74 ACTION OF A SECONDARY WIRE. 



elastic force, chemical authors generally adopting the same signification: "The re- 

 moteness from the unexcited state, a condition expressed by the terms tension or inten- 

 sity." " By tension or intensity is meant the energy or effort with which the current 

 is impelled." ( Turner, Elem. ChetnJ) 



243. This confusion of terms leads to a confusion of facts of a much more serious 

 kind. English electricians uniformly state that the magnetic needle, deviating in the 

 neighbourhood of a current, takes no note whatever of the intensity of that current. 

 Continental writers, almost without exception, regard the deviation as a function of the 

 intensity, and the statements, therefore, appear discordant. While the effect is thus dif- 

 ferently described, all agree as to the facts of the case. In what follows, the term tension 

 will be used as expressive of the elastic force of the current, that power by which it is 

 enabled to pass a resisting medium ; the term intensity will be strictly confined to the 

 acceptation in which writers on analytical mechanics use it. " By the intensity of a force, 

 we understand its greater or lesser capacity to produce motion" (Eoucharlaf) ; and in 

 the case before us, the intensity will be regarded as a function of the quantity and ten- 

 sion conjointly. Thus, the deviation of a magnetic needle does not indicate the ten- 

 sion, but the intensity of a current. 



244. 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 in- 

 terpose 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 deter- 

 mined 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 ab- 

 solute 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 measureable by the deviations of the magnetic needle, or by the 

 voltameter, these instruments may be used to determine the tension, by making quan- 

 tity indirectly the measure thereof. 



245. If, therefore, we send a certain quantity of electricity, as 100 parts, to a resist- 

 ing 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 quantity, 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 cur- 

 rent which passes the resisting wire may be taken as the representative of the tension 

 of that current 



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



247. But it is plain that this amount per cent., of which I am speaking, in introdu- 

 cing this fundamental proposition, is nothing more than the ratio which exists between 

 the quantities passing the large and the little wires respectively. By dividing, there- 

 fore, the quantity that passes the secondary wire by the quantity that passes the large 

 wire, we shall have a numercial representative of the relative tension of the current un- 

 der consideration. 



