﻿306 Dr. S. R. Milner on the Use of the Secohmmeter 



for the unreversed currents through the galvanometer branch 

 during the contact and break periods of the battery respec- 

 tively. When p=r=k, which is in effect equation (1), 

 Gi and C 2 are zero. When r = k only, the exponential teims 

 vanish, and the current becomes 



Ci=(p-»-)J, C 2 =0. 



This is the same as it would be if the capacities were absent 

 and there were the resistances only in circuit; a result uhich 

 might have been inferred without calculation, from the fact 

 that when r—k the operator D cancels out from both sides 

 of the differential equation (5) for the current. 



Curve I. (fig. 4) shows the calculated values of d and C 2 

 with the following values of the variables : — 



E, 2-05 volt; F, -52 O; P, 5000; Q, 4000; R, 5000 ; 

 S, 7650 co; H, I inf.; K, '4314 mf.; t, fa sec. 



The x 's are experimental points taken as follows. By- 

 releasing two opposite brushes of the short-circuiting com- 

 mutator Y from contact with the brass pieces, an instantaneous 

 contact becomes made between the two remaining brushes 

 twice in each revolution of the axle, that is once in each 

 complete alternation of the battery current when the battery 

 is being commutated by X. The phase of the alternation 

 at which this contact is made may be given any value 

 by suitably adjusting the lead of X. The instantaneous 

 P.D. between any two points of the circuit due to the 

 battery current may therefore be measured direct by con- 

 necting through this contact a potentiometer on to the 

 points. The method is rather interesting as it forms a very 

 simple way of illustrating the distortion produced in the 

 current by any arrangement of condensers, &c. for the simple 

 type of alternating E.M.F. given by the secohmmeter. The 

 contact is, however, only approximately instantaneous, it 

 really extends over ^- of the complete period ; the experi- 

 mental results consequently do not show the sudden changes 

 in the value of the current, but they agree well with the 

 calculated mean values during the interval of contact. 



Effect of Galvanometer Reversals. — We see from the curves 

 of fig. 2 that, if 8 is the lead of the battery commutator X, 

 the effect of commutator Y is to reverse the ordinates of the 

 curves for C after the instant t=8r, and by short-circuiting 

 to make them zero from the instant t = (8 — ^)r up to t=8r. 



