Harcourt pentane

In England, 19th century, a type of lamp used in photometric work. It burned a highly refined fraction of gasoline. Early versions were designed to have an output equivalent to one British candle. In the final version, output could be adjusted between 1 and 2 British candles, about candela. Its intensity was sometimes described as a "pentane candle."

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A more perfect solution of the problem [of having a gas of constant composition to burn] has been found, after numerous trials, by Vernon-Harcourt; the combustible employed by him is air carburetted by means of volatile carburets of hydrogen, products of petroleum. The carburet is prepared by a fractional distillation of gasoline, previously washed with sulphuric acid and caustic soda. The liquid decanted is distilled four times, successively at 60°, 55°, 50°, and lastly at 50° again. The product obtained is composed of hydrocarburets of the paraffine series, CnH2n+ 2, principally pentane. C5H12 mixed with its homologues, tetrane, C4H10, and hexane, C6H14. Its specific gravity at 15° C varies between the limits 0.628 and 0.631.

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The burner adopted by Vernon-Harcourt is a candle-burner of yellow copper, because of the ease with which this metal is worked. The opening is circular; it is made in a copper plate 0.05 of an inch thick and should be 0.25 of an inch in diameter.

This dimension may be reproduced within 0.001 of an inch; that is, with an error of less than 0.8 per cent of the total section of the opening. The greatest diameter compatible with steadiness of the flame was chosen. The body of the burner is 4 inches long and 1 inch in diameter.

The normal height of the flame is 2.5 inches measured from the opening of the burner; under these conditions, the normal consumption of gas is between 0.48 and 0.52 cu. ft. per hour. The height of the flame is shown by a platinum wire carried on a rod parallel to the burner.

The gas is measured by means of a small meter, and its consumption is further regulated by a sensitive regulator. Under the above conditions, the Harcourt standard gives a luminous intensity exactly equal to that of a spermaceti candle.

For the needs of industrial photometry, we may obtain a quite intense standard of light, about ten candles, of great constancy, by using the pentane air gas of Harcourt in an Argand burner (Sugg’s Standard London Argand, No. 1) combined with the Methven screen. It is this which has been done by Dibdin among others.

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laboratory form of Harcourt pentane photometric standard

Figure 51.

98. The air-gas lamp, in which the gaseous mixture is prepared at the time of combustion, has undergone some quite important modifications since the time when Vernon-Harcourt brought it out in its first form. This gave results which were little concordant, if we may judge from the numerous measurements of Heisch and Hartley, while the final form presented to the British Association in 1887, reproduced the light standard with very great exactness; this is at least the conclusion from Dibdin's report, to which we shall return later.

Following is the description of the laboratory form of the pentane standard, as made by Woodhouse and Rawson of London (Fig. 51).

The admission of the gas to the burner is made exclusively under the action of gravity and without a regulator. The air and vapor are mixed in a reservoir M, whence they descend to the burner. At a certain point, the diameter of the tube through which the gas flows is reduced, and there is between this reduction and the height of the reservoir such a relation that, when the mixture, in the above indicated proportions, is introduced into the tube, it is burned with a flame 2.5 inches high.

The pentane is introduced in the liquid state into the globe M, whence it flows into the reservoir B, where it is vaporized; the vapor passes through C and H, and next descends by its own weight in the vertical tube which leads to the interior of the reservoir R, where the mixture passes through the cock D and reaches the bumer FG.

To regulate the velocity and regularity of the flow of the mixture, the pentane vapors are passed through a thermometric tube which is stopped up for a greater or less distance by a platinum wire attached to a screw O. We may thus give to the pentane the velocity of entrance necessary to make the flame exactly 2.5 inches high.

The level of the pentane in the reservoir has naturally a certain influence, of which account is taken in the following manner. The square box shown in the figure contains a rubber balloon filled with water and connected by a flexible tube to the reservoir; when the level of the pentane is too low, the balloon is compressed by means of the screw J, and a certain quantity of water is made to enter the reservoir B below the pentane. In the opposite case this screw is
loosened, and the water is allowed to descend again.

The heavy copper disc Y suspended above the flame at a variable height serves to compensate for the influence of the external temperature; the chimney G protects the flame from currents of air.

line drawing of last version of the Harcourt pentane lamp

Figure 52.

99. Vernon-Harcourt has simplified this apparatus still more, so as to make it portable and much less complicated without taking away any of its precision.

Vernon-Harcourt's new lamp, instead of burning a mixture of air and pentane vapor, burns the vapor of pentane alone; the flame is surrounded by a chimney which produces sufficient draught and steadiness; it is as white as in the original form of the pentane standard, which is a very important condition.

The new standard lamp, represented in Fig. 52, has a form analogous to that of ordinary alcohol burners, with a metal chimney in addition. The metallic chimney producing a strong current of air gives the necessary steadiness to the flame and increases its temperature, which also gives it a whiter color.

Use is made of a wick, which would be a serious disadvantage if combustion took place at its end; this is not the case, for it simply serves to raise the pentane by capillarity from the lower reservoir to the place in the wickholder where it is vaporized under the influence of the heat produced by the combustion of the vapor, 5 or 8 cm. higher. The wick enters with slight friction in a tube open at hoth ends and itself surrounded by a metallic covering much larger, intended to keep the temperature more constant. The combustion of the pentane vapor takes place at the end of this outer tube; the whole is surrounded by a third tube much larger, which contracts in its upper part so as to have only the diameter of the glass chimney. This chimney is enlarged at its upper end and is fixed to the metallic envelope of the lamp by means of the two movable arms shown in the figure.

The working of the lamp is as follows: on raising the wickholder and heating the inner tube a little, the pentane vapor is immediately disengaged and lighted. The outer covering with the chimney is then put on; the flame immediately rises, because of the increased draught, and its end enters the chimney. This has, at a height of 10 mm., two horizontal slits diametrically opposite, so that, by looking across, the height of the flame may be exactly regulated.

Since the movable chimney is regulated in height, and the height of the flame is determined exactly by the two slits mentioned above, the portion of the flame included between the lower envelop and the chimney emits a perfectly definite quantity of light.

We know that the quantity of light emitted by the central part of a flame is only very slightly affected by variations in the height of the later. Harcourt and the makers have carefully determined the dimensions of the lamp which correspond to a luminous intensity of one candle and two candles. By varying the height of the chimney, we may obtain any luminous intensity within the limits of the power of the apparatus.

The height of the chimney is verified by means of a special cylindrical gauge of the same diameter as the lower part of the chimney, which is placed between the latter and the outer tube of the base; the chimney is then fixed in this position by means of regulating screws.

The variations in the height of the flame are very slight; they become insensible ten or fifteen minutes after lighting. The base of the apparatus is made horizontal by means of a level, and a small mirror placed behind the slits facilitates observation of the height of the flame.

The new pentane lamp is more easy of manipulation than the acetate of amyl lamp of Hefner-Alteneck, since variations in the height of the flame have only a very slight influence on the luminous intensity; the flame is, further, much whiter.

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 152-158. The first French edition, Traité de Photométrie Industrielle Spécialement Appliquée à L’Éclairage Électrique, was published in 1892.

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