to interpose in some 

 part of its path a resisting obstacle, as, for example, a slender 

 wire ; it is obvious that a certain portion of the current would 

 pass the barrier, a portion determined partly by the character 

 and dimensions of the wire, and partly by the tension or elastic 

 force of the current. Let the wire under all circumstances 

 be the same, the absolute quantity of electricity be constant, 

 but the tension thereof vary. Now, as the tension increases, 

 the quantity that passes the resisting wire will also increase, 

 and as the one diminishes so will the other too. Under these 

 circumstances, the absolute quantity that passes will always 

 be an increasing function of the tension, and as this quantity 

 is under all circumstances measurable by the deviations of the 

 magnetic needle, or by the voltameter, these instruments may 

 be used to determine the tension, by making quantity indi- 

 rectly the measure thereof. 



If, therefore, we send a certain quantity of electricity, as 100 

 parts, to a resisting wire, and find that of these 50 parts can 

 pass the obstruction, we may assume such a current to have 

 a higher tension than one containing the same absolute quan- 

 tity, of which only 30 could pass; and to have a much lower 

 tension than one, of which 70, 80, or 90 parts could pass. In 

 all these cases, the amount per cent, of the main current 

 which passes the resisting wire, may be taken as the repre- 

 sentative of the tension of that current. 



This obstructing, resisting wire, I call a secondary wire. 



But it is plain that this amount per cent, of which I am 

 speaking, in introducing this fundamental proposition, is no- 

 thing more than the ratio which exists between the quantities 

 passing the large and the little wires respectively. By divi- 

 ding, therefore, the quantity that passes the secondary wire, 

 by the quantity that passes the large wire, we shall have a 

 numerical representative of the relative tension of the current 

 under consideration. 



Let us take an example: a single pair of plates developed 



