A. M. Mayer— Experimental Proof of Ohm's Lain. 49 



sliding up or down the disc on the magnet. We now take out 

 plug of resistance box and make resistance of circuit two ohms. 

 The deflection of the galvanometer magnet now becomes one 

 half of that of previous experiment, and successively making 

 the circuit with resistances of 3, 4, 5, 6, and 7 ohms we get, 

 i> i? \i h an d t °f the deflection we got with one ohm in 

 circuit. 



When these experiments are made with the galvanometer in 

 perfect adjustment, and with the precautions indicated below, 

 the deflections arrive one after the other exactly as the law 

 requires. Thus showing with sufficient precision for a lecture 

 experiment that the current is directly as the electromotive 

 force and inversely as the resistance. Indeed generally the 

 closest scrutiny does not detect in the scale reading any de- 

 parture from the law. 



Certain precautions are, however, necessary in these experi- 

 ments. The resistance outside the galvanometer must be of 

 copper wire, for such is the wire of the galvanometer. Also, 

 the whole of the apparatus must be put together the day before 

 we make the experiments, and the room maintained at as con- 

 stant a temperature as possible, so that the temperature of all 

 parts of the apparatus is the same. The deflections should not 

 exceed 15 divisions of the scale. Thus, if we start with 15 divis- 

 ions of deflection for a resistance of one ohm we will get 7*5 ; 

 5 ; 3*75 ; 3 ; 2*5 ; and 2*143 deflections for resistances of circuit 

 of 2, 3, 4, 5, 6 and 7 ohms ; and if with a constant resistance 

 we obtain a deflection of 2 divisions of scale with a ring of one 

 coil, we will get deflections of 4, 6, 8, 10, and 12, with rings 

 having 2, 3, 4, 5 and 6 coils. 



It is necessary that the coils should be removed from the 

 magnet very quickly, otherwise the deflections will not be as 

 the law requires. In other words, the currents produced should 

 be as instantaneous as can be obtained. Instead of rapidly re- 

 moving the coils by the hand, I have sometimes lashed the coil 

 and their handles to a spring board with a hole in it which 

 went over the magnet. By a trigger this spring-board is re- 

 leased. We thus get the same velocity in lifting the coil in 

 each experiment. We have found, however, that the hand 

 of a good experimenter gives precise results. Sometimes I 

 have sent the coil from the magnet by the blow of a stick 

 delivered on the under side of the handle of the coil at its 

 center of percussion. There is no doubt some departure from 

 the law in these experiments, for it is not possible in such ex- 

 periments to obtain what is understood by instantaneous cur- 

 rents ; and the damping of the magnet by the mirror acting 

 on the air must come into play. Yet I have never seen any 



Am. Jouk. Sci.— Third Series, Vol. XL, No. 235. — July, 1890. 

 4 



