126 Mr. Willoughby Smith [June 6, 



the deflection is reduced to about fifteen divisions. The contact will 

 now be removed to the opposite side, or bottom position of the disc, 

 so that the current will be taken at the same distance from the poles, 

 but after that part of the disc has just passed between them; on 

 completing the circuit you see the deflection has increased to 33 

 divisions. Let us note these results as follows : — 



Copper. 



We will now replace the copper disc with one of iron, and repeat 

 precisely the same experiment. We find that the current from 

 position A is so great that it is necessary to insert a resistance of 

 500 Ohms in the circuit to bring the beam of light on to the galvano- 

 meter scale : as before shown, by means of inserted resistance we can 

 obtain any desired deflection. It is now about 98, and that we will 

 take as the right measure at A. We take the top position, or B, and 

 you observe that the deflection is only 7 divisions ; the connection is 

 now placed at position C, or bottom part, and we get but very little 

 more current as the deflection is only increased by one division. The 

 r esults with iron are therefore as follows : — 



Iron, 

 500 Ohms ill circuit. 



8 



If we compare the results of these experiments with those obtained 

 with the same metals in volta-clcctric induction, we see that iron 

 remains fairly uniform in its results in both cases, but that copper 

 behaves dilferently. You must not, please, take the figures given as 



