The Brandon news. (Brandon, Miss.) 1892-1961, 1907-09-19
MEASURING LIGHT.
System for Finding Out the Efficlen.
cy Of Gas.

system for Finding Out the Efficlen.
cy Of Gas.
What does it mean when we say
that gas Is required to be of a GER
tain candle power-twenty-two in
New York, sixteen in Boston, four
teen In London and fourteen and six
teen in other English cities? The
ideas OF most people on the measur-
Ing OF light are extremely vague. OBJ
viously, light cannot be dipped up
with a quart cup or land alongside Of
yardstick, and rule of thumb would
hardly be reliable way of deciding
the amount of light to which great
city Is entitled. Yet until a COM
paratively few years ago 1t was more
rule OF thumb than anything else
that did decide this important ques-
ton. Even casual examination of
the comparative size of the gas
pipes In the old houses and the new
OF this city will show that our fore
bears were content with less salts
factory gas service than we, though,
doubtless, in comparison with the old
fashioned lamps and candles, the
modest gas jet of the SOS seemed
brilliant beyond words.
In these days, however, there is no
guesswork about the measuring of
light. It Is gagged as carefully and
accurately as a dainty lady's ribbons
or a physicians prescription. The
systems by which gas is measured
have all grown up within two gener-
ations.
Photmetry-the measuring Of light
lis a science all by itself The en-
cyclopaedia will tell you that light is
measured in this way:
If point source emits in all oil.
rections a quantity of light M, its
intensity is said to be ma; /. e., the
quantity of light, going out through
a unit solid angle This intensity Is
written t. A small surface of area
a. at a distance r. from the point
source and inclined to the line join
ing it to the point source so that
the angle between this line and a
line perpendicular to the surface
And so on, and so on.
But that, to the average mind nom
scientific, is not ineligible. It is
much more interesting to see how
the thing is actually done.
In laboratories of the gas company
there Is one room set apart for the
photometric appliances. 1t Is dark as
one searches for comparisons-as
Tophet. Everything In the room is
painted dead black, and there is not
bit of bright metal work anywhere
to catch the light.
Along one side of the room stretch
es a high table, with a gas jet at
one end, and at the other end two
standard candles or lamp Of the
exact intensity of light which is to
be used as standard Between the
gas jet and the candle or standard
lied lamp runs a horizontal bar, on
which is a graduated scale, and on
this bar rests a movable sighting
box
In this box are two mirrors set at
angles so that disk placed between
them, and whose plane is in the line
of vision, is visible in each mirror
i. e., both sides of it can be seen at
once. The disk consists Of two
kinds of paper forming star, the
difference in thickness Of the papers
allowing light to pass through the
thin material and be reflected on
the Opaque one, so that some Of the
light passes through a disk from
one source of light and is reflected
from the face of the other disk by
the other source of light. Thus, the
intensity of the two sources Of light
are in juxtaposition, and when the
sighting box containing the disk is
moved to or from either source of
light 1t is found that the figure Of
the star on the disk is invisible when
the two sources of light give equal
illumination on the disk, the light
on the other being reflected from
its surface. On this sighting box is
a hand or pointer, which indicates On
the graduated scale on the horizontal
bar the ratio of the gas light to the
standard.
In order to correct any inaccuracy
Of the readings ten are taken OF
both the indicated candle power and
the gas consumption In consecutive
minutes, and the. average OF these
ten one-minute readings is used in
making the final computations. Where
candles are used they are carefully
weighed before and after the test.
to insure the accuracy Of their stand
ard. Each candle must consume at
the rate of 120 grains of supermaceti
per hour.-Gas Logic.
Mirrors as Detectives.