390 



Mr. F. J. Smith on the Effects of Magnetism 



induction area of the Rowland coil = 4*97 132 ; it has 42 turns, 

 and a mean circumference 167*305 centim.) The value of 

 the horizontal component of the earth field at Oxford at the 

 time of these experiments was ahout 0*179 C.G.S. units. On 

 breaking the circuit hardly any current was indicated, 1 or 

 1*5 divisions at the most. On again closing the circuit at K 

 no current was indicated, and by repeating the closing no 

 swing of the galvanometer-needle could be produced. The 

 direction of the current was next changed by means of the 

 commutator P. Then, on closing the circuit at K, a current of 

 the same value as the former was indicated, but it was now in 

 the opposite direction. On breaking or making the circuit no 

 deflexion could be got from the iron core under examination. 



So that in order to get a transient current from an iron 

 core well annealed its magnetization must be reversed. An 

 examination of cores under stress shows that the phenomenon 

 may be modified. 



15. The relationship between the direction of the magnetiz- 

 ing current in the long helix and iron core current is shown 

 in the diagram (fig. 3). 



Fig. 3. 



16. The experiment was next arranged so that the core 

 might be as long as possible, and yet wound with a moderate 

 length of copper helix. An iron core, A B, 6*3 metres long, 



Fiff. 4. 



4 mm. diam., covered with a non-conducting coat, was 

 coiled into a ring of about 15 centim. diam. It was next 



