New-York tribune., 1907-05-26
ME4SURING LIGHT.
System for Finding ont the Eft.
fciency of Gas.

What does It mean when We say that gas Is re-
quired to be OF a certain candle power-twenty-two
In New York, sixteen In Boston, fourteen In Lon
don and fourteen and sixteen in other English
cities? The ideas of most people the measur-
Ing of light extremely vague. Obviously. light
cannot be dipped up with quart cup or laid along
side OF a yardstick. and rule OF thumb would
hardly be reliable way OF deciding the amount
OF light to which great city IS entitled Yet until
ii comparatively few years ego 1t was more rule OF
thumb than anything else that did decide this I'M
portant question Even casual examination or
the cOmpa rative size OF the gas pipes in the old
houses and the new of this city will show that our
forebears We're content with less satisfactory gas
service than we. though. doubtless, in comparison
with the old fashioned lamps and candles the mod
est gus jet Of the SOS seemed brilliant beyond words
In these days however. there Is no guesswork
about the measuring OF light. 1t IS gauged as
carefully and accurately as dainty lady's ribbons
or physicians prescription. The systems by
which gas is measured have all grown up within
TWO generations
Photometry-the measuring Of lights science
all by itself The encyclopaedia will tell you that
light is measured in this way:
IF point source emits in all directions guam
ity OF Light NJ. its intensity is said to be Ma e.
the quantity of light going out through unit
solid angle This intensity is written T. A small
surface of a at distance r from the
point source and inclined line Joining II to
the point source that the angle between this
line and line perpendicular To the surface
And SO on. and So
But that. to the average mind non scientific Is
not ntelligibe. 1t Is much 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
ls dark as-one searches for Comparisons-as
Tophet Everything in the is painted dead
black. and there Is not bit OF bright metal work
anywhere 20 catch the light.
Along one side OF the room stretches high table.
with Il gas jet at end. and the other end two
standard candles lamp OF the exact intensity
or light which Is TO be used Il standard. Be
tween the gas jet and the candles or standardized
lamp runs horizontal bar. which Is a gradu-
ated scale and on this bar rests a movable sight
ing box.
In this box mirrors set at angles so that
DISK placed between them and whose plane is
IN the line of vision. Is visible in each mirror
J. e, both sides Of fl can been seen at once The
DISK consists of two kinds OF paper forming star.
The difference In thickness OF tho papers allowing
nght 10 pass through the thin material and be
nected on the Opaque one. that OF the light
passes through the disk from source OF light
ana Is reflected from the face OF the other disk
by the other source OF light Thus the intensity OF
the two sources OF light In tuxtapositon, and
when the sighting box containing the disk is moved
he figure Of the star on the disk s invisible when
the two sources OF light give equal illumination
the DISK, the light on Side going through the
DISK and the other being reflected from its
face On this sighting box is a hand or pointer
which indicates the graduated scale on the
horizontal bar the ratio OF the gas light 20 the
standard
In order to correct any inaccuracy OF the read
ings ten are taken or both the indicated candle
power and the gas consumption In consecutive
ut. readings is usel in making final com
putations. Where candles used they are care
!ul!y weighed before and after the test to insure
consume at the rate OF I'D grains OF spermacet per
honr.-Gas Logic.