28 L. B. Fletcher — A Determination of the B. A. Unit 



ter. Then the circuit was broken and the calorimeter allowed 

 to cool for two or three hours, during which time groups 

 of readings were taken as before, the stirrer being kept 

 in motion. While this radiation experiment was in progress 

 another determination of H was made. Finally the thermom- 

 eter was removed and the calorimeter taken out and weighed. 



The mean of each group of thermometer readings, corrected 

 for stem error, gives very exactly the temperature of the ther- 

 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 d and d'. 



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 thermome- 

 ter 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 ther- 

 mometer after breaking the circuit is due to these differences 

 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 0°*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 millimeters, of 

 which about 12 equal 1° 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 0°\L C. one degree occupying about a centi- 

 meter. It had been compared with standard thermometers, 

 its errors plotted and the error for each degree obtained from 

 the curve. The following tables give the reduction to the 

 absolute scale. 



The table for 6165 is condensed from Professor Eowland's 

 paper* on the mechanical equivalent. Change in the zero 

 point has no effect on the differences of temperature used, but 

 the zero points were determined occasionally in order to get 

 the mean absolute temperature. 



The correction for radiation was made in the following man- 

 ner: The groups of thermometer readings taken after break- 

 ing the circuit were reduced to mean readings at mean times. 

 Any two of these mean readings gave the radiation for the 

 * Proceedings of the American Academy of Arts and Sciences, 1880. 



