SOMK MERCURY STANDARDS OF RESISTANCE, ETC. 89 



No measurements for standards purposes were made until the tube had been 

 immersed in the ice shavings for at least 6 hours. Very intimate contact between 

 the ice and glass was lacking owing to the melting of the ice in the vicinity of the 

 tulie and consequent formation of an ice wall some small distance away. The 

 mercury, in consequence, attained the temperature of the ice but slowly. The cooling 

 may \te hastened by the application of pressure so as to produce better contact, and this 

 was often done in the early stages of the immersion. The cooling curve shown in 

 fig. 1 1 applies, however, to a normal case of cooling, the ice being undisturbed after 

 the first 15 minutes. For the production of the curve, resistance measurements 

 were made at equal intervals of time, and the temperature deduced from these 

 observations. The first measurement of resistance was made 25 minutes after the 

 tube had been immersed. 



As an example of a final measurement at 0C., the data for the standard X are 

 given : 



June 18th, 1903. 



Observations of Resistance. Mercury Sum Ian 1 X. 

 No. of observation, 1. 

 Insulation resistance = 20 megohms. 



(.<) Evaluation of QR/S. 



C. 

 Q = Wolff Coil No. 780, temperature - 17-2 



R= 740, . 17-2 



S - 2449, = 17-2 



P= 1690, =17-2 



Balance wjvs obtained when W = 9400 ohms. 

 The value of P at 17 2 C. = 1 -000112 ohms. 

 Hence 



QR/S = 1-000112(1 + 1/9400) - 1-000218 ohms. 



(b) Evaluation of Standard X. 



Standard X substituted for 1690. 



a = 100-013 ohms. 



/3 = 100-015 



Balance was obtained when W = 7480 ohms. 



Thus, since the total value of the corrections due to the leads has been shown to be small, the 

 approfinmte value of X equals 



1-000218/(1 + 1/7480) = 1 -000084 ohms. 



() Evaluation of Leads. 



The luttrry connection to the mercury standard was now broken, and the lead introduced at the 

 junction of R and P r . With these conditions holding, 



QR/S = 1-0002 18 ohms. 

 P + P r = Standard X + Potential point P^ 



Balance was obtained when W = 55070 ohms. 

 And since 



p p + ... 



Pr ''-*O- 



VOL. OCIV. A. N 



