500 
-ye- 
from its maximum value but it supplies no information regarding the rise of pressure or regarding small 
fluctuations of pressure in the ‘fall’ curve, Again, all the above methods are based on certain 
assumptions relative to inertia in the mechanical system which indicates the pressure. Thus in Hilliar's 
methcd, assumptions are made regarding the inertia lags in the steel pistuns (with added water) and in the 
coppers. {In Walker’s ‘spray method’, assumptions are made relative to the size of the spray ‘droplets’ 
and to the forces retarding their motion through the air. In the ‘copper diaphragm' method also, thecry 
is faced with difficulties mainly concerned with the motion of a metal plate strained beyond its elastic 
limit. 1t is not proposed at this stage, however, to discuss these methods beyond the point of showing 
how necessary it is to utilize a more perfect means of indicating the form and magnitude of the explosion 
pulse. Such a method must be free from all mechanical time-lags and must give a ‘true’ record of the 
actual time-pressure variation within the explosive pulse. The nearest approach to such an ideal method 
is that originally proposed by Sir J. J. Thomson® which utilizes the piezo-electric properties of certain 
crystalline substances, e.g. tourmaline, with a cathode ray oscillograph to record the variations of 
electric charge developed on the tourmaline when subjected to the pressure pulse produced by the explosion 
of acharge. The feasibility of the method was first demonstrated by 0. a. Keys # at Admiraty Experimental 
Station, Shandon, Since that time, the method and apparatus employed have been considerably improved and 
during the past two years a large mass of data has been obtained with the improved apparatus and technique. 
It is the object of this report to describe the method and apparatus employed during the past two years for 
obtaining automatic photographic records of the pressure-time curve of explosion impulses, and to consider 
the more important results obtained. it is also a matter of interest, and considerable practical value, 
to compare these results with those deduced by the Hilliar ‘crusher gauge’ method, and to provide a standard 
of comparison for any other methods (such as the ‘copper diaphragm’ method) which might prove of practical 
value in the service as 'damage' indicators. 
The records obtained not only indicate the pressure-time sequence of the pulse, with the derived 
momentum-time and energy-time sequences, but also supply information relating to the reflection of the wave 
from the surface and bottom of the sea and also from the hull of a ship. Relations are obtained between 
the pressure, momentum and energy of the wave, and the size and distance of the charge from the piezo= 
electric gauge. Determinations have also been made of the velocity of the explosion wave in the neighbour— 
hood of the charge, and a method has been developed for recording at high speed the initial stages of the 
explosion wave jn order to obtain accurate data relating to the time of rise of pressure and the fine 
structure of the pressure pulse. 
Part A. Method and Apparatus. 
In this section it is proposed to deal with the more important details of technique and apparatus 
employed in obtaining photographic records of the explosion p/t curve, and to the methods of calibrating 
the piezo-electric gauge and the cathode-ray oscillograph. Consideration will also be given to certain 
precautions which must be observed in order to obtain ‘true’ records. 
General Outline of Method. 
Certain crystals e.g. tourmaline, quartz, rochelle salt etc, possess the property of exhibiting 
electrical charges on their faces when mechanical pressure is applied in certain directions #, The quantity 
of electricity liberated is found to be exactly proportional to the total pressure applied *, Hence a 
crystal of tourmaline may serve as a pressure gauge when used in conjunction with a suitable indicator of 
the electrostatic charge on its faces. if the pressure is of an impulsive character, as in the case of that 
developed by an under—water explosion, then it is essential that the indicator or recorder must be quick— 
acting and deadbeat. The cathode-ray oscillograph fulfils the required conditions, for the moving element 
(the! (elects 
Sir J. J. Thomson Engineering Vol. 107, 1919, pp. 543-54y, 
s 
D. A. Keys, Phit. Mag. 42, pp. 473-488, October (1921) 
4 This phenomenon was termed ‘piezo-electricity’ by Curie who discovered the effect in 1880. 
* This is true up to the point of fracture of the crystal. 
