burgess: wien's spectral equation 107 



A graphic solution of (2) suggested by v. Wartenberg 2 is to 

 plot the curves S = constant with values of A as ordinates and 

 of T-S as abscissae. 



In view of the fact that for any given substance the value of 

 A\, in general, remains constant, it seems to us most convenient 

 for the estimation of temperatures, of the several possible graph- 

 ic methods, to plot the curves, A\ = constant, with the values 

 of the pyrometer reading S\ — 273 as abscissae and the correc- 

 tion T - S as ordinates or its equal: t — (S\ — 273), where t 

 is temperature centigrade. Such a plot is shown in figure 1 

 for a pyrometer using red light of X = 0.65/x and for the value 

 c = 14500. 



The same plot serves also for equation (1) for passing from one 

 known temperature to another, unknown, when sighting on a 

 black body. In this case the ratio Ii/I 2 is usually given directly 

 from a known relation of scale reading to intensity for the pyrom- 

 eter used. When a considerable number of observations is 

 to be taken, it is worth while to construct such a plot for the wave- 

 length used. 



Pirani 3 uses a protractor method for the solution of (2) by 

 means of which a straight edge laid across the paper gives the 

 value of T by intersecting the values of S and A. 



Pirani also, but it would appear incorrectly, gives a similar 

 graphical method for passing from the value of ^4.\i to^4\ 2 , or 

 from the absorption coefficient as measured in terms of one wave 

 length to that of another. He appears to assume that 



— In A\ x is equal to - In A\, 



c c 



which is true, however, only in special cases (i.e., when S\i — 

 Ste), as for instance the limiting case in which T = S\ (equation 

 2), or for a black body, and is therefore of no interest in the gen- 

 eral case for substances possessing values of A differing from unity, 



2 Uber optische Temperaturmessungen blanker Korper. Verb. Phys. Ges., 12: 

 121-127. 1910. 



3 Uber zwei Recbentafeln zum Gebrauch bei optischpyrometrischen Messungen. 

 Verh. Phys. Ges., 12: 1054-1058. 1910. 



