A unit of luminous intensity, late 19ᵗʰ – early 20ᵗʰ centuries, the light emitted by 1 square centimeter of glowing platinum at the temperature at which it freezes. About 20 candlepower.
Jules Louis Gabriel Violle (1841-1923), a French physicist, first proposed as a light standard the incandescence of a metal at the temperature at which it freezes on 21 September 1881 at the International Conference of Electricians in Paris.¹
On 16 October 1882, a group of more than 47 distinguished physicists met at the residence of the French foreign minister in Paris, as an “International Conference for the Determination of the Electrical Units.” A committee was formed to discuss a standard for light. After several proposals were considered,
“Dumas² suggested that the late experiments of Violle upon the light emitted by melting platinum might solve the problems. The point of fusion of a body seemed to him to be as good a fixed point as could be wished. He invited the commission to view the experiments of Violle.
“After witnessing the experiments of Violle, the members of the commission appeared to think more favorably of Dumas’ suggestion.”³
Before adjourning on 26 October 1882, the conference passed a resolution:
“The conference express their hope that the experiments now in process upon the light emitted by melting platinum will lead to a definite standard of light.”³
Violle published the results of his experiments on the use of a platinum radiator at this temperature as a standard of luminous intensity in 1884,⁴ and in the same year it was adopted as a unit by the International Conference for the Determination of the Electrical Units:
L'unité de chaque lumière simple est la quantité de lumière de même espèce émise en direction normale par un centimètre carré de surface de platine fondu, à la température de solidification. L'unité pratique de lumière blanche est la quantité totale de lumière émise normalement par la même source.5
The unit of each type of simple light is the quantity of light of the same kind emitted perpendicular to a square centimeter of molten platinum at the temperature at which it solidifies. The practical unit of white light is the total quantity of light emitted perpendicular to the same source.
The unit was reconfirmed at the International Conference of Electricians (Paris, 21 May 1889), the International Congress (Chicago, 1893), and the International Conference of Electricians (Geneva, 1896).
1. Comptes Rendus du Congrès International des Electriciens, Paris, 1881. Assemblée générale, Troisème Séance, page 50.
2. Jean Baptiste André Dumas (1800 – 1884).
3. The International Conference for the Determination of the
Science, volume 1, no. 4, pages 87-89 (2 March 1883).
A copy in this website here.
For the proceedings in French, see: Procès-Verbaux de la Conférence Internationale pour la Détermination des Unités électriques, Troisième Commission, Séance du 20 Octobre, 1882, page 131.
4. J. Violle.
On the Absolute Standard of Light.
Philosophical Magazine and Journal of Science, series 5, volume 17, page 563 (1884).
Translated from the Comptes Rendus of 28 April 1884.
Annales de Chimie et de Physique, series 6, volume 3, page 373 (1884).
Conférence Internationale pour la Détermination des Unités électriques, séance de la Troisième Commission, 3 April 1884.
5. Procès-Verbaux de la Conférence Internationale pour la Détermination des Unités électriques. Deuxième Session, Troisième Séance, 2 May 1884.
6. C. W. Waidener and George Kimball Burgess.
Comptes Rendus de l'Academie des sciences (France), volume 148, page 1117 (1909).
In order that the photometric standard furnished by the preceding apparatus may be utilizable, the flame must always be the same, the combustible and supporter of combustion must have a constant composition, and combustion must take place under invariable conditions. Now flame standards are sensitive in a great measure to every modification in the state of the supporter of combustion. Thus a candle and a carcel lamp depart very rapidly from the normal condition when they are put in a somewhat small room containing several observers.
Further, the illuminating power of a luminous body depends on its temperature; the former increases very rapidly with the latter, so that the higher the temperature of a luminous body is raised, the more necessary is it to obtain constancy in it, without which the luminous intensity would be essentially variable.
Now, in a flame, constancy of temperature is not easy to realize,for it requires that the mixture of combustible and supporter of combustion should always be made under identical conditions. If this mixture is not perfect and invariable, the temperature varies, and the brightness even more.
Finally, a flame is always transparent, and the quantity of light emitted varies with the degree of transparency. In order that a flame may emit constant luminous radiations, it is not sufficient for the temperature to be invariable; it is necessary in addition that the transparency of the flame and its thickness undergo no change.
To furnish a satisfactory unit of light, a flame should then satisfy an aggregation of conditions quite difficult to realize completely. These are the difficulties which determined the International Commission on Electrical Units to reject definitely, as the absolute standard, standards of combustion, and to adopt the platinum standard proposed by Violle.
The advantages of a standard of light based on the incandescence of a body raised to a high temperature, for instance to the the temperature of fusion of platinum, have been recognized for a long time by all physicists. As early as 1844 Draper indicated the possibility of taking as unity the light emitted by platinum wire made incandescent by the passage of an electrical current, and later, in 1859, Zoellner also conceived the same idea. It was taken up again in 1878 by Schwendler, in Calcutta, who made numerous attempts with apparatus constructed with a view to realizing this photometric standard.
Schwendler’s platinum standard is excellent in theory, but cannot give satisfactory practical results, because of the modifications which platinum undergoes as a consequence of slightly prolonged incandescence. Under the action of the electrical current continuous modifications are produced in the platinum wire to which changes in electrical resistance correspond and, consequently, with the same current, changes in temperature.
Incandescent lamps, as they are now made, are free from a part only of these defects; the luminous intensity for a constant current varies, althongh very slowly; but the energy spent in the lamp is divided differently, according to the nature of the carbon, into calorific energy and luminous energy.
At the International Congress of Electricians in 1881, Violle proposed as the absolute standard of light the light emitted normally by 1 sq. cm. of platinum raised to its fusion point and about to solidify.
The phenomenon here employed has the advantage of being constant and susceptible of being always exactly reproduced. A liquid metal which is on the point of solidifying, and which is furthermore unalterable, like platinum, constitutes a body of fixed temperature. The temperature remains in fact invariable as long as any part of the mass remains liquid. If this metal is unalterable, like platinum, it will always have the same emissive power. With a given surface, it will always emit the same quantity of light. The quality of this light depends on the temperature; platinum, being the most refractory of ordinary metals, will be the one which, at its point of fusion, will give the whitest light.
As a consequence of the researches undertaken by Violle, which have demonstrated the exactness of the arguments by which he supported his proposition before the congress, the International Commission on Electrical Units, in session at Paris in 1883, definitely adopted as the absolute standard of white light the standard proposed by this French savant.
A Treatise on Industrial Photometry with Special Attention to Electric Lighting.
Authorized translation from the French by George W. Patterson, Jr., and Merib Rowley Patterson.
New York: Van Nostrand Co, 1894.
Pages 107-108. The first French edition, Traité de Photométrie Industrielle Spécialement Appliquée à L’éclairage électrique, was published in 1892.
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