The New Zealand Railways Magazine, Volume 1, Issue 8 (January 15, 1927)
The Meaning of “Candle Power” — How to Make a Simple Photometer
The standard of light in use is that of a special sperm candle, weighing six to the pound and burning at the rate of 120 grains of sperm per hour or 2 grains per minute. When it is said that an electric lamp is a sixteen candlepower lamp, it is implied that the lamp radiates as much light as do 16 of these standard candles. There are various ways of measuring candle power, but whatever method is used a great deal of care and skill is required if one would lay claim to accuracy.
For ordinary purposes, however, the apparatus described here will suffice, and has the advantage that it can be made by anyone wishing to experiment. This simple photometer consists of:—
(1) An electric glow lamp.
(2) Candle—weighing one sixth of a pound.
(3) Wedge with sloping surfaces that serve as the photometer (made of white wood or paper).
The side “A” is illuminated by the lamp and side “B” by the candle, in general the lamp side will look brighter than the other. By moving the wedge nearer to the candle a spot can be found at which the observer, looking down on the two surfaces of the wedge (from “C”) cannot see any difference between them in respect of brightness.
They are then equally illuminated; that is to say the candle light falling on “B” is equal in intensity to the electric light falling on “A.”
If the single candle could be replaced by 16 the wedge would be equally illuminated when placed half way between 1 and 2.
The wedge here employed along with a scale to indicate distances from the two sources of light is called a “photometer” or “light measurer” and is one of the simplest.
The wedge should have a sharp angle, say 60£ to 70£, with the edge as sharp as possible. It is important, of course, to exclude all other lights except the two that are being compared.
Certain points in the behaviour of a lamp may be worth mentioning. Take for instance a 16 candle power lamp as a sample. It may be marked 100 v. 16 c. This means that it is intended to be worked at a pressure of 100 volts and that it then should give a light of 16 candles. As a matter of fact it will give 16 candles at this pressure only. If the pressure be increased the lamp gives more light than 16 candles, while if the pressure be reduced the lamp gives less light than 16 candles. In each case the luminous efficiency changes tremendously. Now it may appear from the foregoing that it would be economical to use a 100 volt lamp at 105 volt pressure or more, but that is not so, as such overrunning shortens the life of a lamp. Moreover when overrun, the glass blackens because of an internal coating of vaporised carbon, and this absorbs a good deal of the light produced. Hence it is important to work a lamp at the pressure marked by the maker. For instance, to use a small lamp of a voltage suitable for motor car or train-lighting work on a town supply of the usual 230 volts would ensure that the lamp and also the fuses would be blown out.