308 A. ML Mayer on measuring Electrical Conductivities. 
that, when the spirals are together slid off the magnet, the two 
equal electric currents, thus generated, simultaneously tend to 
traverse the ga vanometer in opposite directions, and therefore 
its needle remains stationary. If we now introduce into the 
circuit of one of the spirals a resistance equal to that introduced 
into the cireuit of the other, the needle will still remain at rest 
when the spirals are slipped off the magnet; but, if the resist- 
ance placed in one circuit is greater or less than that placed in 
the other, there will be a deflection of the galvanometer needles 
when the spirals are remov us, by introducing wires 0 
different metals into the circuits we can readily determine their 
relative conductivities, by making them of such length that 
two wires whose conductivities are to be compared. These 
coils contain equal lengths of the same sized wires and the same 
number of turns ; the direction of the turns being opposed in 
the two coils. The coils having been bound together are placed 
in a plane at right angles to the line of “the dip,” and the four 
terminal wires are so connected with the reflecting-galvanometer 
that the two induced currents tend to traverse it in opposite 
directions. The coils are now quickly rotated through 180°, 
around an axis at right angles to the line of the dip, and if the 
wires present equal resistances the needle remains at rest; if it 
is deflected, the direction and the amount of the deflection 
shows which coil has ae jae resistance and affords a means 
of estimating the sam 
r this general al deshigtan of the method I will present, 
in alee, a be wi of the apparatus used, and of the actions 
which take Re ace in it; the degree of precision of the method ; 
examples of the determinations of electrical-conductivities, and 
experiments on the modification of the method. 
2. Description of the Apparatus. 
The magnet was formed of a rg age of three steel bars, 
separated from each other by slips of wood -2 in. thick. The 
middle bar was 10:4 in. long and its ends projected ‘25 in. be- - 
sg the two side magnets. Hach bar was 27 in. thick we 9 
wide. About three months before this investigation was 
indianc they had been magnetized to saturation by the 
