AQUEOUS SOLUTIONS.—ARCHIBALD. 29 
or 
Determination of the Conductivity. 
The Kohlrausch method with the telephone and alternating 
current was used. The measuring apparatus consisted of four 
resistance coils, and a german-silver 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. It 
was calibrated by the method of Strouhal and Barus,* the 
corrections thus obtained being plotted against length on co-ordi- 
nate paper, and the correction for any point on the wire taken 
off this curve. 
The resistance coils were marked 1, 10, 100, and 1000 ohms. 
As I used only one coil (that of 1000 ohms), and as it was not 
necessary to express che conductivities in absolute measure, I 
did not need to know the relative accuracy of the coils, or the 
absolute value of the one used. 
Two electrolytic cells were used, one for solutions more con- 
centrated than 0.1 equivalent gramme-molecules per litre, the 
other for solutions more dilute. They were of the U-form, 
shown by Ostwald in his Physico-Chemical Measurements, page 
226, fie. 178. 
The electrodes were of stout platinum foil, not easily bent, 
circular in form, and about 3.5 em. in diameter. Care was 
taken tc have the electrodes always in as neariy the same posi- 
tion in the electrolytic cell as possible. No change of resistance 
could be observed for small differences in position, such as could 
be detected by the eye, and avoided. 
The induction coil was small, and had a very rapid vibrator. 
It was kept in a box stuffed with cotton wool, that the noise 
might not interfere with the determination of the sound mini- 
mum in the telephone. A Leclanché cell was found most 
convenient for working the coil. With this arrangement the 
minimum point on the bridge-wire could be determined to within 
0.8 of a division. This would allow an error of 0.12 per cent 
in the determination of the resistance at the centre of the bridge, 
* Wied. Ann., xX (1880), p. 326. 
