the Magnet on Electric Currents. 



229 



gold-leaf strip not covered] by the pieces of brass was about 

 5^- centims. in length, and had a resistance of about 2 ohms. 

 The poles of a high-resistance Thomson galvanometer were 

 placed in connexion with points opposite each other on the 

 edges of the strip of gold-leaf, and midway between the 

 pieces of brass. The glass plate bearing the gold-leaf was 

 fastened, as the first one had been, by a soft cement to the 

 flat end of one pole of the magnet, the other pole of the 

 magnet being brought to within about 6 millims. of the strip 

 of gold-leaf. 



The apparatus being arranged as above described, on the 

 12th of November a series of observations was made for the 

 purpose of determining the variations of the observed effect 

 with known variations of the magnetic force and the strength 

 of current through the gold-leaf. 



The experiments were hastily and roughly made, but are 

 sufficiently accurate, it is thought, to determine the law of 

 variation above mentioned as well as the order of magnitude of 

 the current through the Thomson galvanometer compared with 

 the current through the gold-leaf and the intensity of the 

 magnetic field. 



The results obtained are as follows : — 



Current through 



gold-leaf strip. 



0. 



Strength of 



magnetic field. 



M. 



Current through 

 Thomson galvano- 

 meter. 

 c. 



CxM 

 c 



•0616 

 •0249 

 •0389 

 •0598 

 •0595 



11420 H 



11240 „ 



11060 „ 



7670 „ 



5700 „ 



•00000000232 

 085 

 135 

 147 

 104 



303000000000. 



329 



319 



312 



326 



H is the horizontal intensity of the earth's magnetism 

 = *19 approximately. 



Though the greatest difference in the last column above 

 amounts to about 8 per cent, of the mean quotient, yet it 

 seems safe to conclude that, with a given form and arrange- 

 ment of apparatus, the action on the Thomson galvanometer 

 is proportional to the product of the magnetic force by the 

 current through the gold-leaf. This is not the same as 

 saying that the effect on the Thomson galvanometer is under 

 all circumstances proportional to the current which is passing 

 between the poles of the magnet. If a strip of copper of the 

 same length and breadth as the gold-leaf, but \ millim. in 

 thickness, is substituted for the latter, the galvanometer fails 

 to detect any current arising from the action of the magnet, 



