ON THE MECHANICAL EQUIVALENT OF HEAT 439 



Both these sources of error tend to give too small coefficients of radia- 

 tion, and this is confirmed by looking over the final tables. But I have 

 not felt at liberty to make any corrections based on the final results, as 

 that would destroy the independence of the observations. But we are 

 able thus to get the limits of the error produced. 



During the preliminary experiments a water jacket was not used, 

 but only a tin case, whose temperature was noted by a thermometer 

 above and below. The radiation under these circumstances was larger, 

 as the case was not entirely closed at the bottom, and so permitted more 

 circulation of air. 



3. CORRECTIONS TO THERMOMETERS, ETC. 



Among the other corrections to the temperature as read off from 

 the thermometers, the correction for the stem at the temperature of 

 the air is the greatest. The ordinary formula for the correction is 

 000156n( t"). But, in applying this correction, it is difficult to 

 estimate n, the number of degrees of thermometer outside the calo- 

 rimeter and at the temperature of the air, seeing that part of the stem 

 is heated by conduction. The uncertainty vanishes as the thermometer 

 becomes longer and longer, or rather as it is more and more sensitive. 

 But even then some of the uncertainty remains. I have sought to 

 avoid this uncertainty by placing a short tube filled with water about 

 the lower part of the thermometer as it comes out of the calorimeter. 

 The temperature of this was indicated by a thermometer, by aid of 

 which also the heat lost to the water by conduction through the ther- 

 mometer stem could be computed; this, however, was very minute com- 

 pared with the whole heat generated, say 1 in 10,000. 



The water being very nearly at the temperature of the air, the stem 

 above it could be assumed to be at the temperature of the air indicated 

 by a thermometer hung within an inch or two of it. The correction for 

 stem would thus have to be divided into two parts, and calculated 

 separately. Calculated in this way, I suppose the correction is perfectly 

 certain to much less than one hundredth of a degree : the total amount 

 was seldom over one-tenth of a degree. 



Among the uncertain errors to which the measurement of tempera- 

 ture is subjected, I may mention the following: 



1. Pressure on bulb. A pressure of 60 cm. of water produced a 

 change of about 0-01 in the thermometers. When the calorimeter 

 was entirely closed there was soon some pressure generated. Hence 

 the introduction of the safety-tube, a tube of thin glass about 10 cm. 



