The Topeka state journal. [volume] (Topeka, Kansas) 1892-1980, 1907-09-19
MEASURING LIGHT.
System for Finding Out the EFFICIENCY
of GaS.

What does II mean when We saw thai
gas is required to be OF certain candle
power-22 in New York 16 in Boston.
"a in London, and II and 16 in other
English cities2 The ideas of most
people on the measuring of light. are
extremely vague. Obviously light can
laid alongside of a yardstick. and rule
of thumb would hardly be a reliable
way of deciding the amount of light to
which a great city is entitled Yet I'M
til a comparatively few years ago 1t
was more rule of thumb than anything
else that did decide this important
question. Even a casual examination
of the comparative size of the gas pipes
in the old houses and the new Of this
city will show that our forehears were
content with a less satisfactory 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 or
light 1t is Eauged as carefully and
accurately as dainty ladys ribbons
or a physicians prescription The syS-
TENS by which gas is measured have
all grown up within two generations
PhOtometTy-the measuring Of light
is a science all by itself The en-
cyclopedia Will tell you that light is
measured in this way:
IF a point source ernits in all direc-
tons quantity of light II, its inten-
sfty is said to be M: l. e, the guam
tity of light going out through a unit
solid angle. This intensity is written
I A small surface of area. A at 4
distance from r from the point source
and inclined to the line joining il 70
tween this line and q line perpendicu-
lar 10 the surface"
And so on. and so on
But that. to the average mind non
z-Tez TT--s1, u,
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 II is dark as
one searches for com parisionS"as
Tophet- Everything in the room is
painted dead black and there is not
4 bit Of bright metal work anywhere
TO catch the light
Along one side of the room
stretches a high table. with a gas jet
at one end. and at the other end two
standard candles or a lamp of the ex-
act intensity Of light which IS 10 be
used as a standard. Between the gas
lamp runs & horizontal bar. on which
is a graduated scale. and on this bar
rests a movable sighting box
In this box ara two mirrors set at
angles 80 that disk placed between.
and whose plane is in the line of Wis
ion. is visible in each mirror-i-e.,
both sides of il can be seen at once.
The disk consists OF two kinds Of pa
per forming a 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
the 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 Juxtap0sitiOn. and when
the sighting box containing the disk
is moved TO or from either source of /
light IL is found that the figure of the |
star on the disk is invisible when the |
two sources of light give equal illumi- /
nation on the disk. the light on one
side going through the disk and on
the other being reflected from its sur
face 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 stand |
ard
In order to correct any inaccuracy
of the readings ten are taken OF both !
the indicated candle power and the
gas consumption in consecutive min |
utes, and the average of these ten
one-rninute readings is used in max |
ing the final computation Where
candles are used they are carefully
weighed before and after the test to
insure the accuracy of their standard |
Each candle must consume at the
rate of 40 grains of spermacetf per
hour.-Gas Logic