314 A. M. Mayer on measuring Electrical Conductivites. 
current went through it when the spirals were together slid off 
the magnet. The action of one spiral alone was sufficient to 
deflect the galvanometer needle about 60°. This deflection was 
reduced to 1° 18’ by placing in its circuit a helix of 785 ft. of 
no. 18 copper wire; the mean of six experiments (the range of 
which was only ;; div.) giving 10-4 div. The ends of the 
back spiral were now so connected that an equal current flowed 
through it in a direction the reverse of the other. The mean 
of six deflections, produced by sliding together the spirals off 
the magnet, equalled 10-4 div., the same as in the previous ex- 
periment; thus showing that the mutual inductive action of the 
spirals had no effect on the intensity of the induced magneto- 
electric currents. 
It was also found that on passing the induced current from a 
spiral through another spiral on which rested a third spiral 
whose ends were connected with the galvanometer, that no de- 
flection ensued when the magneto-electric current was passed 
through the inducing spiral. 
However, the magneto-electric currents were of such low in- 
tensity that probably they were not able to produce an induced 
current in the second spiral capable of deflecting the needle, 
and that therefore the experiments here narrated are of little 
value; nevertheless, I think the reasoning given above will be 
supported by experiments made with more powerful magnets 
and with larger spirals. 
4. The degree of Precision of the method. 
‘The degree of precision of this special apparatus was deter- 
mined in the following manner. A copper wire 123 ins. long 
had opposed to it a resistance which was about equal to 120 
ins. of its length and the mean deflection of the Bc mae siverirend 
needles was carefully determined. The copper wire was now 
shortened 1 in. and the deflection again determined; this was 
repeated,—determining the amount of deflection produced after 
each shortening of 1 in.,—until 6 in. had been cut off. These 
experiments showed that a diminution or increase of resistance 
of ;1,; part in one of the wires caused a deflection of -4 div. of 
the scale, or of 3’ of arc, in the galvanometer-needles. But we 
have seen that “1 div. can be read on the scale, therefore, we 
can, with this special apparatus, detect and measure an increased 
or diminished resistance of ;4; part. But as the galvanometer 
can be removed to even twice the distance at which we read its 
within the variations observed in different samples of wires © 
the same lengths and diameters. 
