Since the ratio of the volume of the cylinder to the volume of the
charge may be computed, the pressure of the confined charge may also be
found, and this pressure often exceeds 100,000 lb. per sq. in. The
cooling effect of the inner surface on the gaseous products of
combustion, a vital point in computations of the disruptive force of
explosives by this method, is determined by comparing the pressures
obtained in the original cylinder with those in a second cylinder of
larger capacity, into which has been inserted one or more steel
cylinders to increase the superficial area while keeping the volume
equal to that of the first cylinders. By comparing results, a curve may
be plotted, which will determine the actual pressures developed, with
the surface-cooling effect eliminated.
_Trauzl Lead Blocks._--The lead-block test is the method adopted by the
Fifth International Congress of Applied Chemistry as the standard for
measuring the disruptive force of explosives. The unit by this test is
defined to be the force required to enlarge the bore-hole in the block
to an amount equivalent to that produced by 10 grammes of standard 40%
nitro-glycerine dynamite stemmed with 50 grammes of dry sand under
standard conditions as produced with the tamping device. The results of
this test, when compared with those of the Bichel gauge, indicate that,
for explosives of high detonation, the lead block is quite accurate, but
for slow explosives, such as gunpowder, the expansion of the gases is
not fast enough to make comparative results of value. The reason for
this is that the gases escape through the bore of the block rather than
take effect in expanding the bore-hole.
The lead blocks are cylindrical, 200 mm. in diameter, and 200 mm. high.
Each has a central cavity, 25 mm. in diameter and 125 mm. deep (Fig. 1,
Plate IX), in which the charge is placed. The blocks are made of
desilverized lead of the best quality, and, as nearly as possible, under
identical conditions. The charge is placed in the cavity and prepared
for detonation with an electrical exploder and stemming. After the
explosion the bore-hole is pear-shaped, the size of the cavity
depending, not only on the disruptive power of the explosive, but also
on its rate of detonation, as already indicated. The size of the
bore-hole is measured by filling the cavity with water from a burette.
The difference in the capacity of the cavity before and after detonation
indicates the enlarging power of the explosive.
_Calorimeter._--The explosion calorimeter is designed to measure the
amount of heat given off by the detonation of explosive charges of 100
grammes. The apparatus consists of the calorimeter bomb (Fig. 1, Plate
VIII), the inner receiver or immersion vessel, a wooden tub, a
registering thermometer, and a rocking frame. This piece of apparatus
stands on the east side of Building No. 17.
The bottle-shaped bomb is made of ½-in. wrought steel, and has a
capacity of 30 liters. On opposite sides near the top are bored
apertures, one for the exhaust valve for obtaining a partial vacuum
(about 20 mm., mercury column) after the bomb has been charged, the
other for inserting the plug through which passes the fuse wire for
igniting the charge. The bomb is closed with a cap, by which the chamber
may be made absolutely air-tight. It is 30 in. high with the cap on,
weighs 158 lb., and is handled to and from the immersion vessel by a
small crane.
The inner receiver is made of 1/16-in. sheet copper, 30⅞ in. deep, and
with an inner diameter of 17⅞ in. It is nickel-plated, and strengthened
on the outside with bands of copper wire, and its capacity is about 70
liters. The outer tub is made of 1-in. lumber strengthened with four
brass hoops on the outside. It is 33 in. deep, and its inner diameter
is 21 in.
The stirring device, operated vertically by an electric motor, consists
of a small wooden beam connected to a system of three rings having a
horizontal bearing surface. When the apparatus is put together, the
inner receiver rests on a small standard on top of the base of the outer
tank, and the rings of the stirring device are run between the bomb and
the inner receiver. The bomb itself rests on a small standard placed on
the bottom of the inner receiver. The apparatus is provided with a
snugly fitting board cover. The bomb is charged from the top, the
explosive being suspended in its center. The air is exhausted to the
desired degree of rarification. The caps are then screwed on, and the
apparatus is set together as described.
[Illustration: PLATE IX.
Fig. 1.--Trauzl Lead Blocks.