An important matter in connexion with glow lamps is their photometry. The arrangement most suitable for the photometry and testing of incandescent lamps is a gallery or room large enough to be occupied by several workers, Photometry of glow lamps. the walls being painted dead black. The photometer, preferably one of the Lummer-Brodhun form, is set up on a gallery or bench. On one side of it must be fixed a working standard, which as first suggested by Fleming is preferably a large bulb incandescent lamp with a specially “aged” filament. Its candle-power can be compared, at regular intervals and known voltages, with that of some accepted flame standard, such as the 10 candle pentane lamp of Vernon Harcourt. In a lamp factory or electrical laboratory it is convenient to have a number of such large bulb standard lamps. This working standard should be maintained at a fixed distance on one side of the photometer, such that when worked at a standard voltage it creates an illumination of one candle-foot on one side of the photometer disk. The incandescent lamp to be examined is then placed on the other side of the photometer disk on a travelling carriage, so that it can be moved to and fro. Arrangements must be made to measure the current and the voltage of this lamp under test, and this is most accurately accomplished by employing a potentiometer (q.v.). The holder which carries the lamp should allow the lamp to be held with its axis in any required position; in making normal measurements the lamp should have its axis vertical, the filament being so situated that none of the turns or loops overlies another as seen from the photometer disk. Observations can then be made of the candle-power corresponding to different currents and voltages.
The candle-power of the lamp varies with the other variables in accordance with exponential laws of the following kind:—
If A is the current in amperes through the lamp, V the voltage or terminal potential difference, W the power absorbed in watts, c.p. the maximum candle-power, and a, b, c, &c., constants, it has been found that A and c.p. are connected by an exponential law such that
c.p. = aAx
For carbon filament lamps x is a number lying between 5 and 6, generally equal to 5.5 or 5.6. Also it has been found that c.p. = bW³ very nearly, and that
c.p. = cVy nearly
where c is some other constant, and for carbon filaments y is a number nearly equal to 6. It is obvious that if the candle-power of the lamp varies very nearly as the 6th power of the current and of the voltage, the candle-power must vary as the cube of the wattage.
Sir W. de W. Abney and E. R. Festing have also given a formula connecting candle-power and watts equivalent to c.p. = (W − d)² where d is a constant.
In the case of the tantalum lamp the exponent x has a value near to 6, but the exponent y is a number near to 4, and the same for the osmium filament. Hence for these metallic glowers a certain percentage variation of voltage does not create so great a variation in candle-power as in the case of the carbon lamp.
Curves delineating the relation of these variables for any incandescent lamp are called its characteristic-curves. The life or average duration is a function of W/c.p., or of the watts per candle-power, and therefore of the voltage at which the lamp is worked. It follows from the above relation that the watts per candle-power vary inversely as the fourth power of the voltage.