48 A. M. Mayer — Experimental Proof of Ohm's Law. 



drawn on the table under the vertical center line of the gal- 

 vanometer coil, by means of a long magnetic needle mounted 

 like those used on plane-tables. A line at right angles to this 

 meridian is now drawn so that its point of intersection with the 

 meridian line shall be exactly under the suspending thread of 

 the mirror. In the vertical plane of the line, drawn at right 

 angles to the meridian, is placed the vertical wire in the slit of 

 the lantern, L, and also the zero line of the scale C. The 

 scale is parallel to the magnetic meridian. The galvanometer 

 is now placed in the position given above and the " directing 

 magnet " removed to a distance. The image of the vertical 

 wire at L will now be found on the zero of the scale if there 

 is no torsion in the suspending thread. If it does not come to 

 zero then the head of the rod to which the thread is attached 

 is turned till image of wire coincides with zero of scale, and 

 then the instrument is in adjustment, and it will give deflec- 

 tions as the tangents of the strength of current, or, in other 

 words, the current strength will be directly as the readings on 

 the scale. The magnet M is now placed so that it causes no 

 movement of beam from the zero of the scale. The directing 

 magnet, above the coil, is now so adjusted that the time of an 

 oscillation of the magnets of the galvanometers is above 5 

 seconds. 



The coil, E, over the magnet is put in the circuit of G and 

 R. The wires between E and G and R are twisted and tied 

 together so that no induced current from the earth's magnet- 

 ism may be caused by the motions of this part of the circuit. 

 The image of wire is on zero of scale. Now on rapidly lifting 

 the coil from around the magnet a deflection is produced by 

 the magneto-electric current thus generated. It is sufficient to 

 know that the cause of this current is the quick lifting of the 

 ring with one coil. If we replace this by a ring of two coils 

 we get twice the deflection, and rings of 3, 4, 5, and 6 coils 

 gives 3, 4, 5, and 6 times the deflection given by the ring with 

 one coil. Adopting the conception of the lines of magnetic 

 force, we say that the ring with one coil cuts a certain number 

 of these lines, this cutting of the lines causes the current, and 

 is the electromotive force. The ring with two coils makes two 

 cuts of these same lines, or, cuts double the number of lines, 

 the rings of 3, 4, 5 and 6 coils cut 3, 4, 5 and 6 times the 

 number of lines and hence give 3, 4, 5 and 6 times the electro- 

 motive force. 



In these experiments the resistance of the circuit has re- 

 mained constant. ISTow take the ring with 5 or 6 coils and let 

 us have one ohm as resistance of circuit. On lifting ring from 

 magnet we get a certain deflection, which we may make ex- 

 actly equal to a whole number of the units of the scale by 



