56 DRS. J. A. HARKER AND P. CHAPPUIS ON A 



before the coils were wound, and after the winding the wire itself was also well 

 varnished to improve the insulation and to protect it from oxidation during the 

 annealing. 



We understand that the standard manganine coils issued by the Reichsanstalt arc 

 not tested till a year after their construction, but that after this lapse of time tin- 

 gradual changes they exhibit are very small. 



In our case we were however obliged to begin work with the resistance-box before 

 the coils had been properly aged, and therefore were not surprised at alterations in 

 their values, particularly during the first few months. 



We regret that at the commencement of our work we had not at our disposal the 

 means of comparing the coils with an invariable standard, but could only obtain their 

 relative values in terms of the mean bridge-wire unit, which was even more likely t<> 

 change slightly than the coils themselves ; on this account we are unable to give 

 details of the progressive alterations in their absolute values, and can only indicate 

 the means we adopted to prevent these changes influencing the accuracy of our 

 temperature-measurements. The changes were, as was to be expected, most appre- 

 ciable at the beginning of the work. The first standardization was made as soon as 

 the apparatus was got into order and fitted up at Breteuil, and immediately following 

 this came the comparisons of K. 8 with the mercury thermometers. As soon as tliis 

 series of comparisons was completed a second standardization was at once made. The 

 individual observations of the thermometric fixed points and comparisons were then 

 reduced with both the earlier and new coil-values. At a later stage it was found 

 that, although the absolute value of the mean bridge-wire unit had slightly altered, 

 yet the values of the box coils relatively to one another, with the exception of one of 

 the very low resistances constructed of strip manganine, had not changed by ;m 

 amount large enough to make the two determinations differ materially. It was easy 

 to allow for such small variations as did occur by taking into consideration the date of 

 each experiment and assuming that the change between the two standardizations was 

 proportional to the time which had elapsed since the first. 



Not counting a preliminary series of observations, four complete standardizations 

 were made in all during the course of the work with the thermometers, and we think 

 that no serious errors were introduced into the results by the alterations in the 

 relative values of the resistance-coils. 



Further particulars as to the changes in the values of the diflerent coils are given 

 on p. 5 8 after the description of the method adopted for the standardization. 



For the comparison of the coil-values with one another the following plan \\.-is 

 adopted. Firstly, the values of the smaller coils M and N were determined directly 

 in terms of the bridge- wire by the same process as was employed in the calibration. 

 Next, each higher coil in turn was balanced against the adjustable manganine 

 resistance previously described, which in each case was so adjusted that the position 

 of the contact-maker on the bridge-wire was in the neighbourhood of the zero of the 





