On the Determination of Freezing Points. 155 



into the circulating liquid, and cooled quickly to O5 or O7 . 

 It is then transferred to a copper cylinder lined with polished metal, 

 placed in the centre of the thermostat, an annular space of about 

 1 cm. being left between them. The thermometer whose zero is to be 

 taken is then quickly fixed in position in a spring clamp, the bulb 

 and a considerable length of the stem above the zero being immersed 

 in the water. A crystal of ice is dropped in, and the temperature 

 quickly rises to the freezing point. 



For the details of the arrangement for the illumination of the 

 divisions, and taking the readings through the mass of the liquid 

 containing the ice crystals in suspension, reference must be made to 

 the paper. 



The amount of ice formed in the liquid varies of course with the 

 undercooling. Experiments made with good mercurial thermometers 

 showed that if ice be present in sufficient quantity, the final tem- 

 perature attained by the mixture of ice and water is not influenced 

 perceptibly by variation of the temperature of the circulating liquid 

 within fairly wide limits. As, however, it is extremely doubtful 

 whether the indications of any mercurial thermometer can be relied 

 on beyond O'OOl , it seemed desirable to control this result by some 

 other means. 



A platinum thermometer and bridge were therefore designed, 

 capable of indicating with certainty a change of 0*0001, and a 

 description of the whole arrangement employed to attain this degree 

 of accuracy forms the second half of the paper. The resistances in 

 the bridge were of manganin, whose temperature coefficient is only 

 about -j 1 ^. that of the usual resistance alloys, and the plugs usually 

 employed for short circuiting the coils were replaced by copper bars 

 and mercury contacts of specially low resistance. The thermometers 

 employed were of about 10 ohms resistance, and were provided with 

 the compensating leads, devised by Mr. Callendar. The maximum 

 current which can be used in accurate measurements with these 

 thermometers is about 0'02 ampere, and therefore the galvanometer 

 employed required to be extremely sensitive. The instrument 

 selected was a low resistance astatic one with vertical needle system 

 of the type described by Weiss, and gives at the greatest sensibility 

 at which the zero is steady one scale division for 1 x 10~ 10 ampere 

 at 2500 scale divisions distance. 



With this arrangement the influence of various conditions on the 

 final temperature attained by the mixture of ice and water was 

 studied. The results were found to be in close agreement with the 

 theoretical deductions of Nernst, and it was found that with the 

 right conditions, it was quite easy to keep the temperature in the 

 freezing vessel constant, to within one or two ten-thousandths of a 

 degree for an hour at a time. 



VOL. LX. N 



