Photometers are used to measure the intensity of the light produced by an unknown source in terms of a standard source. The general technique is to locate the two sources so that they give the same illumination to two adjacent surfaces. If we assume that the sources are small compared to the distance from them to the illuminated surfaces, the inverse square law may be used. That is, the intensity of the unknown at a distance x from the surfaces is related to the intensity I(0) of the standard source a distance s from the surfaces by I = I(0) (x/s)².
The standard source was the English standard candle, which was made of spermaceti, a wax obtained from sperm whale oil, consumed at a rate of 7.776 grams per hour with a flame height of 4.5 cm. The diameter of the candle was 2.22 cm.
The photometer below was invented by Charles Wheatstone (1802-1875). It originally had a glass bead stuck to the piece of putty in the upper right-hand corner. When the ivory handle on the crank was turned, the internal and external gearing made the bead travel in the rosette-pattern shown on the left, with each of the lights leaving a trail behind due to the persistence of vision. The distances to the two sources was adjusted until the eye saw equal brightness in each path. This one is unmarked, but a similar device was sold by Max Kohl of Chemnitz about 1900 at 33 marks.
The photometer below, in the Kenyon College collection, is of the Lummer and Brodhun style, and was made by Max Kohl. Otto Lummer and Eugen Lummer of the Physikalish-Technische-Reichanstalt in Berlin developed the instrument in 1889. In this design, the user looked through the eyepiece and, due to internal beamsplitters and mirrors, was able to see the images of both sides of the illuminated disk at the same time and thus compare them. Photometers of this type are still in use today. This instrument cost 125 marks in 1900.
The example of Wheatstone's photometer at the right is in the Millington/Barnard Collection in the University Museum at the University of Mississippi.
It was probably bought by Prof. F.A.P. Bardnard in the second half of the 1850s from the firm of Lerebour's et Secretan of Paris. In the 1853 L&S catalogue it is listed at 30 francs, or about $6.
|| The photometer head at the left is designed
to be used in a dimly lighted room. The inclined mirrors show the piece of
paper with the grease spot that is illuminated from the right and the left
by the known and unknown light sources. The system could be held on an optical
bench. The curtains swing forward and keep the observer from being distracted
by the direct light from the two sources.
This unmarked device probably dates from the nineteen twenties and is in the Greenslade Collection.
REF: Thomas B. Greenslade, Jr., "Two Photometers", Phys. Teach., 15, 44-45 (1977)
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