501 
-5- 
(the cathode-ray stream) possesses negligible inertia, and is capable of responding instantaneously to 
electrostatic changes. The oscillograph is quite free from resonance, being equally sensitive at al) 
frequencies,and will therefore indicate faithfully the fluctuations of electric charge produced by an 
impulsive pressure on the faces of the piezo-electric crystals. The cathode-ray oscillograph consists 
essentially of an evacuated tube in which a beam of cathode rays (high speed negatively charged electrons). 
is generated by means of a hot filament and suitable accelerating potentia}. a fine pencil of these rays 
passes between a pair of electrostatic deflecting plates and the poles of an electro-magnet, and ultimately 
falls on a phosphorescent observing screen or a photographic plate. The electrostatic deflecting plates 
are connected to the faces of the piezo-electric crystal and consequently the cathode stream is deflected 
in proportion to the magnitude of the charge on the crystal faces i.e. to the pressure applied. The 
electro-magnet is employed to deflect the rays at a known rate and in a direction at right angles to the 
electrostatic deflection, i.e. to generate a ‘time-axis’ for the pressure-time* record. The linear 
deflection of the cathode-rays is obtained by making use of the rise of current in the magnet circuit 
containing suitable inductance, resistance and voltage. Jn the early stages the rise of current is 
practically limear and the corresponding deflection of the cathode-rays is, to the required degree of 
accuracy, also a linear function of time. The timing arrangements, to bring the pressure-time (p/t) record 
to the middle of the plate, are rendered automatic by means of a pressure-switch in the sea situated between 
the charge and the p.e. gauges and connected in series with the above magnet circuit. 
The distances of the switch from the gauge and the speed of traverse of the spot are adjusted 
according to the circumstances of the record to be obtained. When the charge is fired the pressure wave 
first closes the pressure switch, thereby starting the cathode-rays on their traverse across the plate, 
and then reaches the p.e. gauge which develops an electric charge and consequently deflects the cathode- 
rays in a direction at right angles to the time axis. 
In this manner, knowing the pressure-sensitiveness of the p,e. gauge and the rate of traverse of 
the time axis, we obtain a p/t record on the photographic plate. It is of course necessary to calibrate 
the crystal vessel at known pressures and to determine the sensitivity and speed of the 'spot' of the 
Cathode-ray oscillograph under different conditions. 
With this information the explosion records obtained by the method just outlined give at once the 
pressure—time curve for the explosion pulse. 
Apparatus. 
The cathode ray oscillograph used throughout the investigation is of a particularly robust type 
suitable for use on board a rolling ship with engines throbbing = it has now been in continuous use ina 
ship for 2 years, and of Several occasions under very severe weather conditions, without breakdown. The 
piezo-electric gauge and the pressure switch have also been specially designed to withstand the forces near 
underwater explosions of large charges. 
(a) The Cathode Ray Oscillograph. 
The cathode ray oscillograph used in these experiments is illustrated in the Figure. The cathode 
consists of a spiral of tungsten or lime-coated platinum wire, which is heated by means of an insulated 
battery. The hot spiral is supported inside a cylindrical sheath which exerts a focussing action on the 
cathode rays as they travel towards the anode, The rays are accelerated by means of an applied voltage 
of 3000, the positive end being connected tc the anode and earthed. after passing through the pinhcle 
tube in the anode the cathode rays travel through parallel electrostatic and magnetic fields and ultimately 
fall on the phosphorescent screen. Removal of this screen exposes a photographic plate to the action of 
the rays. 
The cesses 
is We shall use p/t as an abbreviation for pressure-time* and p.e. for 'piezo-electric’. 
