The Clinical Application of Heat 



67 



FIG. 1. The complete thermoradiometer, consisting of receiver unit (to be placed 

 in the position of the skin area to be irradiated) and millivoltmeter calibrated in 

 pyrons (gram-calories per minute passing through a square centimeter). In addition 

 to the current leads, tubes for water-cooling are attached. 



is likely to be very nearly the same, so that the losses of heat 

 by air conduction are practically the same for the two discs. 

 Since the quantity which is actually measured in this arrange- 

 ment is the fairly small temperature difference between the two 

 discs, all temperature effects due to air conduction are cancelled. 

 The lower, or reference, disc is screened from stray radiation 

 which would falsify the readings, but a small air gap is left 

 between the screen and the main part of the instrument through 

 which a slow convection of air takes place. Without this air 

 gap, layers of hot air might be trapped in the concavity of the 

 water jacket and would falsify the readings. We find that such 

 an instrument registers a final reading in 30-40 seconds, and that 

 it is accurate, certainly within 5%. It should be pointed out 



FIG. 2. Close view of the receiver unit. The circular plate in the center is the 

 actual receiver plate which, like a similar plate facing the water-cooled background, 

 is suspended on the screening block. 



