The apparatus is assembled on scales and weighed before the water is
poured in and after the receiver is filled. From the weight of the water
thus obtained and the rise of temperature, the calorific value may be
computed. The charge is exploded by electricity, while the water is
being stirred. The rise in the temperature of the water is read by a
magnifying glass, from a thermometer which measures temperature
differences of 0.01 degree. From the readings obtained, the maximum
temperature of explosion may be determined, according to certain
formulas for calorimetric experiments. Proper corrections are made for
the effects, on the temperature readings, of the formation of the
products of combustion, and for the heat-absorbing power of the
apparatus.
_Impact Machine._--In Building No. 17, at the south side, is an impact
machine designed to gauge the sensitiveness of explosives to shock. For
this purpose, a drop-hammer, constructed to meet the following
requirements, is used: A substantial, unyielding foundation; minimum
friction in the guide-grooves; and no escape or scattering of the
explosive when struck by the falling weight. This machine is modeled
after one used in Germany, but is much improved in details of
construction.
The apparatus, Fig. 1, Plate XI, consists essentially of the following
parts: An endless chain working in a vertical path and provided with
lugs; a steel anvil on which the charge of explosive is held by a steel
stamp; a demagnetizing collar moving freely in vertical guides and
provided with jaws placed so that the lugs of the chain may engage them;
a steel weight sliding loosely in vertical guides and drawn by the
demagnetizing collar to determinable heights when the machine is in
operation; a second demagnetizing collar, which may be set at known
heights, and provided with a release for the jaws of the first collar;
and a recording device geared to a vertically-driven threaded rod which
raises or lowers, sets the second demagnetizing collar, and thus
determines the height of fall of the weight. By this apparatus the
weight may be lifted to different known heights, and dropped on the
steel stamp which transmits the shock to the explosive. The fall
necessary to explode the sample is thus determined.
The hammers are of varying weight, the one generally used weighing
2,000 grammes. As the sensitiveness of an explosive is influenced by
temperature changes, water at 25° cent. is allowed to flow through the
anvil in order to keep its temperature uniform.
_Flame Test._--An apparatus, Fig. 2, Plate VIII, designed to measure
the length and duration of flames given off by explosives, is placed at
the northeast corner of Building No. 17. It consists essentially of a
cannon, a photographing device, and a drum geared for high speed, to
which a sensitized film may be attached.
About 13 ft. outside the wall of Building No. 17, set in a concrete
footing, is a cannon pointing vertically into an encasing cylinder or
stack, 20 ft. high and 43 in. in diameter. This cannon is a duplicate of
the one used for the ballistic pendulum, details of which have already
been given. The stack or cylinder is of ¼-in. boiler plate, in
twenty-four sections, and is absolutely tight against light at the base
and on the sides. It is connected with a dark room in Building No. 17 by
a light-tight conduit of rectangular section, 12 in. wide, horizontal on
the bottom, and sloping on the top from a height of 8¼ ft. at the stack
to 21 in. at the inside of the wall of the building.
The conduit is carefully insulated from the light at all joints, and is
riveted to the stack. A vertical slit, 2 in. wide and 8 ft. long,
coincident with the center line of the conduit, is cut in the stack. A
vertical plane drawn through the center line of the bore-hole of the
cannon and that of the slit, if produced, intersects the center line of
a quartz lens, and coincides with the center of a stenopaic slit and the
axis of the revolving drum carrying the film. The photographing
apparatus consists of a shutter, a quartz lens, and a stenopaic slit, 76
by 1.7 mm., between the lens and the sensitized film on the rotary drum.
The quartz lens is used because it will focus the ultra-violet rays,
which are those attending extreme heat.
The drum is 50 cm. in circumference and 10 cm. deep. It is driven by a
220-volt motor connected to a tachometer which reads both meters per
second and revolutions per minute. A maximum peripheral speed of 20 m.
per sec. may be obtained.
