101. Money Value of Light. Light is bought and sold almost as
readily as are the products of farm and dairy; many factories,
churches, and apartments pay a definite sum for electric light of a
standard strength, and naturally full value is desired. An instrument
for measuring the strength of a light is called a photometer, and
there are many different varieties, just as there are varieties of
scales which measure household articles. One light-measuring scale
depends upon the law that the intensity of illumination decreases with
the square of the distance of the object from the light. Suppose we
wish to measure the strength of the electric light bulbs in our homes,
in order to see whether we are getting the specified illumination. In
front of a screen place a black rod (Fig. 58) which is illuminated by
two different lights; namely, a standard candle and an incandescent
bulb whose strength is to be measured. Two shadows of the rod will
fall on the screen, one caused by the candle and the other caused by
the incandescent light. The shadow due to the latter source is not so
dark as that due to the candle. Now let the incandescent light be
moved away from the screen until the two shadows are of equal
darkness. If the incandescent light is four times as far away from the
screen as the candle, and the shadows are equal, we know, by Section
100, that its strength is sixteen candle power. If the incandescent
light is four times as far away from the screen as the candle is, its
power must be sixteen times as great, and we know the company is
furnishing the standard amount of light for a sixteen candle power
electric bulb. If, however, the bulb must be moved nearer to the rod
in order that the two shadows may be similar then the light given by
the bulb is less than sixteen candle power, and less than that due the
consumer.