M. J. Tucker and A. R. Stubbs 315 
the specifications of this cable are: 
Over-all diameter...... 0.270 in. 
Weight in water........ 150 lb per 1000 yards 
Breakin oelOaG ey mueriaerene = 2500 Ib 
Resistance per core..... 40 ohms per 1000 yards 
An 8000-yard length would be required to operate gear near the bottom in deep 
water. The weight of the cable alone is half its breaking strain, which is not a 
good factor of safety, even with no equipment on the end. At a towing speed of 
one knot, the wire would stream out at an angle of about 45°, so that only very 
low towing speeds would be practicable, and of course the wire is expensive, 
costing £5,760 for the 8000 yards. 
This is, however, an extreme example, and itis often a practical proposition 
to use such cables at shallower depths. There are also applications, such as 
underwater television, where it would not be possible to bring the information 
back over an acoustic link. 
16.4.2. The ‘‘Pinger’’ 
Though in most applications the "Pinger" is perhaps not used as a telemeter 
in the most conventional sense, it is a device widely used for obtaining infor- 
mation about equipment in the sea. It consists, in its simplest form, of a simple 
relaxation oscillator (Fig. 16.15) in which a capacitor is charged from a battery 
and discharged through a coil round a nickel "scroll" transducer using a gas 
tube as a switch. The pulse of current produces a momentary contraction of the 
ring by the magnetostriction effect and the ring then oscillates at its natural 
frequency, giving a damped train of waves. The circuit illustrated, using a 10- 
keps scroll, has a range of four miles in favorable circumstances. 
In this simple, free-running form, it is used by Swallow [14] in his system 
for deep-current measurement. He has a neutrally buoyant float which is less 
compressible than sea water, so that if suitably weighted it will sink to a pre- 
determined depth and then move around with the water at that depth. The float 
contains a Pinger, and is tracked using a directional hydrophone system aboard 
a ship. A float has been tracked for as long as six weeks in this way. 
NSP I 
360 v 
BATTERY 
TRANSDUCER 
Fig. 16.15. The relaxation oscillator circuit of a "Pinger." 
