MEASUREMENT OF INSTANTANEOUS ELECTROMOTIVE FORCES. 545 



been such that the position of equilibrium of the needle had made 

 an angle of 12, and even of 26, with the magnetic meridian in 

 certain experiments. 



Anomalies of the same kind, but much smaller, have shown 

 themselves in the experiments of 1881. Kohlrausch had pointed 

 out, as a possible source of error, the currents induced in the metal 

 frame of the apparatus. To meet this objection the various parts 

 have been cut and insulated from each other, and connection could 

 be again restored. Experiment has shown that the effect of the 

 currents could almost be neglected, but it appeared to diminish 

 the deflection. 



In the expression for the resistance (9) 



GSco G 2 M o> 



- + Lwtana- 



, 

 2 tan a 2 H sin a R 



the principal term comprises, besides the observed deflection and 

 the angular velocity, the product SG which is given by the dimen- 

 sions of the frame. The second term requires that we know "the 

 ratio of the magnetic moment of the needle to the terrestrial field ; 

 this ratio is small, and may be readily determined with any desired 

 approximation. The two other terms contain the coefficient of self- 

 induction of the frame. We could eliminate this coefficient by two 

 experiments made with different velocities, but it is better to cal- 

 culate it directly or determine it by comparison with a coefficient of 

 mutual induction (1098). In consequence of an error in calculating 

 the coefficient of self-induction, the value of the ohm adopted by the 

 Committee of the British Association is somewhat different from the 

 true value. 



H. F. Weber * has used the same method by making the frame 

 rotate about a horizontal axis in the magnetic meridian. 



1124. MEASUREMENT OF INSTANTANEOUS ELECTROMOTIVE 

 FORCES. This method has been used by Professor G. C. Fosterf 

 in some trial experiments with a rotating coil, which also served 

 as tangent galvanometer. The current I, the velocity of rotation 

 or the resistance R between the points of contact, were adjusted so 

 that there was equilibrium when the electromotive force is at its 

 maximum that is to say, when the frame passes the meridian. A 

 current of the same value passing this in the frame gave a deviation 8. 



* H. F. WEBER. Der absolute Werth der S. Q. U. Zurich, 1884. 

 f CAREY-FOSTER. British Association Report for i88i> p. 2. 

 VOL. II. NN 



