290 Mr. H. F. W. Burstall on the Measurement 



outside and -J inch thick. This tube could be rotated in 

 bearings, and thus the temperature could be kept steady 

 round the circumference. Inside the cast-iron tube, and in- 

 dependently supported, was a thick copper tube in which 

 were placed the two thermometers for comparison, the fine 

 wire of the steel-tube thermometer nearly touching the 

 porcelain tube of the standard. Owing to the great mass of 

 the metal, the temperature in the inner tube changed very 

 slowly, and by regulating the draught a temperature of 500° 

 to 600° C. could easily be obtained. Although this was much 

 better than the gas-furnace, as regards an equal temperature 

 circumferentially, still it could not be assumed that the whole 

 length of the coil of the standard thermometer was at the 

 same temperature as the fine wire on the steel-tube thermo- 

 meter. During any comparison the standard did not show a 

 variation of more than 2° C. Unfortunately the fine wire 

 having such a small thermal capacity as compared with the 

 standard, may not be at quite the same temperature as the 

 standard if there is even the slightest change in the tempera- 

 ture. For these reasons the values of 8 as determined by this 

 method are not so satisfactory as might be wished. 



In the second method the resistance of the steel-tube ther- 

 mometer was determined in sulphur vapour as well as in ice 

 and steam. The sulphur was boiled in an apparatus similar 

 to the first form used by Callendar and Griffiths in their 

 determination of the boiling-point of sulphur *. This con- 

 sisted of a boiler and condenser made of iron tubes. While 

 this method gives very accurate and satisfactory results, it 

 requires extreme care and takes several hours to obtain a 

 steady temperature. 



Results of the Experiments. 



It was not until the summer of 1894 that I was able to 

 make any proper experiments with the wires in the cylinder 

 of the gas-engine. I directed my attention at first to the 

 determination of the best speed for the engine, and to find 

 what conditions were necessary to obtain regularity and to 

 ensure that the measuring-wire should follow the temperature 

 of the expanding gases. I found that to obtain regular and 

 perfect combustion of the gases, the water-jacket of the 

 cylinder had to be kept at a temperature of from 50° to 60° 

 C, otherwise abnormal explosions were frequent. The speed 

 of the engine was kept constant at about 120 revolutions per 

 minute, and the wire was found to follow the cycle at this 

 speed except at the beginning of the stroke. The number of 

 explosions per minute was from 12 to 14, each explosion being 



* Phil. Trans, A. 1891. 



