626 
= ih = 
signal in the manner described later. Type (ii) is being proceeded with 
together with the development of amplifier,and the solution of the cable 
termination problem is being pursued. ‘Type (iii) is being considered but 
at the moment is in abeyance, pending the practicability of reduoing cable 
signal even if its physical interpretation cannot be explained completely. 
In the case of the type (i) gauge, its capacity is about 200 uL F; 
and if the gauge is connected by 250 yards of "standard" cable directly to 
the cathode ray oscilloscope, the total capacity of the oircuit will be 
about 14000 HUF. The piczo-elcotric constant of the tourmaline used was 
about 2.2 x 10717 coulombs/dyne. Knowing this value, and gesuming that 
the gauge is required to record a pressure of 1000 1lbs./in.© from an 
explosion pulse, it can be calculated that the voltage developed aoross the 
cathode ray plates will be 33.3 volts. For the same length of oable, and 
assuming 30 feet of it to be subjected to the same pressure all in phase, 
the cable signal will be < 00 x 30 x 0.172 volts = 0.92 volts. Hence the 
cable signal is approximately 4% of the gauge signal, the direction being 
opposite to that of tho pressure pulse. Since the duration of cable signal 
is long, its effect on impulsc and energy is vory large with this cable. 
To consider the type (ii) gauge, its capacity is only about 20uuF, 
and under similar conditions to those already described, the voltage developed 
across the cathode ray plates will be only 1.11 volts. It is now seen that 
the cable signal may be of the same order as the crystal signal. 
It need only be pointed out that the voltage develcped across the 
type (iii) gauge oan be calculated to be approximately 0.28 volts under 
similar conditions. The cable signal voltage will therefore be over 3 times 
as large as that developed by the crystal. 
It should be noted that the untreated’ "standard" cable referred to 
in this report exhibits a rather large cable signal but the order of 
magnitude of the effect is the same over the range of untreated cables so 
far cxamined. The form of signal obtained with the "standard" cable whether 
the outer covering be T.R.S. or P.V.C. is almost identical to the signal 
obtained in calibrating large piczo-eleotric gauges in the pressure pot, 
except for direction or polarity. 
FURTHER TESTS WITH CABLE (b). 
In order to try and elucidate the properties of oable signal, it was 
next decided to try to protect the cable in the pressure pot. Two and a 
half feet of the cable with the outer P.V.C. covering was sealed inside a 
strong canvassed rubber tubing 3" internal diameter and 14" external diameter 
with a seal at either end to prevent any trace of water seeping through. 
A column of air would therefore surround the portion of cable under test. 
The reduction in cable signal was only 20% and therefore insignificant. 
(It is remarked that probable a metal tube as outer cover to protect the 
cable might have given a bettcr result, since it could withstand the 
subjected pressure without GaPounttony: 
The reinforced canvassed tube was next removed, as well as 2} ft. of 
P.V.C. thus exposing the outer layer of telcothene to the water. The 
cable signal was sensibly of the same order of magnitude although a 20% 
inorease was observed, in two tests. 
A length of cables was then obtained from the manufacturer, closely 
resembling cable (b) except that (i) the P.V.C. outer covering was removed, 
(ii) the thickness of the outer telecothene layer was increased to make the 
overall diameter approximately the same as cable (b), and (iii) a lapping 
of copper foil 0.0015" thick was taped over the outer surface of the inner 
telcothene core, ice, underncath the braid. The cable signal was still 
not significantly reduced, the value obtaincd being about 4 of that 
obtaining for cable (b). 
At cecce 
f# The explanation for this test is given later. 
