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



The mean values of c for the different experiments vary 

 between o, 0035 and 0°-0046, the general mean being o, 0040. 



The mean radiation correction is about 5 per cent, and is the 

 most important source of variable error in the experiment, as 

 the temperature differences are small and errors of reading 

 have a large effect. But a ten per cent error in the radiation 

 would only involve an error of 1 part in 200 in J, and as the 

 errors are irregular they are in a great measure eliminated 

 from the final result. 



The calorimeter was composed of 246 gr. copper, 45 gr. brass, 

 and 6 gr. solder. The specific heat of a mixture of these pro- 

 portions was measured with Regnault's apparatus. Six deter- 

 minations gave the value , 0899db'0005 for the mean specific 

 heat between 24° and 100°. Reduced by Bede's law for cop- 

 per to the mean temperature of my experiments, it becomes 

 •0877. The capacities of the coil and glass rods were calcu- 

 lated from published tables. The whole capacity is as follows : 



Calorimeter 302-1 X '0877=26-49 



Coil " 32-5 X -0324= 1-05 



Glass rods 9-0X -177=1-59 



Thermometer estimated at 1*25 



Total capacity 30*4 



The values of the deflections were obtained by a graphical 

 method. The galvanometer readings fell into 4 groups, lying 

 about 26° and 3° 45' on each, side of the zero point. The 

 readings of each group were plotted separately as functions of 

 the time. From each curve the theoretical mean readings for 

 each interval between two temperatures used in the calculation 

 of J were obtained by measuring a large number of equidistant 

 ordinates, calculating the area of the curve and dividing by 

 the base line. Tf a, b, c and d are the mean readings thus ob- 

 tained, 2d=a—b and 2d'=c—d as the galvanometer was grad- 

 uated from 0° to 360°. Thus the zero point was not used, 

 though observed before and after each experiment. 



Below are the results of one experiment in detail. Each 

 value of J is calculated from the two temperatures found in the 

 same horizontal line. 



Series of December 9th. gr 



Weight of calorimeter and water before experiment.. 1157-2 

 Weight of calorimeter and water after experiment — 1157-0 



1157.1 



Weight of calorimeter - - 343-7 



813.4 



Capacity of calorimeter — 30-4 



843-8 

 Capacity reduced to weight in vacuo 844*8 



