Energy Distribution in Spectra. 61 



of atom, X = wave-length, c = velocity of light. Putting in 

 the values 



r = l-7 x v /2xl0 5 = 2 , 4x 10 5 cm./sec. = mean velocity 

 of a hydrogen atom at 0° C, 



X = 6'563xl0" 5 cm., 



c = 3 x 10 10 cm. /sec, 



we get distance travelled per single vibration = 



2-4 xl0 5 x 6-563 xlO- 5 K OK in 10 



s — — m = 5*2dx 10" 10 cm. 



3 X 1U 10 



Hence the number of vibrations performed by the radiating 

 atom 



4-2 xlO" 4 



5-25x10 



•10 



= 8 x 10 5 . 



A more accurate value could be found by taking the 

 probability distribution of free paths into account, but it is 

 scarcely worth while to perform the laborious quadratures 

 involved until more experimental results have been obtained. 



Change of Intensity with Pressure. 



In fig. 9 (PL II.) are shown the photo-currents due to H a at 

 different pressures with constant discharge current. The 

 curve is plotted from Table I., line 1. 



The maximum effect is produced when jt? = l'3 mm. 

 This maximum may be explained as follows. The light is 

 due to recombination of + H ions with electrons. At low 

 pressures the speed of the free electrons is so great that 

 recombinations in the positive column are infrequent ; at 

 high pressures the free path of the H atom is too small 

 to allow of the full radiation being emitted. At high 

 pressures it will be seen that the curve becomes hyperbolic. 

 This would indicate that the number of +H ions re- 

 combining per c.c. per second is constant, the radiation 

 consequently varying directly as the free path, and there- 

 fore inversely as the pressure. Nutting and Tugman * give 

 a similar curve, using pure hydrogen. 



Calibration of the Photo-Electric Cells in Absolute Units. 

 Benedict f has found that a carbon lamp behaves as 

 a grey body in the visible spectrum, that is, that its energy 



* Loc. ciU p. 58. 



t Ann. d. Phys. xiii. 1915, pp. 641-678. 



