234 Prof. C. Niven on Determining and Comparing 



As an example, the coils of the dynamo formerly used were 

 compared with the field-magnets of an old dynamo of the 

 Ladd pattern. The former of these we shall call A, the 

 latter B ; their coefficients of self-induction L, N. 



b and d were each taken = 100 ohms. 



100 ohms with A in series were balanced by a resistance, 

 the last portion of which is a wire with sliding-contact. B was 

 then put in series with these resistances and slide, and balanced 

 by a rheostat. The new adjustments generally put the other 

 out a little, but by moving the sliding-key the proper position 

 could be found when there was no current, whether the key 

 be up or down. By making S = 25 the two transient currents 

 appeared equal, though the comparison could not be made 

 very exactly on account of the difference of duration of the 

 currents. 



Thus, m= 247±25 =lom 



I have not been able hitherto to try these methods with a 

 ballistic galvanometer, nor do I see at present any means of 

 making the two time-constants equal. 



The result obtained above agrees with an estimate of N 

 made by using a condenser, as in § 3. 



Metltod II. — By the differential galvanometer. 



? B' 



V.NAANfcNW^WvN/Awv^w 



ft UN 



The coils whose coefficients L, N are to be compared are first 

 balanced by rheostats in the two coils of the galvanometer, 

 and points P, Q are then found such that the needle remains 

 unaltered whether PQ be open or closed ; then a resistance S 

 is placed between PQ till the needle gives no deflection on 

 opening circuit. When this is the case, 

 LS-N(S + 2R) = 0. 

 The expression for the flow is 



__ (LS-N.S + 2R)E 



7 ~ (2B + R + B'+G)(SB + R' + 2BR'+GLS + 2R)" 



When it is found desirable to use shunts for the two circuits 

 of the galvanometer, Gr will represent the combined resistance 

 of each circuit and its shunt, and the total flow in the two 



