RESISTANCE OF THE SECONDARY WIRE UNDER VARIATIONS OF TENSION 77 



not only the thick polar wires that come from the electrometer, those which were used 

 being one fifth of an inch thick, but include the electrometer itself, no matter what 

 its character may be if a hydro-arrangement, the plates, exciting liquid, &c. The 

 secondary wires are simply long or slender wires, to obstruct the current ; of these I 

 have occasionally used two, the first 47 inches long, the second 290 : they are of 

 copper, one foot of which weighs 10-65 grs., and are covered with silk. 



262. And, lastly, the measures are sometimes arranged in a form such as this : 



in which the large or upper number represents the quantity passing the primary wire, the 

 under or smaller number the quantity passing the secondary wire, and the decimal on 

 the right hand of the bracket, being the quotient of the former numbers, is, as will 

 presently be shown, the representative of the tension. 



263. We have now to examine the foregoing proposition more minutely. Let us call 

 the primary wire, being that which is in connexion with the electromotoric source, A; 

 and the secondary or resisting wire, B. Now how does B act towards currents when 

 they are of variable character ? There is no current, no matter how low its tension 

 may be, that will not pass along B to a certain extent : this is abundantly proved by 

 such a wire transmitting a thermal current of the lowest tension and amount. But at the 

 other extremity of the scale is there a limiting point ? Can a wire conduct electricity of 

 a certain tension only to a certain amount ? I think not, for a wire of small diameter 

 was found upon trial to conduct a thermal current to the extent at one time of 20, and 

 then of 284 parts, the tension in both cases being the same ; and if it would do this in 

 the case of currents whose tension is so very low, the same might be looked for in 

 hydro-currents ; here, however, when the quantity reaches a certain point, the ignition 

 of the wire ensues, and its physical character is changed. Sir Humphrey Davy's experi- 

 ments lead to the same conclusion (Phil. Mag., Dec., 1821), nor does there appear to be 

 any limit to the conducting power of a wire, either for high or for low tension. If a wire 

 carries a certain amount of electricity, an increase of quantity or of tension will enable 

 it to carry more, and the converse. To this important point I shall presently return. 



264. As it thus appears that any increase of the quantity which A transmits involves 

 also an increase of that which passes B, a second question arises, What is the ratio 

 that will be observed in the two cases 1 If the quantity passing A be doubled, will the 

 quantity passing B be doubled also 1 This is a very important problem ; for if the 

 ratio above mentioned holds, it would show that an observation by the secondary wire 

 will give the tension independent of the absolute quantity. Let a represent the quan- 

 tity traversing A, and b the quantity traversing B. Now, if the tension remains constant, 



and the quantity only is variable, the ratio - is always constant, and is entirely indepen- 



a 



dent of the value of a. 



265. This I have endeavoured to prove experimentally. I took a hydro-electric pair 

 of copper and zinc, each of the plates exposing about two square feet of surface, and 

 dipped them to different depths in dilute sulphuric acid. The following table exhibits 

 one of these results : 



