2Ti> Dr. Draper on the Use of a Secondary Wire as a 



become (n t), then a corresponding change will happen in 

 a and b, admitting the principle that the quantities passing 

 through A and B are increasing functions of {t). If then {t) 

 becomes {n t), a will become (n a). Now if the equation 



a 

 holds ; b =: at; but when the change impressed on (t) has hap- 

 pened, b will be equal to the conjoint values of (w a) and {n t) ; 

 and if these values be substituted in the former ratio, the re- 

 sult is still equal to (w /) ; so that whatever may be the change 

 impressed on {f) the formula b _ 



a 

 will always indicate it. 



Having thus settled, by the foregoing simple reasoning, the 

 fundamental doctrine of investigation, I next proceed to ap- 

 ply it to the analysis of the different processes, by which a 

 change of tension is supposed to be impressed on an electric 

 current; and this leads to the consideration of the second 

 proposition : — 



" That there is reason to doubt, whether the processes 

 usually supposed to affect the condition of an electric current, 

 are ever attended with any such result; but that when changes 

 have apparently taken place, it is probable that they may be 

 directly traced either to a disturbance at the place of genera- 

 tion, or to the development of other currents of a different 

 character, the primary current itself remaining unchanged." 



It is popularly supposed, that if we pass an electric current 

 through a wire of certain length, coiled upon itself, a kind of 

 inductive influence will be exerted, so that the current shall 

 become more and more intense as it goes. Or, if two currents 

 are simultaneously passed into a double helix, they will mu- 

 tually fortify each other. 



{a.) A wire covered with silk, 48 feet long, and arranged 

 as one circular arc, had a current passed through it, which 

 produced a deviation of 35 degrees. The same wire was then 

 coiled round a piece of wood, so as to make 155 circumvolu- 

 tions ; the deviation was still 35 ; and therefore no change was 

 impressed on the current. 



(6.) A thermal current was passed through a straight wire 

 with the following result : — 



22} ••«S8. 

 The wire was then coiled into a helix, the current passed 

 through it, and measured ; a powerful bar magnet was next 

 introduced into the helix, and then a rod of soft iron. But in 



