_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 -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 8 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 circ.u.mference 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 1 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 s.p.a.ce 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 pa.s.s 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 circ.u.mference, 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, 2 in. long and 1 in. in diameter) is enclosed in a piece of paper so that a sh.e.l.l 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 sh.e.l.l 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:
1.--_Physical Examination._
(_a_).--Record of appearance and marks on original package.
(_b_).--Dimensions of cartridge.
(_c_).--Weight of cartridge, color and specific gravity of powder.
2.--_Chemical a.n.a.lysis._
(_a_).--Record of moisture, nitro-glycerine, sodium or pota.s.sium nitrate, and other chemical const.i.tuents, as set forth by the a.n.a.lysis; percentage of ash, hygroscopic coefficient--the amount of water taken up in 24 hours in a saturated atmosphere, at 15 cent., by 5 grammes, as compared with the weight of the explosive.
(_b_).--a.n.a.lysis of products of combustion from 100 grammes, including gaseous products, solids, and water.
(_c_).--Composition of gaseous products of combustion, including carbon monoxide and carbon dioxide, hydrogen, nitrogen, etc.
(_d_).--Composition of solid products of combustion, subdivided into soluble and insoluble.
_3.--A Typical a.n.a.lysis of Natural Gas._
Used in tests, as follows:
Carbon dioxide 0.0 per cent.
Heavy hydrocarbons 0.2 " "
Oxygen 0.1 " "
Carbon monoxide 0.0 " "
Methane 82.4 " "
Ethane 15.3 " "
Nitrogen 2.0 " "
----- 100.00 per cent.
_4.--Typical a.n.a.lysis of Bituminous Coal Dust, 100-Mesh Fine, Used in Tests._
Moisture 1.90 Volatile matter 35.05 Fixed carbon 58.92 Ash 4.13 ------ 100.00 Sulphur 1.04
_5.--An Average a.n.a.lysis of Detonators._
Used on Trauzl lead blocks, pressure gauge, calorimeter, and small lead blocks:
M - l(l/m). Triple-strength exploder.
Charge 1.5729 grammes.
Mercury Chlorate fulminate. of potash.
Specification 89.73 10.27
Used on all other tests:
M - 260(l/m). Double-strength exploder.
Charge 0.9805 grammes.
Mercury Chlorate fulminate. of potash.
Specification 91.31 8.69
_6.--Ballistic-Pendulum Tests._
This record includes powder used, weight of charge, swing of mortar, and unit disruptive charge, the latter being the charge required to produce a swing of the mortar equal to that produced by lb. (227 grammes) of 40% dynamite, or 3.01 in.
_7.--Record of Tests._
Tests Nos. 1 to 5 in Gallery No. 1, as set forth in preceding circular.
_8.--Trauzl Lead-Block Test._
Powder and test numbers, expansion of bore-hole in cubic centimeters, and average expansion compared with that produced by a like quant.i.ty (10 grammes) of 40% dynamite, the latter giving an average expansion of 294 cu. cm.
_9.--Pressure Gauge._