489 



diameter, was nearly proportional to the strength of the current and 

 the length of the wire, any alteration, within certain limits, of the 

 diameter of a bar being attended with only trifling effects, so long 

 as the point of saturation was not nearly approached. The Russian 

 philosophers Lenz and Jacobi had, however, stated that the mag- 

 netism developed was, c&teris paribus, proportional to the diameter 

 of the bar. The discrepancy between the above results is considered 

 by the author to be owing rather to the different circumstances 

 under which the experiments were tried than to any inaccuracies in 

 the experiments themselves. Further, it appeared to him that in 

 any case of induction by electric currents, careful distinction should 

 be made between the several effects, which, compounded together, 

 constitute the total magnetic action. Especially should a distinc- 

 tion be made between the magnetism existing under the inductive 

 influence of the current and that permanently developed so as to 

 remain after the electrical circuit is broken, and therefore the first 

 efforts of the author were directed to ascertain the laws which regu- 

 late this permanent effect, or, as he thinks it may be conveniently 

 termed, the magnetic set. 



In his experiments the magnetism of any bar was ascertained, by 

 placing it vertically with its lower end near a delicately suspended 

 magnetic needle. This was a piece of sewing-needle ^-ths of an 

 inch long, furnished with an index of fine drawn glass tube tra- 

 versing over a graduated circle six inches in diameter. It was sus- 

 pended by a filament of silk. The tangent of the deflection of the 

 needle was found to be the exact measure of the attraction of a bar. 

 In working with this instrument, it was found that the resistance of 

 the air prevented the needle from swinging even once beyond the 

 point of equilibrium to which it always arrived in less than ten 

 seconds. This resistance of the air, so useful for bringing the 

 needle rapidly to a state of rest, rendered it necessary to keep the 

 entire instrument at a uniform temperature, for the slightest local 

 application of heat produced currents of air within the glass case of 

 sufficient strength to occasion considerable deflections. The cir- 

 cumstance points to the possibility of constructing a new and very 

 sensitive thermometer which might be useful, particularly in experi- 

 ments on the conduction of heat. 



The method of experimenting consisted in observing, 1st. the 

 magnetic attraction of any bar when a current circulated through 



