in which the relative intensity of the two lights is
estimated by the intensity or strength of shadows of the same object
which they respectively cast.
Fig. 262. RUMFORD'S SHADOW PHOTOMETER
Fig. 263. RUMFORD'S SHADOW PHOTOMETER
ARRANGED FOR TESTING
A rod is supported in a vertical position. Back of it is a screen of
white paper. The two lights to be compared are arranged in front of the
rod and at a little distance from each other. They are shifted about
until the two shadows appear of equal darkness. The relative intensity
of the lights varies inversely with the square of their distances from
the shadows cast respectively by them on the screen.
The cut, Fig. 262, shows the simplest type of the shadow photometer
the cut, Fig. 263, a shadow photometer
for testing incandescent lamps is
shown. In it E is the lamp under trial supported by a clamp H. A is an
ampere meter in circuit with the lamp, and V is a voltmeter. A candle C
can be moved along a graduated scale G G. R is the vertical rod, and S
is the screen on which the shadows fall.