52 proceedings: philosophical society 



Mr. P. D. Foote then spoke on The "center of gravity" and "effec- 

 tive wave-length" of transmission of pyrometer color-screens, and the 

 extrapolation of the high temperature scale. (Published in full in Journ. 

 Wash. Acad. Sci. 5:526. 1915.) 



The same speaker then presented informally some remarks on 

 A new relation from Planck's law. Numerous displacement laws may be 

 derived from the Planck relation representing the spectral distribution 

 of the energy radiated by a black body. A new displacement law was 

 derived which states, that the product of the absolute temperature # 

 and the X-component of the center of gravity of the spectral energy 

 curve Xc is a constant, as follows: Xc = 0.37021c 2 , where c 2 is the charac- 

 teristic constant of the Planck equation. 



The two communications by Mr. Foote were discussed by Messrs. 

 Swann, C. A. Briggs, Gray, Priest, Crittenden, and Sosman, 

 particularly with reference to details of optical arrangements, the use 

 of absorbing mirrors, and variations between observers. 



Mr. E. F. Mueller then presented a paper on Methods of resist- 

 ance measurement. In the design of a Wheatstone bridge, the 0.01, 

 0.001, and 0.0001 decades were to be provided by varying the shunts 

 on three coils, r%, r 2 , and r 3 , permanently connected in the variable arm 

 of the bridge. The problem is to select suitable values for n, r 2 , r 3 , 

 and for the shunting-coils. The values will be completely determined 

 if the condition be imposed that r\ + r 2 -\- r 3 = p and that the r's 

 be so chosen that the effect of errors in the shunting-coils, clue, e.g., 

 to contact resistances in the dial switches, shall be a minimum. 

 There are, however, obvious advantages in choosing the r's so that 

 the shunting-coils for all decades are the same. This requires that 



r 1 = r2"V 10 = r 3 "10 as may be seen from the formula used: viz., that the 



change in r, due to a shunt R, is equal to — r-=. If r 2 is chosen so as to 



r -f- K 



be divisible by the numbers from 1 to 10, e.g.,r 2 =0.6048, simple values 



for the shunting-coils are obtained. 



The paper was discussed by Mr. White with regard to variations in 



shunts. 



J. A. Fleming, Secretary. 



