If a luminous flame is to be the standard it is essential to find a fuel of known and definite composition and to devise a lamp which insure its combustion under uniform conditions.
in 1884 Zeitschrif 1884, page 20). The performance of this lamp was so promising that it was, after thorough and systematic study at the Imperial Physico-Tech nical Institute in Charlottenburg, reduced to standard form. The modified tyne of amyl acetate lamp thus produced has become recog nized as the most reliable and desirable form of primary standard thus far proposed.
The Hefner lamp in the final form given it, as the result of the experiments made in Char lottenburg, consists of a cylindrical body of brass upon which is mounted the wick tube. This is a vertical tube of German silver, the height, diameter and thickness of wall of which are accurately specified and the dimensions and details of construction of which must be fol lowed with precision if the intensity of the flame is to agree closely with the standard. Upon the accuracy with which it is possible to follow these specifications, in metal working, the performance of the lamp depends. Lamps which are faithful copies of those upon which the original investigations were carried out give results far more consistent as to intensity of light than have been attained with any other form of standard in which a flame is used, and it is this complete and accurate reproducibility which has caused the Hefner lamp to be adopted as the best available primary standard. The light-giving power of the flame of this lamp, like that of all flames, varies not only with the composition of the combustible but likewise with the height of the flame and with the amount of moisture in the surrounding air. The relation between flame-height and Inten sity as determined by Liebenthal to whom our knowledge of the performance of the Hefner lamp is chiefly due, is shown in the following table: Flame heights Intensities 20 mm. 0.38 25 mm. 0.55 30 mm. 0.70 35 mm. 0.85 40 mm. 1.00 45 mm. 1.12 50 mm. 1.25 60 mm. 1.50 To determine the height of the flame a gauge, which consists of a lens throwing a magnified image of the tip of the flame upon a disc of ground glass, is mounted with its axis 40 millimeters above the top of the wick-tube as shown in Fig. 8. The character of the wick does not affect appreciably the intensity of the flame.
A precise relation between the intensity of the Hefner unit and the various standard can dles cannot, owing to the inconstancy of the latter, be said to exist. Since, however, candle power is still the term according to which nearly all artificial sources of light are rated it has been found necessary to adopt some definite ratio. The photometrists of the Physico-Tech nical Institute, using numerous data obtained by various observers, found as a mean value, 1 Vereinskerze= 12 Hefners; also 1 British Standard Candle = -1.14 Hefners, or 1 Hefner =.877 British Standard Candles. Schiele from extensive measurements found 1 Hefner= .881 British Standard Candle. It is probable that the factor .88 gives as fair a ratio rep resenting the average performance of the candle as can be obtained.
Another substance of definite composition the flame from which would be a very desirable standard of light is acetylene. The advantage of such a flame over that of the amyl-acetate lamp, the ruddy color of which makes compari sons with the brilliant sources of light used in modern illumination uncertain, lies in the fact of its far greater intensity and whiteness. Un fortunately acetylene on account of its richness in carbon will burn without smoking only when mixed, before escaping from the burner, with a considerable amount of air. In all practicable forms of acetylene burners thus far devised the openings for the egress of the gas are very small. It is on this account difficult to produce burners all of which will give precisely the same candle-power. Owing likewise to the minuteness of these apertures the burner is subject to partial choking by small particles of lime, etc., deposited from the gas. Thus the amount of gas flowing under a given pressure is reduced and what is quite as serious, the pro portions of the mixture of acetylene and air upon which both the brightness and the color of the flame depend is changed. Because of the difficulty of burning acetylene under completely controllable conditions, there is at the present day no standard burner for use in photometry, but as a secondary standard, subject to repeated comparison with some reliable primary source, the acetylene flame has been found to be of great value.