'122 ON THE CALCULATION OF THE CONDUCTIVITY OF 
bridge-wire, about three metres long, wound on a marble drum. 
The wire was divided into 1000 parts, and had a resistance of 
about 1.14 ohms. I calibrated it by the method of *Strouhal 
and Barus, and applied the corrections thus determined to the 
measured resistances. (The greatest correction that had to be 
applied during the experiments was one division). 
Four coils, marked 1, 10, 100, 1000 true ohms formed part of 
the apparatus, and were guaranteed correct, to0.1l percent The 
range of the resistances measured during the experiments, how- 
ever, was so small that I needed to use only one of these coils 
(that of 100 ohms). Hence it was not necessary for me to test 
the relative accuracy of the coils. Nor did I need to test the 
absolute accuracy of the 100 ohm coil, as it was not necessary 
for me to express conductivities in absolute measure. 
The cell used was a U-shaped one, with enlargements for the 
electrodes, of the kind shown in Ostwald’s Physico-chemical 
Measurements, p. 226, Fig. 178. The cell and also the electrodes 
(each of which had an area of about 7 sq. em.), were smaller 
than ordered, and the latter were so thin as to be easily bent. 
No change of resistance, however, could be noticed for small 
bendings of the plates, which could be readily detected by the 
eye and avoided. The induction coil was quite small, and had a 
specially rapid vibrator. It was kept inan adjoining room, that 
the noise might not disturb the operator ; but, after some practice 
it was found that measurements could be made without difficulty, 
even with considerable noise. Ditterent kinds of batteries for 
working the coil were tried. The most satisfactory was found 
to be a small dry battery, made by the Mamisburg Electric Co., 
of the kind used for electric bells. With this apparatus the 
“minimum ” point on the bridge could be determined by the 
telephone to within } division. This, at the centre of the 
bridge, meant a possible error of 0.2 per cent., and at the point 
of the bridge furthest from the centre, used in my experiments, 
a possible error of 0.3 per cent. 
*Wied. Ann., X, p. 326, 1880. 
