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, to 0.1 per cent 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. cm.), were smaller 

 than ordered, and the latter were so thin as to bo 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 in an 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. Different 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 J division. This, at the centre of the 

 bridge, meant a possible error of 0.2 per cent., and at the point 

 of the bridge farthest from the centre, used in my experiments, 

 a possible error of 0.3 per cent. 



*Wied. Ann., X, p. 326, 1880. 



