Carcel unit

An obsolete unit of luminous intensity, 19ᵗʰ century, primarily used in France, approximately 9.61 international candles, approximately 0.98 candela. It is the horizontal intensity of a Carcel lamp (an Argand-type lamp with mechanical feed of oil) burning 42 grams of rape-seed oil per hour.



The carcel lamp is a simple modification of the Argand lamp. In 1787 its Genevese maker produced a revolution in illumination, by replacing the flat wick burning openly in the air by a round wick giving passage through it and around its edge to a double current of air, produced by a metal chimney placed above the flame. Soon this metal chimney was replaced by a glass tube having at the top of the flame a constriction which forces the air into closer contact with the flame and which thus brings about complete combustion.

In 1800, Carcel made an important modification of the Argand lamp, giving it a very regular feed of oil. In this modification the oil reservoir is placed in the base of the lamp and the oil is raised to the level of the top of the wick by means of clock-work which operates two small pumps in the instrument. The quantity of oil raised should be greater than that which is required for combustion, and the excess falls back to the reservoir; the wick, constantly wet with oil at the point where combustion is taking place, is charred very slowly and gives an almost constant light.

Since the mechanism is apt to get out of order, a regulating lamp is preferably employed, in which the pressure of a spring on a piston produces the same effect as the clock-work; the flow of oil is rendered sensibly constant by means of a small tube, fixed to the piston, in which there fits a fixed regulating rod which offers less obstruction in proportion as the piston is lower and the pressure of the spring less.

In their photometric studies concerning the lights in light-houses Arago and Fresnel employed the oil lamp. Fresnel showed that by using certain precautions great constancy may be obtained within certain limits. It is well to remember the following interesting detail to show what care should be used in photometric measurements. Fresnel insisted on cleaning and caring for the lamps which he used himself; further, he took the greatest precautions to insure the constancy and comparability of this standard.

The carcel lamp was next adopted by Dumas and Regnault for the photometric tests of the gas [used to] illuminate Paris. The good results obtained in this application and the authority of the two savants led to a general adoption of this photometric unit by the gas companies in France.

The dimensions and working conditions of the carcel standard, as given by Dumas and Regnault in their practical instructions for gas testing, are given below (Fig. 40).

Extreme diameter of the burner . . . . . . . . . . . . . . . . . 23.5 mm.
Interior diameter (for the interior current of air) . . . . . 17.0
Diameter of exterior current of air . . . . . . . . . . . . . . . 45.5
Total height of the glass chimney . . . . . . . . . . . . . . . .290.0
Distance of the bend from the base of the chimney . . . 61.0
Exterior diameter at the bend. . . . . . . . . . . . . . . . . . . .47.0
Exterior diameter at the top of the chimney . . . . . . . . .34.0
Mean thickness of the glass. . . . . . . . . . . . . . . . . . . . . . 2.0

The wick adopted is the medium one, called the light-house wick; the strand is composed of 75 threads and weighs 3.6 grams per decimeter. The wicks should be kept in a dry place, or preferably in a box with a double bottom containing unslaked lime.

The carcel lamp burns well-purified rape-seed oil. According to Crova, the composition of the rape-seed oil which is used in these lamps is apt to undergo only insignificant variations, for it is furnished by the grain of a particular vegetable, and its purity is more easy to control than that of other combustibles, such as stearic acid, spermaceti, paraffine, petroleum, and gas. This oil is purified by adding a small quantity of sulphuric acid, which coagulates the mucilage which it naturally contains and renders it more limpid and more fluid.

This opinion of Crova seems somewhat optimistic, although its author has had occasion to employ the carcel lamp frequently in the course of his excellent photometric work. In fact, vegetable products are rarely of consistent composition and are easily affected by external causes and even by the action of time alone, and the manufacturing process is not absolutely definite.

74. For any combustible there is in general no definite relation between the quantity of material burned and the light produced; but adopting the dimensions given above for the burner and the chimney of the carcel lamp, and using a medium wick, the quantity of light increases in direct proportion to the consumption of oil, when this consumption is in the neighborhood of 42 grams per hour; however, the consumption should not be below 38 grams, nor above 46 grams, for this proportion to hold good.

Two lamps having the same diameter of wick and the same capacity may differ in their consumption of oil and their illuminating power; further, it is known that the temperature, the movement of the air, the duration of the illumination, and the fullness of the lamp, have an influence on the quantity consumed. We should then, before using a carcel lamp as a [photometric] standard, submit it to a series of tests to ascertain the exact conditions under which it should be placed to obtain a nearly constant consumption.

