631 
ADDENDUM 
THE ELECTRIC POTENTIALS PRODUCED IN THE CONDUCTORS 
OF A SUBVWERGED CABLE BY UNDERWATGR EXPLOSIONS. 
SUMMARY. 
Further work on the lines of this report has resulted in the practical 
elimination of unwanted signals induced directly by underwater explosions in 
cables used for piezo-clectrio measurements. 5 
DISCUSSION OF RESULTS. 
(1) Further development has proceeded with a view to reducing "cable 
signal" in the oase of the cable shown in Plate 1 of the above report. 
Two changes have been made, namely, (a) graphiting the outer surface 
of the inner core of the oable underneath the braid with a much 
thicker coating than hitherto, and (b) graphiting the inner surface 
of the inner core which is in contact with the central conductors. 
(2) The manufacturers producing specimens of this cable for experimental 
research at this Establishment have now been able to manufacture on 
production scale an equally satisfactory cable as a hand-made 
specimen. 
(3) The magnitude of the cable,signal for 1 ft. of this cable when 
subjected to 1000 lbs./in.© pressure in a circuit of capacity 
2500 nu F was found to be 0.172 volt. In the previous report 
it was observed that this value had been rcduced to 1 millivolt 
for a production length whereas a hand-made length gave a value 
of about $ millivolt. The new figure achieved is of the order 
of 40 microvolts under the same pressure and circuit conditions 
when both cffects (a) and (b) above have been introduced, showing 
a reduction in "cable signal" of some 4000 times compared with 
untreated cable. The method of carrying out the tests is the 
stendardised one described in the above report. 
(4) The implications of this improvement may be illustrated by the 
following calculations. Assuming a total length of cable 250 yards 
( capacity 14000 micro=micro farads) of which 30 fte is immersed 
and subjeoted to a pressure of 1000 lbs. per square inch is used 
with a two ply gauge $" in diameter and i" thickness then the ratio 
of cable signal to gauge signal will not cxcecd 0.1%. If the 
crystal dimensions ere reduced to 3/16" diameter and 3/32" thickness 
the ratio will be about 0.56. 
CONCLUSIONS. 
The investigations reported in the report and this addendum have 
resulted in the production of a cable in which the unwanted parasitic 
Signals due to the impact of the explosive pressure on the cable have been 
reduced to negligible dimensions. 
26 This work may be regarded as a necessary and successful preliminary 
to the development of piezo-electric gauges of dimensions small enough to 
be regarded as suitable for recording the pressure history at a point. 
