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



right of the centres of the break periods of X. In every 

 alternate section thus produced the ordinates of carves I. and 

 II. become reversed, while during the short-circuiting periods 

 themselves the galvanometer current is cutout altogether and 

 the ordinates of the curves become zero. Curves III. and IV. 

 show in the resistance and in the capacity case the resulting 

 curve for the galvanometer current at lead 8=1 (X and Y 

 reversals simultaneous) ; V. and VI. show the same thing at 

 a lead of \ (Y reversals midway between those of X). The 

 mechanical effect on the galvanometer is proportional to the 

 mean ordinate of the curve in each case. As the lead is 

 continuously increased the mechanical effect will pass through 

 a maximum value (approximately when the whole area of the 

 current curve, thus reversed and cut out, is above the axis), 

 diminish to zero (equal areas above and below), and increase 

 again to a maximum in the opposite direction (whole area 

 below) . We see from curves III. and V. that when resistances 

 only are in the bridge-arms, the mechanical effect has its 

 maximum values at leads S = and 8 = 1, and is zero at 

 3 = 1*. When capacities are present, however, the leads at 

 which the mean galvanometer current is a maximum and 

 zero respectively will no longer have these fixed values. 

 Nevertheless in every instance they will have definite values 

 for given values of the resistances, the capacities, and the 

 speed, or more generally for a definite relation between these 

 variables. Conversely, if the lead be given any value and 

 the existing galvanometer deflexion be reduced to zero by a 

 suitable adjustment of the variables, the result of the adjust- 

 ment is to bring about the corresponding relation. 



These considerations give a qualitative explanation of the 

 results detailed on p. 300, and, generally, of incomplete balances 

 obtained at any lead. The methods of adjusting to the com- 

 plete balance, which are described below, are based on the fact 

 that, by making suitable arrangements in the bridge, the 

 relation resulting from an incomplete balance, which is very 

 complicated in the general case, degenerates for suitable 



* This gives a simple practical means of marking out accurately a scale 

 of 8. With non-inductive resistances only in the arms (the effect of the 

 inductance of the galvanometer should be made negligible bj r inserting a 

 high resistance in series with it or with the battery) a suitable deflexion 

 of the spot is obtained. Then, if while the secohmmeter axle is kept in 

 rotation the plate to which the brushes of the commutator X are attached 

 is turned slowly round by hand, the positions of the plate which corre- 

 spond to the zero and maximum deflexions are easily ascertained. These 

 positions 8=0, \, and 1 may be marked once for all by scratching a line 

 on the plate and three corresponding ones on the brasswork of the instru- 

 ment. I have used a uniform paper-scale pasted on to the brasswork to 

 read the intermediate values of d. 



