512 MB. J. E. PETAVEL ON THE HEAT DISSIPATED BY 



meter. By the switch, S, one terminal of the galvanometer can be thrown over from 

 the zero point of the bar to the sliding contact, E, which has been previously set at 

 any desired division. 



If n be the number of divisions at which the sliding contact is set, n X 2 X '00000773 

 is the change in resistance of the bolometer film,* which would produce an equal 

 galvanometer deflection. Where the total radiation received is large, a zero method 

 can be used, and the change in resistance measured directly by the position of the 

 slider, E, on the bar, B. 



As regards sensitiveness, the deflection obtained with a candle at 1 metre from 

 the instrument was 360 millims. The total electromotive force on the terminals of 

 the bolometer was 2 volts, the time period of the galvanometer 10 seconds, and 

 distance of the scale 1 '3 metres. Each film before being mounted was tested at a 

 pressure of 30 volts, so that any electromotive force up to 60 volts could be used 

 without danger, but no advantage would be gained by increasing the pressure above 

 10 or 15 volts. The main object in view has not been to obtain excessive sensitive- 

 ness, but to construct an instrument which would give thoroughly reliable results. 

 In almost all cases a much smaller degree of sensitiveness than that given above has 

 been found amply sufficient. 



The law of thermal radiation may be studied by two entirely distinct methods. 

 We can either measure the heat lost by the radiating surface when in a nearly perfect 

 vacuum, or the heat received by the sensitive surface of some type of radiometer. 



The determinations by the first method will exceed the true value by some quantity, 

 w.R, representing the heat carried away by any gas or vapour remaining in the 

 enclosure. 



The numbers obtained by the second method will fall below the actual value of the 

 radiation by some quantity n.R, where n.R represents the part of the radiation not 

 absorbed by the irradiated surface. 



If R be, at any given temperature, the true value of the radiation, and O, and O 2 

 the observed values obtained by the two methods described above, we have : 



I. O, = c t (R + m.R), 

 II. 2 = c 2 (R - nB), 



where c, and c. 2 are constants depending merely on the experimental conditions and 

 on system of units chosen. 



In Equation I., m represents the ratio between the heat dissipated by convection 

 and conduction and the heat radiated. Now it is well known that this ratio decreases 

 as the temperature increases. The slope of the curve of radiation obtained by this 

 method will therefore always be somewhat less steep than the true curve of radiation. 



* The change in the resistance of the bolometer film produced by the radiation being thus known, it 

 is an easy matter to express the heat received in gramme- degrees. In the present case, however, as we 

 are only dealing with relative values, little advantage would be gained by the use of absolute units. 



