270 Dr. A. D. Denning on a Simple Method of 



mark a is much easier and more accurate with the plunger 

 than is the usual one by means of blotting-paper applied at A, 

 and in addition the horizontal arm permits of its employment 

 in the common thermostats with opaque sides, while reducing 

 to a minimum the amount of contained liquid outside the 

 bath. 



The University, Birmingham. 



XXXIY. _A Simple Method of Determining the Radiation 

 Constant: suitable for a Laboratory Experiment. By 

 A. D. Denning, M.Sc, Ph.D., Demonstrator in Physics 

 in the University of Birmingham*. 



THE following experiment was suggested to me by Prof. 

 Poynting, F.P S., as a laboratory experiment, and as 

 it is by no means difficult to carry out and appears to give 

 good results, it may be useful to give an account of it. 



The principle of the method followed may be thus briefly 

 described : — A hemispherical radiator, blackened inside, was 

 quickly placed over a flat silver disk of known dimensions 

 and which formed one of the junctions of a constantan-silver 

 thermoelectric couple. From observations, at definite in- 

 tervals of time, of the deflexions of the moving coil of a low- 

 resistance d' Arson val galvanometer included in the circuit, 

 the initial rate of rise of temperature t of the disk was 

 obtained and this result, when substituted in the equation (a), 

 given below, gives <x the radiation constant. 

 For, suppose m = mass of silver disk, 

 s = its specific heat, 

 dT/dt = initial rate of temperature change, 

 then m s . dT/dt = initial amount of heat received by disk. 

 But if A = exposed area of disk, 



P = radiation from that area per unit of time, 

 and P, = radiation from blackened hemisphere, 

 then the gain of heat by the plate 



= (Ri-B)A, 



=ms . dT/dt. 



Using Stefan's Law, we may write : 

 P = o-T 4 and R^o-Tj 4 

 where <r = the radiation constant, 



T & T x = the temperatures of disk and hemisphere, 

 respectively, measured from —273° C. 



* Communicated by the Physical Society: read May 12, 1905. 

 t The initial rate of temperature rise being taken to avoid gain of 

 heat bv disk from conduction and convection effects. 



