128 J. Trowbridge — A Standard of Light. 



of the conception, for it is well known that machines, however 

 well finished, tend to heat until the bearings have adjusted 

 themselves to one another. The friction cannot be indefinitely 

 reduced by this mutual action because the dust produced by 

 the abrasion is sufficient to cause constant fresh inequalities in 

 the surfaces.* The character here attributed to a frictional 

 surface is substantially that which various writers have assigned 

 to it ; some of the logical results however seem to me to have 

 escaped attention. 



[To be continued.] 



Art. XVIII. — A Standard of Light ;f by John Trowbridge. 



The discussions in the Paris Conference of 1881-84 upon the 

 subject of a standard of light, which resulted in the adoption 

 of the light emitted by a surface of platinum at the point of 

 solidification, seemed to assort ill with the discussions which 

 led to a reaffirmation of the value of the C. G. S. system of 

 absolute physical units, and a recognition of the relations 

 between work and heat, and electrical energy. 



The solidification point of platinum may be a fixed point in 

 nature ; but it has not been shown how this fixed point can be 

 connected with the great web of physical measurements which 

 has been woven by Weber, Helmholtz, Thomson, Maxwell, 

 and other physicists. It is true that during the discussions of 

 the Conference reference was made to a proposition of 

 Schwendler, that the light emitted by a strip of platinum 

 rendered incandescent by a known electrical current should be 

 taken as a standard. This proposition, however, received little 

 support; and the Conference finally adopted the light emitted 

 by solidifying platinum as a standard. 



It seems highly desirable that any standard of light which 

 may be adopted should be connected with the present system 

 of absolute measurements. The suggestion of Schwendler, 

 therefore, seems to merit more attention than it has received. 

 The suggestion of employing the light from a strip of platinum 

 rendered incandescent by an electrical current is really due to. 

 Dr. John W. Draper, of New York, who in 1847 enunciated it 

 as follows : "A surface of platinum of standard dimensions 

 raised to a standard temperature by a voltaic current will 

 always emit a constant light. A strip of that metal one inch 



* As I pointed out in my former paper, page 158, the friction of ideally lubri- 

 cated surfaces, in which the solid surfaces do not come in contact, is a very 

 different matter. The statement as there given has since been amply confirmed 

 by the report of a committee of the British Association. 



f Read before the American Academy of Arts and Sciences, May 26. 1885. 



