where K is the constant of the galvanometer; a, b, R and x are the resistances in the bridge circuit, G is the galvanometer resistance, E is the E.M.F. of the battery. k' == Gb + aG + bR + aR + ab, k" = aRG + RGb + abR, At is the time the galvanometer is in circuit. Where Ax is small compared with x, This last equation was tested experimentally with several galvanometers, using known values for the variation Ax in the fluctuating resistance. It was found that if the At was left constant that

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... ft constant that for a large range of values of Ax the deflection varied as Ax, and if Ax was kept constant, then the deflection varied as At, the length of time that the galvanometer was in circuit. Then knowing the value of the constant C in equation (2) it should be possible to calculate Ax, where Ax is small. But if Ax is large then equation (1) should be used. However we have found it most convenient in practice to merely substitute known varying resistances in place of the fluctuating resistance and to so adjust the known resistances as to get a range of deflection covering those obtained with the fluctuating resistance. We have thus obtained the average resistance of a selenium cell for a .02 sec. interval at 0.01 sec. to 20 min. after illumination, as desired. This method works well even when Ax is .99 of the value of x.