AQUEOUS SOLUTIONS. ARCHIBALD. 295 



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, fig. 378. 



The electrodes were of stout platinum foil, not easily bent, 

 circular in form, and about 3.5 cm. in diameter. Care was 

 taken to have the electrodes always in as nearly 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 Leclanch^ cell was found most 

 convenient for working the coil. With this arrangement the 

 minimum point on the bridge-wire could be determined to within 

 0.3 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., x (1880), p. 326. 



