[ 150 ] 

 XVIII, Proceedings of Learned Societies P t 



ROYAL SOCIETY. 



[Continued from p. 76.] 



April 27, 1871. —General Sir Edward Sabine, K.C.B., President, 

 in the Chair. 

 rpHE following communications were read : — - 

 * " On the Increase of Electrical Resistance in Conductors with 

 rise of Temperature, and its application to the Measure of Ordinary 

 and Furnace Temperatures ; also on a simple Method of measuring 

 Electrical Resistances/'— The Bakerian Lecture. By Charles Wil- 

 liam Siemens, F.R.S., D.C.L. 



The first part of this Paper treats of the question of the ratio of in- 

 crease of resistance in metallic conductors with increase of temperature. 



The investigations of Arndtson, Dr. Werner Siemens, and Dr, 

 Matthiessen are limited to the range of temperatures ^ between the 

 freezing- and boiling-points of water, and do not comprise platinum, 

 which is the most valuable metal for constructing pyrometric instru- 

 ments. 



Several series of observations are given on different metals, in- 

 cluding platinum, copper, and iron, ranging from the freezing-point 

 to 350° Cent., another set of experiments being also given, extending 

 the observations to 1000° Cent. These results are planned on a 

 diagram, showing a^ratio of increase which does not agree either with 

 the former assumption of a uniform progression, or with Dr. Mat- 

 thiessen's formula, except between the narrow limits of his actual 

 observations, but which conforms itself to a parabolic ratio, modified 

 by two other coefficients, representing linear expansion and an ulti- 

 mate minimum resistance. 



In assuming a dynamical law according to which the electrical 

 resistance of a conductor increases according to the velocity with 

 which the atoms are moved by heat, a. parabolic ratio of increase of 

 resistance with increase of temperature follows ; and in adding to 

 this the coefficients just mentioned, the resistance r for any tempera- 

 ture is expressed by the general formula 



r=aT*+/3T+y, 



which is found to agree very closely both with the experimental data 

 at low temperatures supplied by Dr, Matthiessen, and with the 

 author's experimental results, ranging up to 1000° Cent. He admits, 

 however, that further researches will be necessary to prove the limits 

 of the applicability of the law of increase expressed by this formula 

 to conductors generally, especially when nearing their fusing-point. 



In the second part of this Paper it is shown that, in taking advan- 

 tage of the circumstance that the electrical resistance of a metallic 

 conductor increases with an increase of temperature, an instrument 

 may be devised for . measuring with great accuracy the temperature 

 at distant or inaccessible places, including the interior of furnaces, 

 where metallurgical or other smelting-operations are carried on. 



