122 Mr. S. Bidwell. On the Changes produced by 



the axis of the coil, while the direction of the magnet bisected the 

 axis of the coil at right angles. The galvanometer and the coil were 

 connected in circuit with the box of resistance coils, the tangent 

 galvanometer, and the battery, the connexions being so arranged that 

 the electromagnetic actions of the magnetising coil and the galvano- 

 meter coil nrged the galvanometer needle in opposite directions. The 

 iron rod being temporarily removed from the coil, the galvanometer, 

 which had a resistance of 1400 ohms, was shunted with a few centi- 

 metres of German silver wire, and the length of the shunt was 

 adjusted by trial until, when a strong current was passing, the action 

 of the galvanometer coil upon the needle exactly balanced that of the 

 magnetising coil and the connecting wires of the circuit, so that, on 

 depressing the contact key, no movement of the needle occurred 

 except (with the strongest currents) a slight momentary kick due to 

 induction. 



The iron rod being replaced inside the coil, a strong current was 

 caused to circulate round it for two or three seconds. The rod was 

 thus sub-permanently magnetised as in the former experiments. The 

 line of light was then brought to zero of the galvanometer scale by 

 means of the controlling magnet. Great care was taken in setting 

 up the scale to place it accurately at right angles to the magnetising 

 coil, and in such a position that the perpendicular upon it from the 

 middle of the galvanometer needle met it exactly at the zero point, a 

 specially made T-square being used for the purpose. The magnetic 

 field in which the galvanometer needle hung was the resultant of 

 those due to the controlling magnet, the horizontal component of the 

 earth's force, and the sub-permanent magnetism of the experimental 

 rod. No attempt was made to determine its strength. 



Currents of gradually increasing strength were successively passed 

 through the coil, and the deflections corresponding to the temporary 

 moments of the rod were noted. When the circuit was opened, the spot 

 of light returned in most cases accurately to zero, and the permanent 

 deviations from zero never exceeded two or three divisions, equal to 

 one-fortieth of an inch. The results are given in Table III, and shown 

 graphically in the curves in fig. 3. In each separate experiment the 

 galvanometer deflections are proportional to the temporary moments ; 

 but these deflections and the ordinates of the curves are purely arbi- 

 trary, and as regards the absolute values of the moments, they give 

 no information, nor are the ordinates of one curve comparable with 

 those of another. 



The relative changes in the values of the temporary moments with 

 increasing magnetising forces are, however, clearly shown, and to 

 ascertain the nature of these was the sole object of the experiment. 

 The distance between the galvanometer magnet and the three rods 

 was respectively 25 cm., 36*5 cm., and 53 cm. 



