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candle

1

One of several obsolete units of luminous intensity. Originally actual, standardized candles were used, often made of spermaceti. Real candles, however, do not make very good physical standards; candles taken from the same package were reported to vary in light output by as much as 15%. The extent of this variation became significant as related measurements became more precise, and whales becoming scarce, the candles were replaced by

• various standardized lamps burning vegetable oil (see carcel), then by
• various standardized lamps burning pure chemical compounds (see hefner or Harcourt), then by
• blackbody radiators at the temperature of freezing platinum (see violle and candela), 
  and finally by
• the redefined candela, the unit of luminous intensity in SI.

According to the current national standard in the United States¹, none of the candles, and including candlepower, may be used. The candela should be used instead.

1. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.

sources

1

It is difficult to express numerically the relative luminous intensity of various candles, there being as many different values as observers. One of the principal causes of these differences should be sought in the lack of constancy in the luminous intensity of these photometric standards.

We have already said that Schwendler found variations of 40 per cent in luminous intensity; we must believe, however, that he took account neither of the height of the flame nor the consumption of the candle.

At the other extreme is Krüss, who claims to have been able to maintain the constancy of the light emitted by the candle within 3 per cent; this figure is evidently more favorable than can be obtained in industrial tests, where one can scarcely observe all the minute precautions used by this renowned German specialist. But let us mention the results of other observers.

Dibdin, after extended investigation of luminous standards, in his report to the Metropolitan Board of Works of London, claims that candles give uniform results only by accident. Thus, in 454 measurements made with candles, 154, or 34 per cent only, gave results differing by less than 10 per cent from the mean.

Heisch and Hartley, in an investigation of the same subject, found deviations of from 1.3 to 16 per cent from the mean, the mean deviation being about 7 per cent. On the other hand, Foucart found that, in his experiments, the intensity of the Star candle varies from 9.9 per cent above to 13.9 per cent below the mean, the total variation being 23.8 per cent.

An English commission, including Williamson, called attention, in its report to the Board of Trade, to the fact that candles taken in two different packages, or even in the same package, coming from the same factory, may give variations of from 14 to 15 per cent.

We see then that it is scarcely possible under these conditions to give exact figures for the relative values of the different candles. So we shall limit ourselves to briefly indicating in the following table the relative values obtained by different observers.

We give in conclusion the results obtained by Monnier relative to the value of the luminous intensity of ordinary candles, indicating the height of the flame and hourly consumption; all the numbers are expressed in terms of the normal carcel lamp.

2

Hence, the candle is becoming a thing of the past, both as a practical illuminant and as an actual standard in photometry. We have got rid of the article itself, why should we retain the name? It is like continuing to reckon lengths in “barleycorns,” three of which were said to make an inch; and if the candle is no longer in use in practical photometry, it will soon have to be expunged from the Statute book as the legal unit of light. At the present time it may almost be called archaic, a thing to be preserved in museums, but not to have its name perpetuated as a unit of light in every way too small for modern purposes. A unit of light of convenient magnitude for the purposes of electric lighting is that which is given by the 30-watt glow-lamp or by the Harcourt pentane lamp as adopted by the gas referees.

Instead of calling this standard of luminous intensity 10 candles, why not call it 1 lamp ? The word lamp is a short, common word existing both in French and German, and, therefore, not presenting anything strange in sound. A light which we now call 10-candle power would be called 1-lamp power, and similarly lamps of 20 c.p., 50 c.p., and 100 c.p., would then be called lights of 2-lamp power, 5-lamp power, and 10-lamp power. These simple multiples are more convenient than the present 8, 16, and 32-candle multiples which are in use for glow-lamp classification. These last multiples were only adopted originally because at the outset electric-lighting people copied gas-lighting people in everything. We put our wires originally into gas brackets, fixed our electric lamps to gas chandeliers, and selected as the standard glow lamp one which gave the same light as an argand gas burner consuming 5 cubic ft. per hour.

J. A. Fleming.
The Photometry of Electric Lamps.
The Electrician, vol 50, no 15 (issue No. 1289) January 30, 1903.
Pages 599-600.

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Last revised: 1 November 2016.