E. Merritt — Light from Incandescent Lamps. 171 



thermometer was inserted through the lid of the calorimeter, 

 (the flow being stopped) so that the bulb was just covered by 

 the liquid. The water being about 4° warmer than the air, its 

 gradual cooling was shown by the thermometer. Since the 

 sides and bottom cooled more rapidly than the top, there were 

 no convection currents, and the fall in temperature shown by 

 the thermometer was due to the radiation from a layer of 

 water at the top whose depth was equal to the length of the 

 thermometer bulb. The greater part of this radiation must 

 have been from the upper surface. Some heat escaped from 

 the sides of the layer, but, since the sum 11, + R 2 was known, 

 this could be allowed for, and H, was determined. The radia- 

 tion from the cylindrical surface of the calorimeter during an 

 actual test, depended also on the mean surface temperature. 

 After each experiment, therefore, the surface temperature was 

 measured at seven different points by means of a thermometer 

 let down into the calorimeter and placed as close to the sides 

 as possible. A curve was then plotted showing the distribu- 

 tion of surface temperature. The mean ordinate of this curve 

 gave the mean temperature of the cylindrical surface. The 

 temperature of the bottom was taken as equal to that of the 

 water entering, and the temperature of the top as equal to that 

 of the water leaving. 



To test the accuracy of the whole method we made several 

 experiments with the lamp surrounded by a copper case, which 

 allowed the water to circulate freely and yet shut off all the 

 light. The heat given to the water should in this case be ex- 

 actly equal to the electrical energy consumed. The mean of 

 two determinations showed the agreement between the heat 

 energy and electrical energy to be within 025 per cent. 



In computing the results we used Lord Rayleigh's determin- 

 ation of the ohm, viz : 



1 B A unit = 0-98677 true ohms 

 and Rowland's value of the mechanical equivalent of heat : 

 1 calorie = 4-179 10 7 ergs, at 20°. 



The correction for the absorption of light by the calorimeter 

 was determined photometrically. Curves were found connect- 

 ing candle power and potential when the lamp was in the cal- 

 orimeter and when outside. From these the absorption was 

 calculated. It was found to vary from 25 per cent to 30 per 

 cent. 



Another needed correction was due to the fact that water 

 does not absorb quite all of the dark heat from a highly heated 

 source. It seems, as Tyndall first pointed out,* that the pene- 

 trating power of dark heat increases as the temperature of the 



* " Contributions to Molecular Physics." Tyndall, p. 264. 



