C04 Prof. Richardson and Mr. Lownds on tlie Magnetic 



The secondary of the ring was joined up in series with the 

 secondary of a standard mutual inductance-coil and a 

 D'Arsonval ballistic galvanometer. 



A known standardizing current was then sent through 



the primary o£ the mutual inductance-coil and the throw of a 



spot of light (reflected from the mirror of the galvanometer 



on to a transparent scale) on reversing the current was noted. 



Then if 



M = the value in c.G.s. units of the standard mutual 



inductance ; 

 7 = the value of the standardizing current ; 

 d = the throw of the spot of light ; 

 k = some constant ; 

 we have 



My = Jed 



or 



k= 



My 



w 



The primary of the standard mutual inductance-coil was 

 then disconnected, and a current was sent through the 

 primary of the ring and an adjustable resistance. This current 

 was then varied ' by steps/ and the throw corresponding to 

 any change By in the current strength was noted. 

 Then if 



E" = the number of turns in the secondary of the ring ; 

 S = the mean sectional area of the ring ; 

 SB — the change in the induction in the ring correspond- 

 ing to a change hy in the current strength ; 

 d' — the throw of the spot of light ; 

 we have 



SB.S.JST=M' 



or 



BB = k 



d!_ 



(?) 



Combining (1) and (2) we obtain the equation 



M. 7 d' 



SB = 



d 



S.N 



The Primary Circuit. 



The ring was wound with a primary coil of 42 turns of 

 copper wire insulated with asbestos-paper. 



The primary was connected to the two mercury cups 1 & 2 

 of the rocker D (vide fig. 1). 



To the cups 3 & 4 of this rocker were connected the ter- 

 minals of the primary of the mutual inductance-coil M. 



