ON THE SPECIFIC RESISTANCE OF MERCURY. 



been passed once through the still at the University Chemical Laboratory, and then 

 treated with nitric acid. It was clear, from the appearance of the mercury, that it 

 was impure ; but it was thought of interest to determine a value for mercury in the 

 purifying of which no special trouble had been taken. The impurity shows itself at 

 once in the results, for, while the mean value of R for VI. is '99996, for this filling 

 R = -99989; while for VIII. the resistance of the filling is '99990 B.A. unit, against a 

 mean of 1 '00006. The corresponding values for r are '95348 and '95329. 



This mercury was then treated with nitric acid, &c., redistilled in our own still, and 

 used again in the fillings VI. 8 and VIII. 7 ; the values of R were '99996 and 

 1-00004, and of r '95360 and '95361 respectively. 



Thus, the impurity has been clearly removed by the distillation and acid treatment. 



In some cases a tube was filled on one afternoon, and its resistance determined. The 

 whole was then allowed to stand over night, being re-pucked in ice in the morning, 

 and the resistance again measured, but no appreciable change was noted. Thus, for 

 VI., on January 9, the value '99994 was found, while the same filling, re-packed on 

 January 10, after the mercury had stood for 16 or 18 hours in contact with the 

 platinum of the terminals, gave R = '99996 ; the difference is within the temperature 

 errors of the coils. 



Some experiments were made on the effect of known impurities on the mercury in 

 altering its resistance. In one case, about one two-thousandth part of zinc filings was 

 added to the mercury. On mixing, the surface of the mercury was made foul ; the 

 mercury was then passed through a filter paper and used in VL , but the effect on the 

 resistance was not appreciable. It is probable, of course, that the filtering had 

 removed a large portion of the zinc, but the experiment gives some idea of the 

 amount of impurity which the resistance measures will detect. 



In another filling of VL, a mixture of mercury with a small percentage of tin was 

 used. The resistance was much too small to be measured on the bridge the bridge 

 wire has a resistance of '05 B.A. unit approximately. This mercury was treated in 

 the usual way, distilled in the Laboratory still, and then in our own, and on being 

 again used gave as the value for R '99991 B.A. unit. Thus our treatment was 

 sufficient to remove the tin from the mercury. 



Some observations were also made on the change of resistance of mercury with 

 temperature. 



Thus, on January 5, tube II. was placed in a trough in water at about the tempera- 

 ture of the room, and its resistance measured. The tube was then packed in ice and 

 measured. Similar observations were made with tube III., and the results are given 

 in the Table IX. Other observations confirmed the results there given. 



3 B 2 



