624 
Eyes 
Road Research Laboratory working on the theory that cable signal arises from 
frictional charges in the oable dielectrio. 
The cable based on this research, was produced by Messrs. Callender 
and was tried here, but, although apparently entirely suitable for blast and 
small scale underwater work, it provided unsuitable for the conditions of 
trial required in this Establishment. 
The present report therefore deals with the continuation of the 
rescaroh, applying the general prinoiple to the particular cable defined above. 
The principle used in measuring oable signal is as follows; the cable 
is scalcd at one end and a length of several feet is subjected to hydrostatio 
pressure. The pressure is suddenly released by bursting a copper diaphragm 
and the signal received, amplified by mcans of a tvo-yalve amplifier, and 
then recorded on a cathode ray oscillograph. The amplifier had a reasonably 
flat response between 100 c/s and 50 ko/s. 
The use of long cables, properly terminated, together with the design 
of en amplifier for use with both large and small gauges, is being proceeded 
with; but in this conneotion it need only be stressed that for the tests 
described in this report the length of cable was kept approximately constant 
and as short as possible, and that the same amplifior and recording equipment 
was used throughout these tests. 
It will be showm that the minimun size of gauge thet oan be used for 
pressure-time measurements of an underwater explosion is entirely dependent 
upon the cable signal reduction that oan be achieved. To use small gauges 
without reducing cable signal to the order of 1% of the peak pressure expected 
from the explosion wave can cause incorreot interpretation of the reoords, 
particularly in impulse and energy determination because of the long duration 
of the cable signal. The oable signal value for any cable used with piezo- 
electric gauges must therefore be known. If it is not small, a correction 
for it may cause considerable ambiguity in determining the three main 
propertics from an explosion wave record. 
EXPERIMENTAL PROCEDURE AND RESULTS. 
The work that has been carried out on cable signal has been 
intermittent. For example, the cable developed for use with piezo- 
electric gauges has been brought to this final design for reasons explained 
in the report already referred to in the introduction; other cables of a 
standard type have been tested also for cable signals, in order to aim at an 
understanding of the causes of oable signal. Reference to those will not 
be made except where evidence helps to explain this phenomenon. 
(a) Test with "standard" cable; tough rubber sheath outer 
covering (1.8.3) 
Omitting the graphite layer, Plate 1 illustrates the cable used in 
the test. One end was completely scaled. The central conductors were 
sealed inside the inner teloothene core, and the braid inside the outer 
telcothene core. The outer tough rubber sheath was then rubber taped to 
prevent water from seeping through to the surface of the outer telcothene 
layer. The insulation resistance of the cable under test (capacity 
0.0014 F) was infinite, both between the central conductor and the braid, 
and between the braid and the outside pressure calibration pot or laboratory 
carth as measured by a 2500 volt megcer. Eightcen feet of the cable was 
placed inside the pressure pot and then subjected to a pressure of 
1000 lbs./in.*. The cable had to be shunted with a capacity of 0.075, F 
so that the deflection on the cathode-ray tube, expressed in volts, did not 
exoeed the linear output voltage of the 2 valve amplifier (amplification 625). 
Later as the cable was withdrawn from the pressure pot, the shunting capacity 
was reduced to 0.0204 F. The cable signal records obtained by a sudden 
release of pressure, ise. by bursting the copper diaphragms, are shown in 
Plate 2 sees. 
