1871.] Dr. C. W. Siemens on Electrical Resistance* 443 



Jprin7 } 1871, 



General Sir EDWARD SABINE, K.C.B., President, in the Chair, 



The Bakerian Lecture was delivered by Charles William 

 Siemens, F.R.S., D.C.L., " On the Increase of Electrical Re- 

 sistance in Conductors with rise of Temperature, and its appli- 

 cation to the Measure of Ordinary and Furnace Temperatures - } 

 also on a simple Method of measuring Electrical Resistances." 

 The following is an Abstract. 



The first part of this Paper treats of the question of the ratio of increase 

 of resistance in metallic conductors with increase of temperature. 



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

 sen 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 instruments. 



Several series of observations are given on different metals, including 

 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 uni- 

 form progression, or with Dr. Matthiessen'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 ex- 

 pansion and an ultimate 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 temperature 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 ex- 

 perimental 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 advantage of 

 the circumstance that the electrical resistance of a metallic conductor in- 

 creases 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. 