These conditions obtained, we may proceed to definite photometric measurements. For each experiment we should insert a new wick which is trimmed even with the wick-holder; next the lamp is filled up to the beginning of the gallery, it is afterward lighted, keeping the wick at first 5 or 6 mm. high, and then the chimney is put on.

The consumption of oil is regulated by raising the wick to a height of 10 mm. and the chimney so that the bend may be at a height of 7 mm. above the level of the wick. To easily realize these conditions, we make the lower point of the small contrivance which is fitted to the wick-holder level with the wick itself, and the upper point level with a diamond scratch on the neck of the chimney.

The consumption and intensity increase slowly during the first half-hour, because of the heating of the burner; at the end of this time a constant state is established which lasts more than an hour; it is during this period that the photometric observations are made; then the consumption and intensity begin to decrease slowly in proportion as the wick becomes charred.

75. The lamp should consume 42 grams of oil per hour; when the consumption falls below 38 grams or rises above 46 grams, the test results should be discarded. When the consumption is maintained between these limits, the luminous intensity is reduced by a simple proportion to that which corresponds exactly to 42 grams per hour.

To regulate the consumption of the lamp, we suspend it at the end of one of the arms of a balance having on the other arm a counterweight; equilibrium being obtained at any given time, a weight of 10 grams is placed on the side of the lamp; when this weight of oil is consumed, the balance returns to its position of equilibrium; now the point of the balance has a notch which at the moment of equilibrium causes the fall of a hammer; this, striking a bell, notifies the experimenter, who reads on a seconds counter the time necessary for the burning of 10 grams of oil.


Opinions are much divided as to the practical value of the carcel standard. While French engineers think its qualities unrivalled, in other countries other photometric standards are used. In connection with this, the discussions which took place at the International Congress of Electricians, in 1881, and at the International Conference of 1882, are very interesting.

The French representatives, Dumas, Allard, Crova, etc., all insisted on the advantages of the carcel lamp, —advantages scarcely recognized by foreign savants, who put this standard on a par with the candle.

The numerous experiments of Crova showed that two very different carcel lamps, compared within an hour, give indications whose variations amount to from 2 per cent to 3 per cent at most.

According to Leblanc*, the employees charged with testing the gas at the municipal bureau of Paris easily acquire such experience that they regulate the consumption of the oil very exactly between 41 and 42 grams per hour. It follows, from the measurements made daily in this bureau, that the carcel lamp, well cared for and carefully manipulated, can give good results, quite comparable with one another.

However, this agreement does not happen with lamps, wicks, and rape-seed oil of different origins; it is probably to this that we must attribute the great differences which have been several times found to exist by foreign engineers who have used the carcel lamp in their photometric comparisons, but without following exactly the very minute directions given by Dumas and Regnault.

It is of the greatest importance to observe exactly all the precautions mentioned by these two scientists; only then may we obtain results truly comparable to those of other observers, working under analogous conditions.

Leblanc recommends the use of two lamps, which are used on alternate days; when a lamp remains unused for several days, the oil thickens and the mechanism gets out of order.

We have considered so far only the opinions of those who favor use of the carcel lamp, after having used it for a long time. However, we must not be blind to the fact that this standard has also serious inconveniences.

To give an idea of them, we cannot do better than to cite the following passage from a communication of Hartley† to the British Association of Gas Managers, in 1880:

“The very great number of tests that I made in 1867 with a carcel lamp does not encourage me to give credit to the indications of any lamp in which vegetable oil is used. I say vegetable oil because some recent experiments with lamps burning paraffine oil have shown great uniformity in their illuminating power.

“The objections which I have to standard lamps are that they must be kept in a state of perfect cleanliness; the wick must be renewed very often, if not each time the lamp is used (this last point is essential with the carcel lamp); the wick as well as the chimney must be adjusted with the greatest care and exactness, and finally, when all this has been done, there is no certainty that the quantity of oil consumed will not be greatly in excess of the regulation quantity.

“This variation in the consumption would have no effect if, as my experiments have shown me, the quantity of light emitted did not often increase in a much more rapid proportion than the consumption of oil. It is, furthermore, very difficult to keep the consumption of the lamp as low as the regulation rate.”

* Procès-Verbaux de le Conférence Internationale, 1882, p. 145.

Journal des usines à gaz, 1882.

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.
Selections from pages 111-118. The first French edition, Traité de Photométrie Industrielle Spécialement Appliquée à L’éclairage électrique, was published in 1892.
The Pattersons' translation is not always a happy one; we have taken the liberty of revising some passages for the sake of clarity.

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