518 
Pees 
(3) irregularities in P/T Records. 
In addition to the regular rise and fall of pressure which alone has been considered up to this 
point, the majority of p/t records exhibit irregularities of a more or less serious character. These 
irregularities have in some cases been found to originate in the charge itself, it others in the pre. 
gauge, whilst in others they have been traced to the influence of bodies external to both the charge and 
the gauge. 
(i) 
@) 
(8) 
ty) 
The irregularities are classified as periodic and non-periodic. 
The periodic fluctuations of pressure in the records have been traced to the following 
Causes. 
Resonant vibrations in the gauge mounting. When the tourmaline crystals are mounted 
inside a cast brass ring (as in Figure 4a) it was found in every instance that the p/t 
record showed a superposed damped oscillation of frequency about 4000/second. The 
initia) amplitude of this oscillation was a considerable proportion (10% or so) of the 
total pressure amplitude applied directly to the tourmaline. With a gauge mounting of 
ebonite or Xylonite these oscillations, if present at all, are negligible, Record g 
Sheet 3 shows the result obtained with an ‘ebonite' and a ‘brass' gauge exposed to the 
same shot (184 lbs. G.C.). The oscillations due to the brass gauge are very pronounced. 
Resonant oscillation of the tourmaline slabs. A certain number of records, but not all, 
show indications of high frequency small amplitude oscillations which may possibly be 
ascribed to longitudinal oscillations of the tourmaline slabs in the direction of the 
electric axis. These oscillations are shown in Record abc ef h j and k on Sheet 3, 
In a bce and f the gauge used was 7 inches circular broadsideon; in "h’ the gauge was 
a ‘strip’ 15 inches long used end-on; in j ‘strip’ and 7 inches circular both broadside 
on; and in 'k' 7 inches circular edge-on and 7 inches circular broadside-on. 
It will be seen therefore that the high frequency oscillations indicated by ‘dots‘ 
on the record, occur whatever the orientation or the gauge with respect to the charge. 
In some cases, but not in all, it is possible to reconcile the frequency of these 
oscillations with the simple theory of resonant oscillations in the tourmaline slabs, 
Thus in record 'j' the period of the dots in the first peak is 0.31 x 107" second, 
Taking 6000 metres/second as the velocity of elastic waves in tourmatine and the thickness 
involved 2.2 cms. the calculated period should be 0.37 x 107" second in fair agreement 
with the record. Such agreement, however, is not always obtained and we must look for 
other causes also. It is interesting to note, in passing, that the dots in records a 
b c Sheet 3 appear in pairs. 
It is difficult to explain why these oscillations should appear in some records but 
not in others - it is possible that some slight initial variation in the form of the 
incident p/t pulse may produce resonance in the tourmaline on one occasion but not in 
another. An examination of this point and a test of gauges having tourmaline slabs of 
different thicknesses would be interesting. 
Electrical Resonance in p.e. Circuit. A possible cause of high frequency oscillation 
on the p/t record Is the existence of inductance and capacity in the electrical circuit 
of the p.e. gauge and oscillograph. The inductance (calculated) of the single core 
cable with earth return as used in the present experiments is approximately 0.5 x 1074 
henry per 100 feet. With a 300 feet length of cable, of total capacity of 1078 farad, 
the frequency of the superposed oscillations wil) be 10° ~/second approximately, which 
is of the same order of frequency as the oscillations due to resonance in the tourmaline 
slabs (see (G) above). The cause of the high frequency oscillations recorded may, 
therefore, be due either to (Z) or (y) or both, and can only be distinguished with 
certainty by varying L and C or by varying the thickness of the tourmaline. A simple 
test would be to use two gauges, having tourmaline of different thickness, In the same 
circuit for a record of the same shot. 
In any case the amplitude of these oscillations is too small to have any practical 
significance, as the records show. 
Chit) reretetareis 
