Evening star., 1897-03-06


for long while. but minute intervals OF
time offered a more complex problem.
From a physical point Of view. the time
unit. unlike the units of length or mass.
is in a sense a derived unit Unlike the
standard meter or gram, which can be d.
rectly examined. compared and copied. we
have no direct model of a second but must
derive the unit by observations upon the
motion of some standard body. such as a
pendulum or the earth. This process of OBJ
serving the motion Of the standard body :n-
wolves the recording of the exact instant
between two or more points of the motion
of the body which excessively repeat
themselves. Ultimately. then. the relia bill
ity of such measurement depends upon
ability to produce a record practically ST.
multaneously with the occurrence of the
stantaneous modes of motion we have are
hight and electricity. The old chTonograph,
used for the measurement of the speed of
projectiles, was electrical device, IN
Which series Of wire screens were placed
in the path of the projectile ut intervals.
SO that the latter would cut the screens
and thus break the circuit The firing Of
the gun likewise caused the release Of a
bar. which. as 1t fell, passed a series of
knives. AS the projectile in its flight cut
the succe ssive wires. the breaks IN the
circuits caused these knives to fly out and
lightly mark the falling bar. Thus, by
knowing the interval between the success
s.ve wire screens and the time interval Of
the falling bar. 1t was possible to compute
the speed OF the bullet or smell.
The Velocity of n Bullet.
But this was crude affair. The new
Kraag-Jorgensen rifle adopted for the
United States army gives a bullet an in
itial velocity Of about 1000 feet per Sec
ord. and the h!gh-power gun which shoots
a distance of twelve or fifteen miles devel-
ops muzzle velocity often of 5000 feet per
second. Incidentally, 1t was a desirable
thing to know whether the velocity of a
projectile was greatest at the muzzle or
some distance therefrom. and the exact
point of this maximum To effect such a
measurement required dealing with unit
Of time so minute as to bear about the
same relation to a second as second does
to the time required for walking, leiSur aly,
a mile. Extraordinary as it may seem, a
rev instrument has been devised at the
United States artillery school at Fortress
Monroe, in Virginia, for measuring such
velocities at intervals of five feet for any
desired distance. 1t Is known as a polarlZ-
ing photo-chronograph. and compared with
its mechanism its name Is simplicity ltSel1
IL would require on the part of the reader
a very clear knowledge of physics ana the
principles of the polarization of hight Tor
an understanding OF any technical descrlp-
ton Of the apparatus.
Roughly. however. it may be said that
a series of screens are set up in the path
of the projectile at the proving grounds
u
back TO the chronograph In the laboratory.
The chronograph itself consists Of be-
wildering combination of electric lights.
prisms. coils analyzers. lenses. cameras.
timing forks and electrical motors The
machine depends for it's action upon the
use of polarized light A sensitive pho
tcgraphic plate is made to rotate at a
known speed in light-tight box and light
is admitted to the plate through narrow
slit by means of ''massless'' shutter.
! The latter consists of pair of Nicol
prisms, between which is unique device
for the rotation of the plane of polariza-
tion This rotation Is accomplished electri-
cally, with the result that the light Is
admitted and cut on without the move
| rent of any material. or at least pondera-
BILE, thing.
With this wonderful apparatus it Is pos
sible to project upon an exceedingly sensi-
tive camera plate series of Impressions,
from which accurate measurements can be
made. These are little more than a series
of minute dots in a circle. caused by the
breakage of the circuit. due to the cutting
of the different wire screens in the path
of the projectile. The breakage of the
circuit Is communicated directly to the
transmitter placed between the polarizer
| and analyzer OF the chronograph. with
the result that the rotation of the plane Of
| polarization Is momentarily but instantly
i stopped From this IL further results that
| the beam of light passing through the
! transmitter is interrupted in its progress
| toward the camera plate. and SO its record
| on the plate Is a series Of broken circles
Now. if a tuning fork be likewise placed
| in the path of the ray of light so that
| its vibrations will make a record upon the
| camera plate. 1t will be possible with q
/ scale closely to compare the two records
! And the rate of the vibration of the turl-
| ing fork being known. the Intervals OF
| time between the breakage of the success
| sive circuits by the projectile can be quick
! ly computed The distance between the
screens being also known. IT is a mere mat
ter of calculation to discover the rate CT
| the pro1ectile's flight.
| Complicated as this instrument seems In
| the description. 1t Is comparatively simple
| In operation. and. of course, instantane.
| Gus. 1t Is manipulated with a pressure OF
| the thumb on electrical key, and its
work Is literally done In a flash.
| Other Instruments.
1t Is In electrical work proper however
that by far the most astonishing degree
of minuteness OF measurement has beer
| obtained. At the electrical exposition hello
In New York year ago a meter was
shown which would measure a current O,
10000 amperes. Lord Kelvin devised ar
instrument which, as now perfected, will
| register current of one fifty y-four-mil
honths Of an ampere; and such q current
| as Is formed by dipping the tip Of q brass
| pin and of steel needle, connected will
/ Ralv0nOnAtar Int -