Determining the Radiation Constant. 275 



ether near the disk, it was possible to take several sets of 

 observations in fairly quick succession — especially if a current 

 of air were maintained across the disk. Owing to the small 

 range of temperature prevailing when the ice was used and 

 the possibility of disturbances due to convection, such close 

 agreement with the other results as was actually obtained 

 was hardly to have been expected. It has been thought 

 advisable to include all the experiments made to indicate 

 what kind of accuracy can be expected. 



A glance at the values for a given in the last columns of 

 both tables will at once show that two of the values are very 

 much lower than the other nineteen. The most probable 

 explanation for this is that somewhere in the galvanometer- 

 circuit there was poor contact and a consequent increase in 

 the circuit-resistance — most likely in the mercury commutator, 

 since these at times are very unreliable. Indeed, deter- 

 minations of the equivalent of the galvanometer readings 

 made from time to time during the course of the first series 

 of experiments showed variations in the sensibility of the 

 galvanometer. Hence it were better to measure the sensi- 

 tiveness of the galvanometer directly before taking a series 

 of observations by arranging that the simple reversal of a 

 commutating-switch might place the galvanometer in circuit 

 with a low resistance, the fall of potential along which is a 

 constant small fraction of the E.M.F. of a storage- cell. 



Ignoring these two values, we find uniformity and agree- 

 ment among the remainder — the average of the residual 

 nineteen results being about 5*3. The mean of Kurlbaum's 

 more elaborate experiments was 5*32. The simplicity of 

 construction of the apparatus and the principles embodied in 

 its use will, it is hoped, recommend the experiment as a 

 laboratory method of illustrating an important and funda- 

 mental law. 



Appendix. 



Note on Motion of Galvanometer Coil. 



In order to take into account the effect of the moment of 

 inertia of the moving coil and the viscous resistance of the 

 medium, let us suppose that a steady difference of 1° C. 

 between the thermo-junctions gives a steady deflexion of the 

 galvanometer- coil of X radians, then a steady difference of 

 temperature of T' would give a steady deflexion 



= \T. 



