Comparison of Platinum Thermometers. 431 



It will be seen that for the first and third wires the results 

 are in agreement, almost within the limits of error of observa- 

 tion by direct comparison. In the second wire the agree- 

 ment is. not so complete, which is not at all remarkable when 

 it is observed that the temperature-coefficient of this wire is 

 33 per cent, less than that of the standard. The fourth wire 

 (the most impure) does not agree at all when reduced by this 

 method*. 



9. Effect of Annealing. — As before stated, considerable 

 changes of zero were found in some of the wires on first 

 heating them to 1000° for a couple of hours, in one instance 

 a rise of no less than 2°. In this extreme case, there seems 

 to be no reason to doubt that the mica used was largely the 

 cause, as the wire after prolonged use became slightly dis- 

 coloured. A specimen of the mica used was tested chemically 

 and found to be slightly acid. The standard wire, which was 

 chemically very pure, was also slightly affected in this 

 pyrometer; and hence the observations were disregarded. 

 Afterwards some white mica was obtained, from the Instru- 

 ment Co., Cambridge, and when proper precautions were 

 taken in winding and annealing;, the standard wire never 

 showed changes of zero greater than *2 or of F.I. 

 greater than 0°-03, after heating to 1000° C. Most of the 

 impure wires used, however, showed changes somewhat 

 greater than this. In the case of the wire «?=1670, a fall of 

 1°*2 was observed in the zero on first annealing at 1000 J 

 for one hour. This was doubtless due to the fact that, in 

 winding the coil, the wire, as bent on the frame, was in a state 

 of unequal tension, and when heated to 1000° this was relieved, 

 with a consequent fall of zero on cooling. For accurate 

 work, too great care cannot be exercised in winding the 

 pyrometer-coil. A fall of zero may be easily produced as 

 above, or, if the wire be wound too tight originally, a thicken- 

 ing of the mica may cause a rise of zero, as suggested by 

 Heycock and Neville. After the coil is wound, the pyrometer 

 should be annealed for some time at a temperature beyond 

 that for which it is to be used, and allowed to cool slowly 



* Footnote added by Prof. Callendar. — In order to illustrate the nature 

 of the agreement of the observations with the difference-formula and with 

 Heycock and Neville's method, I have inserted in fig. 2 a series of points 

 representing the values of the differences pt'—pt for a wire d= l - 650 cal- 

 culated by the two methods, by the difference-formula, by Heycock 

 and Neville's method. It will be observed that up to 600 pt both methods 

 agree about equally well with the observations. Above 600° the observed 

 difference is much less than that calculated by the difference-formula. 

 The second method agrees very fairly up to 800°, but tends to diverge 

 from the linear formula beyond that point. 



