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LIV. The Minimum Temperature of Visibility. By P. L. 

 Gray, B.Sc, Assoc. R.C.S., Lecturer in Physics, Mason 

 College, Birmingham* . 



THE title of this paper is, for convenience, somewhat 

 elliptical. Fully, it should be a The Minimum Tem- 

 perature at which- a Strip of Platinum, either bright or lamp- 

 blacked, becomes visible in the dark." 



Apparently no exact work has been done on the point since 

 that of Draper ("On the Production of Light by Heat," Phil. 

 Mag. 1847, xxx. p. 345), who used a somewhat similar method 

 to that described in this paper. This work seems to be gene- 

 rally little known, vague statements, usually without authority, 

 being given when the subject is referred to at all. It only 

 fell under the present writer's notice after the observations, 

 detailed below, had been completed. 



Draper looked at a small strip of platinum, only ^V inch 

 wide, in a dark room, gradually heating the metal by an 

 electric current until it became visible, and estimating its 

 temperature from its linear expansion, measured by a mag- 

 nifying-lever method. Taking a mean coefficient of expansion, 

 from Dulong and Petlt's results, with a lengthening of 222^ 

 he arrives at a temperature of 525° 0. This is obviously too 

 hio-h. as no account is taken of the increase of the coefficient 

 with temperature. Taking the most recent result f for pla- 

 tinum, 



a= 10-9(8901 + 1-210, 



or 



^- = 10-9(8901 + 1-210; 



and integrating, we have 



k-Z =10-9x 8901*+ 10-9x1-21^, 



or * 



^=•0045045= „ „ „ 



whence £ = 490° C. 



As this formula was only tested experimentally up to about 

 73° C, it is most likely that another term, in t 2 , should be 

 added, which would somewhat reduce this value, and thus 

 produce a satisfactory agreement between Draper's result and 

 the maximum obtained by myself. 



* Communicated by the Physical Society : read April 13, 1894. 

 f J. R. Benoit, Travaux et Memoir es du Bureau International des 

 Poids et Mesures, vi. 1888, p. 90. 



