TABLE 40 (continued) Q7 



PROBABLE VALUES OF THE GENERAL PHYSICAL CONSTANTS 



The equation for evaluating // is that just used, except that now the energy 

 (P) to pull an electron out of the metal is no longer negligible compared to hv. 

 Hence we write 



Ve-hv-P. 



To eliminate P, light of varying frequencies is used, measuring for each the 

 critical voltage V at which ionization starts. V is plotted against v ; the result- 

 ing curves should be linear, with a slope 



dV/dv = h/e. 



With V measured as V int. volts, and v as v cm -1 , we have 



d V/dv = (pai o 8 d V')/( c 2 dv' )=h/e 



Lukirsky and Prilezaev use the metals Al, Zn, Sn, Ni, Cd, Cu, and Pt. Six 

 curves, three with Zn, two with Al, and one with Ni, were the best. Un- 

 fortunately these investigators give no detailed data, and no indication of the 

 actual equation used. Their final value of h is 6.543 x icr 27 erg • sec, the indi- 

 vidual results being 6.539, 6.542, 6.540, 6.556, 6.536, and 6.546. We take 



h— (6.543 ±0.010) x io -27 erg • sec. 



These investigators estimate their final error in h as 0.1 to 0.2 per cent. 



(e) Wien's displacement law; Planck equation. — h may be had from radia- 

 tion constants in two different ways. The first is by means of c 2 , in the Wien 



displacement law, 



\ max T = c 2 /p = A. 



where /? = 4-965i (root of e~P + (3/5 — 1 =0). The radiation constant c 2 occurs 

 also in Planck's black-body radiation law in the form 



c 2 — hc/k. 



c = velocity of light, £(Boltzmann constant) =R /N , R (gas constant per 

 mole) =v n A„/T , and N (Avogadro's number) =Fc/e. Substituting the 

 values of v n , A n , T , F and e previously adopted, 



N = (6.0644 ±0.0061) x iq23 mole -1 . 

 R Q = (8.3i36±o.ooio) x io 7 erg • deg. -1 • mole -1 . 

 k = ( 1 .3709 ± 0.00 1 4 ) x 1 0" 16 erg • deg. -1 . 



In 1919 Doctor Birge asked Coblentz what in his opinion was then the best 

 value of c 2 . He recommended 1.433 cm • deg. ; this value was adopted. In a 

 long critical review of the radiation constants, three years later, Coblentz 1 gives 

 1.432 as the most probable value. No probable error is given but the four 

 results, obtained by four investigators, were 1.436, 1.430, 1.430 and 1.43 18, 



1 Bur. Standards Bull., 17, 7. 1922. 

 Smithsonian Tables 



