78 Royal Society. 



by the author to be owing rather to the diiFerent 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 

 iniiuence 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 pomts 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 

 its spiral ; 2nd. the attraction still subsisting after the circuit was 

 broken ; 3rd. the attraction of the other pole of the needle on the 

 reversal of the current ; and 4th. the attraction remaining after 

 this reverse current was cut off. The sum of the 1st and 3rd 

 observations gives the total change in the magnetism of a bar by 

 the reversal of the current. The sum of the 2nd and 4th gives the 

 total permanent change of magnetism, or the magnetic set. 



The experiments were made with iron bars of the several dia- 

 meters, ^, pj:^, ^, \, \, and one inch, the length being in each 

 case one yard; and also with iron bars -g-, |^, \ and one inch diame- 

 ter, of the length of two yards. In all the bars of small diameter 

 up to i of an inch, the magnetic set obtained by the use of feeble 

 currents was found to be proportional to the square of the current 

 employed in producing them. This law was found to subsist 

 through a long series of electric intensities ; but when the current 

 was increased to a certain amount, the set, as observed in the bars 

 of ^ and yyrj of an inch diameter, increased in a much higher ratio, 

 so as to vary, in some instances, with the 4th and 6th powers of 