When the cannon is charged, the operator retires to the dark room in
which the recording apparatus is located, starts the drum, obtains the
desired speed, and fires the shot by means of a battery. When developed,
the film shows a blur of certain dimensions, produced by the flame from
the charge. From the two dimensions--height and lateral
displacement--the length and duration of the flame of the explosive are
determined.
The results of flame tests of a permissible explosive and a test of
black blasting powder, all shot without stemming, are shown on Fig. 2,
Plate IX. In this test, the speed of the drum carrying the black powder
negative was reduced to one sixty-fourth of that for the permissible
explosives, in order that the photograph might come within the limits of
the negative. In other words, the duration of the black powder flame, as
shown, should be multiplied by 64 for comparison with that of the
permissible explosive, which is from 3,500 to 4,000 times quicker.
_Apparatus for Measuring Rate of Detonation._--The rate at which
detonation travels through a given length of an explosive can be
measured by an apparatus installed in and near Building No. 17. Its most
essential feature is a recording device, with an electrical connection,
by which very small time intervals can be measured with great exactness.
The explosive is placed in a sheet-iron tube about 1½ in. in diameter
and 4 ft. long, and suspended by cords in a pit, 11 ft. deep and 16 ft.
in diameter. This pit was once used as the well of a gas tank, Fig. 2,
Plate VIII. In adapting the pit to its new use, the tank was cut in two;
the top half, inverted, was placed in the pit on a bed of saw-dust, and
the space between the tank and the masonry walls of the pit was filled
with saw-dust. The cover of the pit consists of heavy timbers framed
together and overlaid by a 12-in. layer of concrete reinforced by six
I-beams. Four straps extend over the top and down to eight “deadmen”
planted about 8 ft. below the surface of the ground.
The recording device, known as the Mettegang recorder, Fig. 2, Plate
VII, comprises two sparking induction coils and a rapidly revolving
metallic drum driven by a small motor, the periphery of the drum having
a thin coating of lampblack. A vibration tachometer which will indicate
any speed between 50 and 150 rev. per sec., is directly connected to the
drum, so that any chance of error by slipping is eliminated. The wires
leading to the primary coils of the sparking coils pass through the
explosive a meter or more apart. Wires lead from the secondary coils to
two platinum points placed a fraction of a millimeter from the periphery
of the drum. A separate circuit is provided for the firing lines.
In making a test, the separate cartridges, with the paper trimmed from
the ends, are placed, end to end, in the sheet-iron tube; the drum is
given the desired peripheral speed, and the charge is exploded. The
usual length between the points in the tube is 1 m., and the time
required for the detonation of a charge of that length is shown by the
distance between the beginning of two rows of dots on the drum made by
the sparks from the secondary coil circuits, the dots starting the
instant the primary circuits are broken by the detonation. At one end of
the drum are gear teeth, 1 mm. apart on centers, which can be made to
engage a worm revolving a pointer in front of a dial graduated to
hundredths; by means of this and a filar eyepiece, the distance between
the start of the two rows of spark dots on the drum can be measured
accurately to 0.01 mm. As the drum is 500 mm. in circumference, and its
normal speed is 86 rev. per sec., it is theoretically possible to
measure time to one four-millionth of a second, though with a cartridge
1 m. long, such refinement has not been found necessary.
The use of small lead blocks affords another means of determining the
rate of detonation or quickness of an explosive. Each block (a cylinder,
2½ in. long and 1½ in. in diameter) is enclosed in a piece of paper so
that a shell is formed above the block, in which to place the charge. A
small steel disk of the same diameter as the block is first placed in
the shell on top of the block, then the charge with a detonator is
inserted. The charge is customarily 100 grammes. On detonation of the
charge, a deformation of the lead takes place, the amount of which is
due to the quickness of the explosive used (Fig. 3, Plate VIII).
Sample Record of Tests.
The procedure followed in the examination of an explosive is shown by
the following outline: