Dr. Ritchie's Remarks on the Bakerian Lecture. 209 



matical, prove nothing beyond these general results. Now 

 as the object of the paper was obviously to determine the 

 strict laws of conduction, it is in this point of view that its 

 claims ought to be scrutinized. 



In accumulating a current of voltaic electricity so feeble as 

 to produce scarcely any permanent deflection on a needle, 

 the method uniformly employed was to make the needle os- 

 cillate, or swing, by repeated impulses given at the moment 

 it returned to the conducting wire. 



Mr. Faraday was the first who employed the method of esti- 

 mating the intensity of electric forces by comparing the arcs 

 through which a magnetic needle was made to oscillate by 

 single electric impulses. As he viewed the forces proportional 

 to the arcs themselves, instead of the sines of half the arcs, it 

 is obvious that he employed this mode as an approximation, 

 his object being merely to ascertain general results*. This 

 principle, when employed as<Mr. Faraday has done, as a sim- 

 ple and obvious mode of exhibiting the differences of electric 

 intensity, is exceedingly valuable; but when extended, as Mr. 

 Christie has attempted, to investigations of the most delicate 

 and accurate kind, may lead to general conclusions exceedingly 

 wide of the truth. 



Mr. Christie sets out with assuming that a magnetic needle 

 suspended by a fibre of silk directly above a conducting wire, 

 and parallel to it, will, when acted upon by currents of mag- 

 neto-electricity, begin to move with a velocity proportional to 

 the intensity of the current. " The intensity of the current 

 will therefore vary as the velocity with which the needle is 

 impelled at the commencement of its motion, and this velocity 

 will be the same as that which it would acquire in descending 

 from its highest point by the force of terrestrial magnetism 

 acting upon it. 



" Let V be the angular velocity with which the needle begins 

 to move; A the whole arc described by the needle; v the an- 

 gular velocity corresponding to any arc 0; and m the mag- 

 netic intensity acting on either pole in the direction parallel 

 to the wires of the galvanometer, and reduced to the di- 

 stance 1. Then, since the force in the direction of the tan- 

 gent is m sin 0, we have 



v dv = — m sin dd. 

 Integrating from = to 9 = A 



V = 2 s/ nTsin {A (a)" 



"Let I', I" be the intensities of the currents corresponding 



* Miilosophira! Transactions for 1833, Part I. p. 50. 

 Third Series. Vol. 4. No. 21 . March 1834. 2 E 



