627 
-5- 
At this stage it was thought that balanced-line cables might be 
selected to give a cable signal of the order of some 10% of the maximum 
cable signal of a single-conductor cable. But from considerations of 
the magnitude of cable signal as compared to the gauge signal, already 
calculated and described above, it was realised that the order of reduction 
which might be achieved by the use of balanoed-line cables was not 
sufficiently significant, in particular, due to the difficulty in obtaining 
electrical balanco, 
TESTS ON OTHAR CABLES. 
In order to gather evidence about the origin of cable signal, tests 
were carried out on other cables. Details are not given but the following 
general conclusions were drawn from the tests. 
(1) In the case of telcothene cables, unless the braid was also covered 
with telcothene, the cable signal appeared to be inconsistent in 
value for identical repeat shots even though the insulation 
resistance between the braid and carth was kept high by choice of 
& water-proof P.V.C. covering. 
(2) When the braid was exposed to the water, the polarity of cable 
signal was reversed: this was also accompanied by some reduotion 
in the value of cablc signal. 
(3) On one particular ocoasion, the cable signal reversed in polarity 
and increased considerably in magnitude, without any trace of 
water being able to penetrate to the braid through the P.V.C. 
covering; the only change made was in washing the pot and 
refilling with new water. 
(4) A similar cable to (3) but with a considerably less overall 
diameter showed a reduction of 2 : 1 in oable signal voltage 
between observations over two successive days. 
(5) One non-telcothene cable in particular was tested - a cable 
being used extensively, it is believed, by experimenters working 
with miniature gauges. It consisted of scveral central conductors 
enclosed in a soft synthetic thermo-plastic or rubber-like 
composition, the outside of which is bound with cloth; a tinned 
copper braid surrounded this and a cotton tape was wound over it, 
and then apparently a soft P.V.C. compound served as an outer 
covering. 
With successive subjecting to hydrostatic pressure the cable signal 
exhibited a form of hysteresis effect, becoming gradually worse. When 
water penetrated to the braid, the cable signal reversal and decrease was 
again observed. The cable signal varied in value between 1/30th and 1/10th 
of that obtained for the "standard" cable. 
DISCUSSION OF RESULTS. 
It was realised that the problem of cable signal was a most difficult 
one, and might mean a long-term research programme with special facilities 
for its elucidation. But by the following reasoning, an attack was made 
which enabled a practical solution to be achieved and a reduction of cable 
signal by a factor of some 500. 
The pressure applicd to the "standard" telcothene oable apparently 
oaused an electrostatic charge to be induced, and thus a potential 
difference to be developed between the central conductor and the braid. 
This electrostatic charge was shown from the experiments already described 
to be directly proportional both to the length of cable subjected to the 
pressure, and to the pressure itsclf. Also it was found that the 
potential eese..« 
