A. M. Mayer on measuring Electrical Conductivities. 317 
wires leading to the galvanometer and the coil placed in a plane 
at right angles to the line of the dipping needle. On quickly ro- _ 
tating the coil through 180° the needles were deflected 25°, an 
by making the rotations correspond in direction and time with 
the oscillations of the needle, I found that six rotations brought 
the deflection to over 45°. Faraday (Exp. Res. 202-213 and 
3145 et seg.) has shown that the intensities of the magneto-elec- 
tric currents induced in wires of different metals are as their 
electrical conductivities, therefore a coil of iron wire similar in 
all respects to the above copper coil will give a deflection of 
about 4° for the first rotation; but by increasing the number of 
turns of the coil to 10 or more and by using a galvanometer 
with a shorter and thicker wire coil, the angle of deflection can 
no doubt be doubled. 
mens of wire to be compared. The coils are placed on each 
180°; but if the wire of one coil offers a greater or less resist: 
ance than that of the other the needle will be deflected. By 
the time of its oscillation, and by reversing the motion of rota 
tion of the coils so as to correspond to the swing of the needle ; 
thus after several reversals a motion is given to the needle 
doubt whether this method will be pean’ adopted ; but after 
the conception of the idea it appeared worth investigating; this 
