Research on some Standards of Light. 



481 



Preliminary Research on the Molten Platinum Standard of Light. 



The use of molten platinum as a standard of light was first proposed 

 by J. Violle* at the Congres International des Electriciens on the 21st 

 of September, 1881. f In 1881 Violle published an account of his 

 experiments with the new standard, which was adopted in the same 

 year by the Conference Internationale pour la Determination des 

 Unites Electriques on the 2nd of May, 1881. J The adoption was con- 

 firmed by the Congres International des Electriciens on the 21st of 

 May, 1889, when it was specified that the practical unit should be the 

 bougie decimate, and by the International Congress in Chicago in 1893, 

 and the Congres International des Electriciens at Geneva in 1896, 

 when the use of the Hefner lamp as a practical standard was recom- 

 mended. 



Since the adoption of this unit the aim of most of the experimental 

 work on the subject has been not so much to find a suitable method of 

 using the Violle standard as to obtain a substitute for it. These 

 efforts have resulted- in the well-known instrument devised by W. Sie- 

 mens§ for fusing platinum foil, and in the proposal made by C. R. 

 Cross|| to use as a practical standard the light emitted by a thin 

 platinum wire at its melting point. 



Physiological considerations render such a thing as an instantaneous 

 photometric reading an impossibility, and herein lies the principal 

 reason why the above proposals have had to be abandoned. Apart 

 from this fact, it is extremely doubtful if either in the case of fine wire 

 or thin foil the break occurs actually at the temperature of fusion of 

 the metal.U Cross admits that there were considerable differences in 

 the quantity of light emitted per unit area when the diameter of the 

 wire was changed. In connection with the present work a number of 

 experiments were tried, both with wire and foil, but the results were by 

 no means encouraging. 



* For Violle's researches see ' Comptes Rendus,' vol. 88, p. 171, 1879; vol. 85, 

 p. 543, 1879; vol. 92, p. 866, 1881; 'Luniiere Electrique,' vol. 14, p. 475, 1884; 

 'Annales de Chiniie et de Physique,' ser. 6, vol. 3, p. 373, 1884. 



t ' Comptes Rendus du Congres International des Electriciens,' Paris, 1881. 

 Assembled generale, Troisienie Seance, p. 50. See also ' Proces-Verbaux de la 

 Conference Internationale pour la Detercnination des Unites Electriques.' Troi- 

 sieme Commission, Seance du 20 Octobre, 1882, p. 131. 



% 1 Conference Internationale pour la Determination des Unites Electriques.' 

 Deuxieme Session, Troisieme Seance, Mai 2, 1884, pp. 23, 115. "L'unite de 

 chaque lumiere simple est la quantite de luniiere de nieme espece emise en direc- 

 tion nonnale par un centimetre carre de surface de platine fondu, a la temperature 

 de solidification. L'unite pratique de lumiere blanche est la quantite totale de 

 lumiere emise normalement par la menie source." 



§ ' Elektrotechnische Zeitschrif t,' vol. 5, p. 244, 1884. 



|| < Electrician,' vol. 17, p. 514, 1886. 



*Hi See 1 British Association Fourth Report of the Standards of Light Committee,' 

 App. II, 1888, p. 47. 



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