516 



MR. J. E. PETAVEL ON THE HEAT DISSIPATED BY 



ness to Professors DEWAR and FLEMING, and to Dr. A. SCOTT, who throughout this 

 work have most kindly helped me with their advice on the more difficult theoretical 

 points, and with their long experience of all the details of experimental work. 



NOTE ADDED HTH SEPTEMBER, 1898. 



It has been suggested that the standard resistance referred to on p. 502, owing to 

 the large currents which were used, might in reality rise to a temperature much in 

 excess of that indicated by the thermometer, and thus by the consequent change in 

 resistance materially affect the accuracy of the observations. The following experi- 

 ment was carried out to ascertain to what extent this actually took place. 



A platinum resistance was constructed similar to the standard manganin resistance 

 referred to above. This new resistance was calibrated as a platinum thermometer. 

 It was then placed in the same oil bath as used for the manganin standard. An 

 electric current of increasing intensity was passed through it, and after each succes- 

 sive increase of current the temperature of the oil was read on an ordinary mercury 

 thermometer, and the temperature of the platinum strip was calculated from^ its 

 resistance. The stirrer was, of course, kept in motion during the entire experiment. 



The results, reduced so as to apply to the manganin standard, are given in the 

 following table : 



Current through the standard. 



amperes. 

 18-6 

 39-6 

 57-0 

 74-3 

 102-0 



Probable rise of the manganin strip 



above the temperature indicated 



by the mercury thermometer. 



0-3 

 1-4 

 3-1 

 4-9 

 9-3 



This standard resistance was rarely used for currents above 50 amperes, and never 

 for currents above 70 amperes. For larger currents it was replaced by a '001 ohm 

 resistance. However, taking an extreme case, and supposing 74 '3 amperes to be 

 passing (the temperature coefficient being '0003 per cent.), it is clear that the rise of 

 temperature would not involve an error of more than '0015 per cent. 



