8 Dr. Fletcher on the Determination of the B.A. Unit 



mometer for the mean time of that group. The difference 

 between any two of these mean temperatures, corrected for 

 radiation, gives by multiplication into the capacity of the 

 calorimeter and contents the heat generated in the interval. 

 Hence any two groups give a determination of J when com- 

 bined with the proper values of and 6'. 



I have combined groups taken 18 to 25 minutes apart, the 

 rise of temperature being 6° to 8°. 



In this calculation the differences of temperature of coil, 

 water, and thermometer are assumed to be constant for this 

 interval. The water is cooler than the coil, and the thermo- 

 meter cooler than the water. Both differences depend upon 

 the rate of generation of heat, and may be put approximately 

 proportional to the square of the current. The rise of the 

 thermometer after breaking the circuit is due to these dif- 

 ferences, and was found to be less than o, 05. The variation 

 of this quantity during the interval in question would be 

 about 3 per cent., as the current changed 1*5 per cent. Hence 

 the variation is o, 0015; and as the rise of the thermometer is 

 6° or 8°, the error is negligible. 



Two thermometers were used, designated as Baudin 6165, 

 and Baudin 7320. The former is graduated in millimetres, of 

 which about 12 equal l c C. It had been used by Professor 

 Rowland in his determination of the mechanical equivalent, 

 and compared several times with the air-thermometer. Baudin 

 7320 is graduated to o, l C, one degree occupying about a 

 centimetre. It had been compared with standard thermo- 

 meters, its errors plotted, and the error for each degree obtained 

 from the curve. The following tables give the reduction to 

 the absolute scale : — 



Baudin 6165. 



Reading 



Temperature 

 on absolute 



Reading 



Temperature 

 on absolute 



in mm. 



scale from 0° C. 



in mm. 



scale from 0° C. 



35 



o 



00 



320 



24%47 



50 



1-368 



330 



25-365 



100 



5839 



340 



26174 



150 



10-183 



350 



26-981 



200 



14-450 



360 



27-782 



250 



18-709 



370 



28-584 



260 



19-557 



380 



29-376 



270 



20401 



390 



30170 



280 



21-242 



400 



30-965 



290 



22076 



410 



31-786 



300 



22-907 



420 



32581 



310 



23-731 







