OF THE NATIONAL PHYSICAL LABORATOKY. 



357 



within 1 part in 10,000 of its nominal value, and an inspection of the table shows 

 that the cumulative effect of individual coil errors is very small. No corrections were 

 therefore applied to any of the potentiometer readings. 



Special precautions were taken to avoid the effect of temperature variation both on 

 the accumulator furnishing the compensating current of '01 ampere and on the 

 standard cells, both being placed in double-walled boxes surrounded by a thick layer 

 of cork clippings. Under these conditions the daily temperature range in the boxes 

 was found to be reduced to about one-fifth of the value outside them, and the 

 compensating current could generally be kept to within yorWo f ^ s value for an 

 hour at a time without adjustment. 



XIII. The Thermoj unctions. 



The thermoj unctions used in this research were composed of pure platinum with 

 platinum containing 10 per cent, of rhodium, and were all "6 millim. diameter. They 

 were obtained from HERAUS of Hanau, through Dr. HOLBORN of the Reichsanstalt, 

 and were compared by him with the standard junctions of the Reichsanstalt at a 

 number of fixed points. 



In a letter to the Director of the Laboratory, Dr. HOLBORN says : " Two elements 

 were compared at four points, the melting-points of Zn, Sb, Ag, and Cu, and gave 

 the following results'" : 



Before commencing comparisons with the gas thermometer, three independent 

 determinations of the freezing-point of silver were taken in an electrically heated 

 crucible furnace with one of these junctions N.P.L. 2 ; in these experiments two 

 different observers took part, and the three results were : 



9087] 



9092 > microvolts. 



9082J 



The mean of these, 9087, agrees very closely with the datum given above. 

 Junction N.P.L. 2 was selected for the comparison, while N.P.L. 1 was reserved as a 



* In all experiments with thermojunctions here referred to, it is to be understood that the cold 

 junction is at C, 



