511 
-15- 
(a) Variation of P, max with Distance. 
Two distinct methods of determining the variation of maximum pressure with distance from the charge 
have been employed. In the first method a number of charges are fired and records made with a piezo— 
electric gauge at different distances from the charge. From measurements of the maximum deflections of the 
individual records it is possible therefore to plot a curve connecting P max and distance. As we have 
already pointed out, however, the maximum pressure varies considerably from charge to charge (apparently 
similar) and it is necessary to make a number of records at each distance in order to obtain a reliable 
average. 
Observaticns with a large number of 24 1b. and 184 1b. gun-cotton charges and a number of larger 
T.N.T. Charges, indicate that the pressure falls off inversely as (distance) 1-9 within the limits of 
experimental error. 
In view of the fact that charges are so variable and that slight errors in calibration of gauges 
and the osci lograph might affect the deductions, it seemed very desirable to devise a method which is quite 
independent of the differences between one charge and another and eliminates any assumption with regard to 
the gauge sensitivity or the oscillograph circuit. The method involves only a knowledge of the distances 
of tne gauge from the charge and the deflections on the oscillograph record, 
The principle of the second method is as follows:— 
Two gauges A and 8, connected to one oscillograph, are laid at distances Dy and 0, respectively froma 
charge, and a 'slow-traverse' record is taken when the charge is fired. Each gauge will record its pressure 
peak on the plate in the oscillograph. Suppose 8, and 3, are the measured amplitudes of these peaks. 
Then, assuming the pressure to vary as 0% we have 
S a x ‘I > x 
3, = ak, Dy ; 3, ak, 0, 
wnere a involves all the parameters external to the gauges (charge, oscillograph sensitivity, capacity of 
circuit) and Ky Ky depend only on the gauges, dividing 3, by 8, we eliminate a and obtain 
x 
lebih bal i 
= 1 
Ly Ly \ERh, 
Similarty, if a second shot be recorded with the gauges A B at distance D Dy giving peak amplitudes 
5 3 
b, and by respectively 
x 
pee ab % 
5, kK \ 0, 
Dividing (1) by (2) we have 
x 
ot °y =i) a thereby eliminati 
= —) — ereby e imina ing K K 
8, $, rae sa? 
whence 
8, 8 Da wo 
x = Toa} <= r hOgt eee ee (3) 
Bivad Beas 
In the practical application of this method three similar gauges A B C were employed simultaneously, 
and three 184 1b. G.c. Charges © By were fired, the gauges being interchanged (AC, CAB, BCA) in the 
respective shots. This was done to obtain as much data as possible with the fewest shots. The 
results are given in the following table;- 
Shots « cesses 
