740 



MR J. D. HAMILTON DICKSON ON 



In this set the two points on the thread were E = 397|-, £ = 250, and E = — 157|-, 

 t= -100. The result was that dE/dt =1*586 at the temperature 7|° C. 



The second set shows the determination of the point on the curve at which the 

 tangent is parallel to the axis of t : — 



Deviation. 



Upper 

 Temperature. 



Lower 



Temperature. 



Readings for 

 the Tangent. 



Temperature 

 of Contact. 



100 x tangent. 



2 mm. 



-661 



- 97 



\ 



- 8175 



\ 



4 „ 



-61 



-103 





-82 





6 „ 



-561 



-108 







-82-25 



o 



8 „ 



-53 



-111 



-82 



10 „ 



-50 



-114 





-82 





12 „ 



-97 



-116-7 



, 



-81-9 



, 



Mean temperature of contact = -81-98°. 



This second set shows how remarkably closely the several temperatures obtained 

 from the six pairs of readings agree, and justifies the accuracy with which the method 

 is credited. 



From the A-curve for platinum, by the method described, sixteen points on the 

 Tait-line were determined, the lowest being at — 173 - 2° and the highest at+ 86*8. ° The 

 equation of the parabola was then calculated by the method of least squares (such 

 calculated curves will be referred to in future as the B-curves), assuming the observa- 

 tional value of the slope of its axis to the axis of E.M.F., and the equation of the Tait- 

 line was deduced from it. The Tait-line was constructed from this equation, and the 

 sixteen observed values of dE/dt previously determined from the A-curve were plotted 

 in their respective positions, and gave a coincidence which fully justified further 

 calculations on the theory of the parabola with a slant axis. 



On this coincidence of hypothesis with observation I made similar examinations of 

 the observations on copper, gold, cadmium, zinc, all with fairly satisfactory results. 

 Meeting Lord Kelvin in the spring of 189/, I told him of these results. He was at 

 once deeply interested, and his first remark showed how completely he was acquainted 

 with all current scientific investigations, for he immediately asked if the results were 

 referred to centigrade or platinum temperatures. On hearing that platinum tempera- 

 tures had been used, he said, with vigour : " Have nothing to do with platinum 

 temperatures — get centigrade." It was natural to accept without hesitation his advice ; 

 the work accomplished was set aside, and my attention was given to all the best work 

 on platinum temperatures, for the purpose of finding the most approximate formula 

 possible for the reduction of low platinum temperatures to centigrade temperatures. 

 The result of this investigation was published in two papers in the Philosophical 

 Magazine* for December 1897 and June 1898. The latter of these contained a tahle 

 of conversion from platinum degrees to centigrade degrees for each 5 degrees from + I "0 



* Phil. May., Ser. V., Vol. xliv. p. 445 j and Vol. xlv. p. 525. 



