510 
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Method of Ex teriment. 
Whilst the trot is being laid the oscillograph is evacuated and adjusted until a suitable recording 
*spot' is obtained. A general preliminary test of the various circuits~deflecting magnet, relay, E.S. 
plates etc. is then made. When the trot is laid and the p.e, cable and pressure switch cable joined to 
their respective circuits, after leaving a sufficient tength in the sea to ensure the ship being at a 
safe distance from the charge, a careful measurement is made of the insulation and capacity of the complete 
p.e. circuit. 
The insulation must be good (say > 50 megohms) and the capacity must be adjusted, by adding standard 
capacity, until it is estimated from the experimental conditions that the deflection of the cathode rays 
will be satisfactory. A final test of insulation and capacity is made just before firing the charge, after 
all cables have been made fast and the capacity is steady. The deflecting magnet circuit is adjusted to 
the required speed of traverse of the spot and a signal given 10 seconds before firing. At this signal the 
second vessel keep$ the trot wire at a steady tension, a kinema* is started, and an operator stands by the 
firing key which is in the oscillograph room. At another signal the door of the oscillograph camera is 
opened and immediately afterwards the charge is fired. The insulation and capacity of the p.e. circuit are 
now checked — if possible, for very frequently the cables break and an ‘earth’ appears — before the trot is 
weighed. A record is also made of the time scale (at known frequency) the conditions of the deflecting 
circuits remaining exactly as in the exptosion experiment. The operation of opening camera door, firing 
and reclosing the door should not take mae than a fraction of a second. The tonger the door is open, the 
greater is the danger of fogging the plate from diffuse cathode radiation. 
im some cases where a very slow traverse of the spot (e.g. .01 to .1 second) is to be made, as ina 
bottom echo record, it is necessary to employ an auxiliary switch, closing with the firing switch, to cause 
the spot to jump on the left-hand edge of the plate at, or before, the instant of firing. This switch 
introduces a resistance shunting the auxiliary deftecting magnet, thus reducing the initial deflection of 
the spot to the left. In this manner, the spot is on the plate for a short interval just before the pressure 
switch is closed. Otherwise, it would not, in many cases be possible for the pressure switch to bring the 
spot on the plate before the arrival of the pressure wave at the gauge. This device has proved very 
serviceable in many cases where the pressure switch alone would have been of little use, 
On certain occasions simultaneous records have been made with two cathode ray oscillographs. 
Section B. Experimental Results. 
after a preliminary series of experiments with smal) charges during which the technique of the method 
was greatly improved, it was possible to count on obtaining a satisfactory record of atmost every shot and to 
examine the effect of the size and orientation of the gauge on the recorded pressure-time curve, and of the 
proximity of air cavities or resonant structures near the gauge. 
Variation of Maximum Pressure, Momentum and Energy in the Pressure Pulse with 
Weight and Distance of Charge. 
Data obtained from a large number of small charges were examined ta eliminate accidental irregularit— 
ies in the charges and were used as a Check on the results obtained from a number of large charges. The 
latter series of large charges consisted of specially designed T.N.T. charges of welghts varying from 20 
to 2000 Ibs. and a few 300 1b. amatol depth charges and H.2 mines. The special charges were al.) prepared 
from the same melt of T.N,T. of density 1.67 and every effort was made to render them similar. 
The experimental results given in what follows refer only to the region where the maximum pressure 
developed in less than 2 tons per square inch. 
(a), acces 
, jn some cases a kinema is used to photograph the spray and 'plume* phenomena accompany the 
explosion. Otherwise the 10 second signal is not required except as a genera) warning signal. 
