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



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 se- 

 condary 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 

 28'i 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 Humphry Davy's experiments 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. 



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 ob- 

 served in the two cases ? If the quantity passing A be doubled, 

 will the quantity passing B be doubled also ? 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 quantity traversing A, and b the quantity tra- 

 versing B. Now, if the tension remains constant, and the 

 quantity only is variable, the ratio 



a 



is always constant, and is entirely independent of the value 

 of a. 



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. 



