viii THERMO-ELECTRIC CRYOSCOPY 165 



vantageous in maintaining the resistance of the galvano- 

 meter itself constant. 



The galvanometer employed was one of the Ayrton- 

 Mather pattern, manufactured by the Cambridge Scientific 

 Instrument Company. Its resistance was 20*7 ohms. The 

 deflection of the spot of light for one micro-volt, when 

 the screen was 1 metre distant from the mirror, was 10 mm., 

 and for one micro-ampere 206 mm. A translucent screen 

 was used to receive the spot of light from the galvano- 

 meter-mirror, which was illuminated with a Nernst-lamp 

 (Fig. 28, L). 



Where one observer is using the apparatus, it is 

 convenient to have the galvanometer leads so long that 

 the petroleum key may be placed close to the screen, while 

 the freezing-bath and thermocouple, etc. (F), may stand 

 at a level 50 cm. below the screen and somewhat nearer the 

 observer. This disposition brings the key (K), the supports 

 of the thermocouple, and the stirrer of the freezing-bath 

 close to the observer, and he is in a convenient position 

 for reading the position of the image of the cross-wire. 



The apparatus should be set up, and the thermostat 

 and galvanometer adjusted, on the day before an observa- 

 tion is made. Once set up, no readjustment should be 

 necessary. 



The freezing-bath is contained in the large cylindrical 

 glass vessel, shown in Fig. 25, H, with thick walls. To 

 prepare the bath the vessel is about a quarter rilled with 

 salt solution, and then finely divided ice is added till the 

 vessel is filled up to within about 3 cm. of the brim. A 

 stout brass wire stirrer of the usual form is used to mix 

 the brine and ice. Salt is added till the desired tempera- 

 ture is attained. This should be about T5 below the 

 freezing-point of the solution to be examined. If the 

 proportion of ice to the liquid is large, this temperature 

 may be maintained constant by occasionally adding a 

 little salt. A brass lid is fitted to the freezing-vessel, and 



