204 Profs. Richardson and Cooke on the Heat developed 



On account of the greater consistency o£ the measurements 

 the value for oxygen is probably considerably more reliable 

 than that for hydrogen. With the exception of the single 

 high value, which has been omitted, the five remaining 

 determinations for oxygen agree with one another within 

 the limits of observational error. 



It is interesting to compare these values of <£ with the 

 values of the corresponding quantity deduced from experi- 

 ments on the variation with temperature of the negative 

 thermionic emission from hot platinum. This method of 

 deducing the value of <f> has already been explained by 

 0. W. Richardson. A little further explanation will 

 perhaps not be superfluous. The coefficient /> in the formula 

 A6 x ' 2 e~ h l®, which represents the variation of the thermi- 

 onic emission from a hot metal with the temperature, 



(be 

 is V, , where e is the charge on an electron and R is the 



gas constant reckoned for a single molecule. Hence if n is 

 the number of molecules in 1 c.c. of a gas under standard 

 conditions of temperature and pressure, 



_<j>e__ (f>t>e 



— i > — l > > 

 tt 1 v i 



where R t is the constant in the equation j>v = B. l reckoned 

 for 1 c.c. of gas at 0° C. and 700 mm. pressure, ne is clearly 

 the quantity of electricity required to liberate half a cubic 

 centimetre of H 2 at 0° U. and 760 mm. pressure in a water 

 voltameter, and is = *4327 electromagnetic units. The value 

 of Ri is 3-72 x 10 :] erg/°C. Substituting these values, we find 



<£ = 8\59 xl0- 5 x/. volts. 



The value of <f> given by Richardson * in his first series 

 of measurements is 4*1 volts. This is a little lower than 

 what the value of h from which it is calculated requires, on 

 account of approximate values of R and e having been used. 

 Recalculating from the experimental value of b, using the 

 relation given in the last paragraph, we find <£ = 4'26 volts. 



More recent work has shown that this value is smaller 

 than that from pure platinum, as the results there given 

 indicate that the metal used in these experiments was probably 

 not free from traces of hydrogen. The best value hitherto 

 obtained is probably one given by H. A. Wilson f for a wire 

 carefully freed from hydrogen by treatment with nitric acid. 



* Phil. Trans. A, vol cci. p. 497 (1903). 

 t Phil. Trans. A, vol. ccii. p. 243 (1903). 



