148 Mi*. H. A. Rowland on Magnetic Permeability, 



a small horseshoe magnet, on one branch of which was a coil 

 of wire; and by sliding this back and forth, induced currents 

 could be sent through the wire, which, when properly timed, 

 soon brought the needle to rest. This arrangement was very 

 efficient ; and without it this form of galvanometer could hardly 

 have been used. To compare the magnetism of the ring with 

 the known magnetism of the earth, and thus reduce it to ab- 

 solute measure, a ring G supported upon a horizontal surface 

 was included in the circuit; when this was suddenly turned 

 over, it produced an induced current, due to twice cutting* the 

 lines of magnetic force which pass through the ring from the 

 earth's magnetism. The induced current in the case of either 

 coil, F or G, is proportional to the number of the lines of force 

 cut by the coils * and to the number of wires in the coil, which 

 latter is self evident, but may be deduced from the law of Gau- 

 gainf. It is evident, then, that if c is the deflection from coil 

 G, and h that from helix F, the number of lines of force passing 

 through the magnet E, expressed in the unit we have chosen, 

 will be 



Q'=2?i'(6'27sin74 o 50')7rR 2 i^-, . . . (9) 



c ATI 



where n! is the number of coils in the ring G, n the number in 

 the helix F, R the radius of G, 6*27 the total magnetism of the 

 earth, and 74° 50' the dip. The quantity 2n' (6-27sin74° 50')7rR, 2 

 is constant for the coil, and had the value 14*51. This is the 

 number of square metres of a unit field which, when cut once 

 by a wire from the galvanometer, would produce the same de- 

 flection as the coil when turned over. 



The experiments being made by reversing the magnetism of 

 the bars, a rough experiment was made to see whether they had 

 time to change in half a single vibration of the needle ; it was 

 found that this varied from sensibly to nearly 1 second, so that 

 there was ample time. It was also proved that the sudden im- 

 pulse given to the needle by the change of current produced the 

 same deflection as when the change was more gradual, which 

 has also been remarked by Faraday, though he did not use such 

 sudden induced currents. As a test of the method, the hori- 

 zontal force of the earth's magnetism was determined by means 

 of a vertical coil ; it was found to be 1*634, while the true quan- 

 tity is 1*641. 



It is sometimes assumed that some of the action in a case like 

 the present is due to the direct induction of the helix around the 

 magnet on the coil F. I think that this is not correct ; for when 



* Faraday's Experimental Researches, vol. iii. series 29. 

 t Paguin's TraiU de Physique, vol. iii. p. 691. 



