800 
Sela 
This report therefore deals primarily with the development and 
re-design of the miniature gauge, and it is now believed that finality has 
been reached, although it is just possible that further trials may revoal 
minor improvemonts. It is hoped to measure peak pressures, momenta and 
onergies rosulting from an underwater explosion with high accuracy and 
very near to the cxploding charge. 
EXPERIMENTAL WORK: PREFACE 
(i) Mothod of suspending charge and gauge. 
The altoration in dosign of the small gauge has been brought about 
as a rosult of oxperimental work oarried out during the last few months 
under controlled water conditions. The depth of water is about 36 feot, 
the surface area boing 850 feet long and 120 feet wide, A boom, shown in 
Plate 1, was constructed to enable the charge and gauge to be suspended in 
the water about mid-way. 
(441) Distanoes and dopths and sizes of charges. 
The distance between charge and gauge was varied from approximately 
8 to 14. foot, but mainly kept constant at about 10 feet, and the depth of 
gauge and chargo was varied from 5 to 15 feet, but generally kept constant 
at about 10 fect. This report does not deal with aotual magnitude of the 
pressuros reoordod but only with the form of the pressure-time signatures 
in order that roliable data far explosive charges may be ascertained in the 
futuro. ‘The charges were generally if lb. blocks of T.N.T., but a 
considerable number of 14. oz. P.E. spherical charges were used latterly: 
the former charge was used without its oase on many occasions, 
(iii) Orientation of gauge. 
After some initial experiments, means were provided to enable the 
small gauge to be either face-on or edge-on to the charge, the two faces 
and opposite edges being separately directed at the pressure wave. The 
length of cable attached to the gauge would naturally hang vertically, 
but later on provision was also made for the two faces of the orystal to be 
both parallel to the water surface end at right angles to it, the cable 
attached to the gauge being then parallel and leading away from the 
direction of the pressure wave. The above cight orientations were possible 
by fixing the oable and gauge to a Tebar about 25 feet long which was 
fixed to a rotating attachment on the boom. 
(iv) Connection of gauge to amplifier. 
The gauge was connected to the amplifier via a 150 $o, 200 feet 
length of non-signeal cable doveloped at this Establishment(1) having a 
shunt capaoity not less than 5 times that of the gauge and cable. A 
resistance equal to the surge impedance of the cable was inserted between 
the oéntral conductor of the cable and the common point of the shunt 
condenser and the grid of the first valve. This method of connection 
shown in Appendix 1, is partioularly useful to avoid end refleotions when 
very long lengths of cable envisaged in field trials are being used. 
EXPERIMENTAL WORK: PART I. 
The numerous tests oarried out and desoribed below ran parallel 
with the development of perfecting the teohnique of construction of the 
Miniature gauge. Aftor a few early tests it was realised that thickness 
of electrodes should be reduced to a minimum, and that the best method 
was to spray conducting films on to the crystal faces. The effeot of 
insulation, in giving distortion to the signature of the pressure-time 
pulse from an underwator explosion, soon manifested itself. The gauge 
construotion finally develonod is described in Appendix 2. 
The first eccve 
