503 
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(varying in shape and size) suitably mounted - care being taken to observe that al} the pieces have their 
polarity in the same direction. The design of the gauge itse?f is a matter of considerable importance, 
as we Shal? see later, both the material and the dimensions of the mounting having an important bearing on 
the results obtained. Experiments have been made with a number of different types of gauge, but in al? 
cases the following important considerations have been kept in mind in the design:= 
(Note:= In the original paper follows a detailed discussion of the following points:— 
(i) Insulation 
(ii) absence of air cavities 
(iii) Material of the mounting 
(iv) Dimensions of gauge) 
(c) The Pressure Switch. 
AS we have already stated in the general outline of the p.e. method, the function of the pressure 
switch is to close the magnet deflecting circuit in the oscillograph, thereby starting the cathode rays in 
their traverse across the plate. It is essential that the switch should close quickly (in a time of the 
order of 10" second) should not re-open, and should be sufficiently robust to withstand the blow of the 
explosion wave at a short distance from the charge, The type of pressure-switch finally adopted is shown 
in Figure 6a. It consists essentially of a pair of stainless stee} coned points, electrically connected 
by means of a twin-core cable to the ship, mounted in a massive brass cylinder as shcwn. A lead disc 2s 
inches thick is clamped by a screwed cap in the end of the cybinder and just clear of the tips of the 
stainless steel points. The body of the gauge behind the points is filled with ebonite and insulating 
Compound, the whole being water-tight and free from air cavities. The lead disc is sufficiently stiff to 
prevent making contact across the points by average hydrostatic pressure (say up to 40 feet depth of water), 
but is easily forced into contact by the explosion pressure which often reaches 2000 1bs/sq. inch or more. 
These large pressures shearthe lead disc at the clamping edge and impale it on the steel points, thus 
closinu the magnet deflecting circuit of the oscillograph. The pressure—switch can be used again for the 
next explosion, after cleaning and fitting with a new lead disc. In the majority of cases the pressure— 
switch was arranged at a distance of 8 to 10 feet from the gauge on the side nearer the charge. Taking 
the velocity of the explosion wave as 5000 feet per second this gives a time interval between 1 and 2 
thousands of a second before the pressure pulSe reachee the p.e. gauge. Combined with a knowledge of the 
rate of traverse of the spot across the plate this provides the necessary information to tocalise the p/t 
record in a suitable position on the photographic plate. tn special cases, of course, it was often 
necessary to vary the distance of the pressure switch from the gauge, time intervals being reckoned as 
before, i.e. 5 feet per .001 second (allowing 2 or 3 x 10” second for the lag in closing the pressure 
switch)s On account of an element of uncertainty in the electrical resistance of the pressure switch, 
which would throw doubt on the actual rate of traverse of the spot across the plate it was considered 
advisable to insert a quick-acting relay® in the circuit, the "local' contacts of the relay to act in 
parallel with the pressure switch. The operation of the pressure switch in this case not only starts 
the cathode ray moving across the plate but also actuates the relay, the local of which closes (and remains 
closed) at least .001 second before the pressure wave reaches the p.e. gauge. In this manner the time 
constant of the deflecting circuit during the p/t record is rendered perfectly definite and any complications 
due to a possible re-opening of the pressure-switch are obviated. 
(Note:- In the original paper follows 
(d) Description of the electrical circuits and auxiliary apparatus, including 
WP ppace- 
bs A converted Brown reed telephone with a tag, under the conditions of working, of not 
more than ,0005 second, 
