platinum light standard

Caution: a number of photometric standards employ platinum; this is the only one with “platinum” in its name. Compare: violle, new candle, candela.

A photometric standard proposed by Louis Schwendler in 1879, a consequence of his researches into the feasibility of electric lighting for railways in India. Leaving aside the details of his apparatus, he defined the unit as:

“6.15 webers passing through a piece of platinum 2 millims. broad, 36.28 millims. long and 0.017 millims. thick, weighing 0.0264 grms., having a calculated resistance = 0.109 S.U. and a measured resistance = 0.143 S.U. at 66°F, gives the unit for light-intensity.”

In interpreting this definition, for “webers” read “amperes”, and “S.U.” are Siemen’s units of resistance. Schwendler determined that his unit was equal to 0.69 Sugg’s candle. Sugg was a maker of British standard candles.

Abbreviated, “P. L. S.”.

Louis Schwendler.
On a new Standard of Light.
Philosophical Magazine, vol. 8, (1879) pages 392-403.



The causes which influence the quantity of light emitted by a platinum strip heated by the electric current are too numerous for one to count on the constancy of the light emitted, if one is limited to controlling simply the constancy of the current. The variations in the emissive power, as well as the diminution of the section of the platinum strip under the influence of a prolonged incandescence, are factors of which it is difficult to take account. These circumstances have, from the beginning, inspired a certain distrust of the Schwendler standard, so that its use has not spread either in industrial measurements or in scientific researches.

A. Palaz.
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 122-123. The first French edition, Traitè de Photomètrie Industrielle Spècialement Appliquèe à L’èclairage èlectrique, was published in 1892.

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