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EXPERIMENTAL WORK: . PART 2. 
It was realised that whilst the effects of thickness, material and 
method of attachment of electrodes were of fundamental importance, the fact 
that the steep front associated with the pressure wave from an underwater 
explosion had failed to manifest itself, indicated that the method of 
insulating the gauge with telcothene and rubber tape was unsatisfactory. 
It made the gauge into a mechanically robust unit, but at the expense of 
distortion of the pressure pulse. The amplifier had been proved to be 
capable of dealing with a very sharp wave: front). 
When calibrating a piezo-electric gauge in the pressure pot, as . 
previously described the record normally shows a time of rise of the 
order of a millisecond which is the duration associated with the bursting 
of the copper diaphragm but in the pressure pulse from a small underwater 
explosion, this time of rise is of the order of a microsecond. Hence, any 
insulation or clectrode variations are not likely to reveal themselves 
during calibration. 
Tests were therefore commenoed with changes in the mode of insulating 
the gauge. 
(a) Singlo-ply gauge without elecothene insulating layer 
The gauge had annealed copper foil clectrodes 0.022" thick, iee. as 
used in 1(s) and 1(t) above, except that the insulation consisted of three 
coatings or good quality rubber tape only, the inner telcothene layer 
having been removed. 
(b) Results with single-ply gauge, having only rubber insulation. 
The records were still far from being satisfactory, and for comparison 
with Record 6 of Plate 5 with the gauge perpendicular to the water surface, 
Record 1, Plato 6, is shovm. The only point to emphasise is that the 
telcothene layer had an effect on the form of the pressure pulse.- 
(c) Single-ply pause with rubber solution as insulator. 
The gauge had annealed copper foil electrodes 0,022" thiok as for 
(a) and (b) above; the rubber tape was replaced by rubber solution. 
Mechanically it did not prove a satisfactory construction. 
(a) Results with singlo-ply gauge having insulation of 
rubber solution. 
Plate 6, Recaqrds 2 and 3, show the pressure pulse obtained with the 
gauge face-on and edge-on respectively whereas Record 4, Plate 6, shows 
the pulse when the gauge had its faces perpendicular to the water surface. 
It will be apparent, for the first time, that the face-on record shows 
a tendency to a very steep front whereas the edge-on record and the record 
taken With the crystal face perpendicular to the water surface show somewhat 
equal and finite times of rise. Whilst the oscillations at the peak had yet 
to be eliminated, there was considerable improvement in the signatures. 
EXPERIMENTAL WORK: PART 3. 
Whilst evolving methods of sintering a metallic conductor on the 
surface of the tourmaline crystal and providing it with a thin rubberoid 
insulation, described later in Appendix 2, experimental tests proceeded 
with the insulation used in the large type of gauge, namely a mixture of 
25% paraffin wax and 75% vaseline. Tho miniature gauge was insulated by 
dipping it in this hot mixture, and allowing the minimum thickness to 
adhere on cooling. 
(a) igsccne 
