CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY, 



HARVARD COLLEGE. 



ON THE CORRECTION FOR THE EFFECT OF THE 

 COUNTER ELECTROMOTIVE FORCE INDUCED IN A 

 MOVING COIL GALVANOMETER WHEN THE INSTRU- 

 MENT IS USED BALLISTICALLY. 



By B. Osgood Peikce. 



Presented March 14, 1906. Received April 12, 1906. 



At the present time, when on account of their many good qualities 

 ballistic galvanometers of the moving coil type are in such general 

 use, and when some of them fall into the hands of persons who are not 

 acquainted with their vagaries, it seems desirable to emphasize the 

 fact that the counter electromotive force induced in the coil as it swings 

 in the field of its own permanent magnet is often large, and that in 

 certain classes of absolute measurements the result might be in error 

 by many per cent if the effect of this electromotive force were not 

 allowed for. Of course, in comparing condenser charges, mirror, ballis- 

 tic d'Arsonval galvanometers are usually entirely satisfactory, and, 

 within wide limits, the "throws" of the coil, as measured on the 

 scale of the instrument, are nearly proportional to the quantities of 

 electricity sent impulsively through the coil. If, however, a moving 

 coil galvanometer, the resistance of which is known, be connected in 

 series with a resistance r, wound non-inductively, and an earth inductor, 

 permanently mounted at a given station, it will usually be found that 

 if /• be varied and if no account be taken of the electromotive force 

 induced in the galvanometer coil, throws ■will be obtained from a given 

 movement of the earth inductor, the relative values of which can only 

 be explained by assigning to the resistance of the galvanometer a value 

 much greater than the correct one. The amount of this difference 

 between the apparent resistance of the galvanometer and its real re- 

 sistance depends, it is clear, upon many things, such as the whole 

 effective area of the turns of the coil, the strength of the field about 

 the coil, the stiffness of the gimp by which the coil is suspended, the 



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