505 
-9- 
H.I1| Mines. These are of the standard service pattern shown in Figure 8 No. (12), fitted with 
an inner case containing 320 lbs. of 40/60 Amato?. This inner case is surrounded by an air space of 
considerable volume and a further iron container of spherical form having a diameter of 38 inches. The 
thickness of the wall of the inner charge is A inch and of the outer spherical shel) 4 inch. The 
influence of the large air space is referred to in Section IV of the report, In this case also the mine 
was fitted with a C.E—. primer and standard electrically-fired detonator. 
Special T.N.T. Charges. A number of specially designed T.N.T, poured filled charges were prepared 
by woolwich for the main purpose of explosion research = using all forms of gauge under widely conditions. 
Reference has already been made to the principal features of these charges which are shown diagrammatically 
to scale in Figure 8. It will be observed that the charge cases are completely filled with T.N.T. the 
linear dimensions of the inside of the case being approximately proportional to the cube root of the weight 
of the charge. The density of the T.N.T. filling is 1.67 app.» Three sizes of primet, of the same type 
and fitted with the same standard detonator, are shown in Figures 8, NOs 8, 9 and 10 and also fitted to 
the T.N.T. Charges Nos. (1) to (7). 
Capacity and Insulation of the Piezo Blectric Circuit. 
The voltage V developed in the circuit when a pressure p Ibs/sq. in. is applied to a total area a 
Sq- ins. of tourmaline crystals is 
V = 8.65 x 10° 
PeA 
Cc 
where C is the capacity of the circuit measured in farads. Since the deftection § of the cathode rays is 
directly proportional to Vv it will be seen that this relation has an important bearing on the interpretation 
of the oscillograph record. Not only is it necessary that the value of C should be known accurately, but 
it is also important that it should be kept as small as pcssible so that the oscillograph deflections 8 & 
V should provide an accurate measure of the pressure. Since C is determined mainly by the length of cable 
between the gauge and the ship carrying the oscillograph, this latter condition implies that the cable should 
be as short as possible. against this, however, is the fact that the charge must be placed at a distance 
from the Ship not less than the ‘minimum safe distance’ (determined by experience). The ‘safety’ limit, 
fixes the minimum length, and consequently capacity, of cable 4 to be used. This being known, sensitivity 
must be increased by increasing the area of tourmaline crystals or the sensitivity of the oscillograph. 
For certain reasons, the latter method was not practicable, consequently the diameter of the gauge generally 
employed was fixed at 7 inches, the effective area of crystals in this case giving sufficient sensitivity 
whilst meeting the conditions relating to the safety of the ship. 
The type* and length of cable having been decided upon it is necessary to know accurately the value 
of the capacity of the circuit at the time of firing the charge. This quantity is measured immediately 
before.and after firing on every occasion - the a.c. Dridge shown in Figure 7a being employed for the 
purpose. Capacity measurements are considered accurate within 1%. 
Now the total electrical charge generated on the tourmaline crystals is a very small quantity although 
the exphosion pressure is high. Thus the explosion of 100 1b. T.N.T. at a distance of yo feet froma 7 inches 
p.e. gauge develops a charge of the order of 1076 coulombs, a very small electrical quantity. This charge 
only exists for a period less than 0.001 second during which it must be conveyed, without loss, to the 
oscillograph on the ship. It is evident, therefore, that the insulation of the circuit must be good in 
order that as little as possible of this charge shall leak away. From the point of view of leakage it is 
indeed fortunate that the electrical charge only exists for a very short period and that the capacity of the 
circuit is considerable. Thus if Ve is the voltage (© Q@p) in the circuit at any instant and Ve is its 
value at a time 't* after. 
Vt = 
+ Aerials were avoided whenever possible on account of the difficulties involved in fitting 
at sea. 
- The cable used with the p.e. gauges is pattern No. 1389A. H.T. Cable, having a capacity 
to earth of .0042 mfd. per 100 feet, the insulation resistance being very high. 
