3 117 
‘ 
The foregoing brief description gives no account of the difficulty of handling 
so much gear with the restricted facilities of a small drifter. Plenty of opportunities 
for fouling and confusion presented themselves when the lines were being streamed, 
and it was due to the skill and experience of the working party that everything went 
without a hitch in all except two or three shots. 
In any future trials it is recommended that the drifting method of suspension 
should be adopted, with a bottom-line as described in the first part of this section. 
The system is convenient to lay out and gives accurate distance, and it renders the 
experiments independent of tide. Suppose that a new explosive is to be tested 
against a standard, such as T.N.T., the following conditions would be suitable :— 
weight of charge, of each kind, 300 lbs. ; number of charges, of each kind, 3; the 
charges should be in depth-charge form, not surrounded by an air chamber; depth 
of charge, not less than 30 feet; two gauge lines, one each side of charge, at a 
horizontal distance of 50 feet from centre of charge; gauges on each line as follows— 
GF, GX (3-inch piston), GY (144-inch piston), GY (1,%-inch piston), GZ (24-inch 
piston), GZ (1g-inch piston), GA (44-inch piston), GA (83-inch piston); gauges 
1 foot apart, middle gauge at same depth as charge ; the depth of water should not 
be less than 20 fathoms. 
(22) Alternative Methods for determining the 
Time-pressure Curve; Hopkinson’s Pressure-bar Method ; 
Sir J. J. Thomson’s Piezo-electric Method. 
The results obtained with the gauges described in Sections 18 and 19 are so 
consistent and lead to such coherent conclusions that there is no doubt in the writer’s 
mind of their substantial correctness ; at the same time it is very desirable to check 
them if possible by some entirely different method. Two other methods are known 
for determining the time-pressure curve of an explosion. The first of these is 
Hopkinson’s pressure-bar method, described in the Philosophical Transactions of 
the Royal Society, 1913. In this method the pressure to be investigated is allowed 
to act on one end of a steel bar, in which it generates a corresponding pressure wave, 
which travels with the velocity of sound in steel, about 17,000 feet per second. At 
a given moment the space-distribution of pressure along the bar is a copy of the 
time-distribution of the pressure which has acted on the end of the bar. The steel 
bar is divided toward its further end, the opposed faces of the cut being carefully 
surfaced and held in firm contact. The compression wave passes this joint unaltered, 
and reaching the end of the bar beyond the joint is reflected as a wave of tension. 
Ata given moment the pressure at any section of the, bar is the algebraic sum of 
the effects of the forward-travelling compression wave and the returning tension 
wave. As long as the amplitude of the compression wave at the joint exceeds that 
of the tension wave the part of the bar beyond the joint, known as the time-piece, 
is held in contact, but as soon as the amplitude of the tension wave at the joint equals 
that of the compression wave the time-piece is free to separate, and flies forward 
with a definite momentum. This momentum, which is determined by a ballistic 
pendulum, is a measure of the time-integral of the force (pressure multiplied by area) 
that has acted across the joint up to the moment of separation. By a series of 
experiments with time-pieces of different lengths it is possible to determine the 
whole time-integral of the pressure, the maximum intensity of the pressure, and 
the time during which the pressure exceeds any given value. The results do not 
give the exact form of the time-pressure curve ; they give no information for example 
as to the relative rapidity of the rise and fall of the pressure ; if, however, the pressure 
is assumed to reach its maximum intensity instantaneously, as is probably very 
nearly the case, this limitation disappears and the time-pressure curve is definitely 
determined. 
Hopkinson’s method has been extensively used for investigating the pressure in 
the immediate neighbourhood of small charges, say, a few ounces or a few pounds of 
guncotton ; it is in fact the only accurate method available for the purpose. In these 
cases, however, the pressure is of the order-of 100 tons per square inch and is all 
over in a ten-thousandth of a second or less, while the present problem is to measure 
pressures of the order of one ton per square inch lasting several thousandths of a 
second. The difference in both respects is against the method. With a pressure 
so low as 1 ton per square inch the velocity of the time-piece would be very small, 
O AS 7498 
