502 
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The oscillograph is described in Proc. Phys. Sec. Vol. 35. February 15, 1923. {t will be seen 
from the illustrations that the instrument is of very rohust construction, ccnsisting mainly of strong 
metal parts and a minimum amount of glass. The oscillograph is evacuated to a pressure less than 0.001 mm 
of mercury by means of a Gaode mercury pump with suitable backing pump (a Fleuss hand pump or a rotary oi) 
*box' pump). Under normal conditions the desired vacuum can be obtained after about 10 to 20 minutes 
continuous pumping, commencing at atmospheric pressure. The high tension 0,C, supply is obtained from a 
5000 volt generator. An automatic cut-out is provided which prevents the space current in the tube from 
rising above 20 milliamps, and incidentally protects the operator. The vacuum conditions in the apparatus 
are examined qualitatively by means of an auxiliary test bulb and spark coil. 
A question of considerable importance in using the oscillograph for photographic purposes is that of 
the most suitable photographic plate. Since cathode rays of velocity corresponding to 3000 volts can only 
penetrate extremely thin Tilms of matter, e.g. 10% mm. of gelatine, it is important that the energy of the 
ray should be expended in the most efficient manner. Consequently a special type of photographic plate, 
known as the Schumann plate", tas been employed in preference to the ordinary gelatine-coated plate. 
Analysis of the film to determine the ratio of Ag Br to gelatine in an ordinary plate and a Schumann plate 
has shown that the Schumann plate has about 100 times the proportion of ag Br to gelatine that is found in 
the ordinary plate. On these grounds it would be anticipated that the cathode rays would produce much 
greater photographic effect in a Schumann plate than in the ordinary gelatine-coated plate, and this is 
strikingly borne out in practice. The Schuinann plate gives excellent photographic results even when the 
cathode rays are deflected at the highest speeds. For comparatively slow records, however, Paget Half 
Tone plates have been found to give fairly satisfactury results. 
The electro-magnet shown in the Figure is used to deflect the rays at a known rate across the plate 
i.e. to generate the time axis of the p/t record. For certain reasons it is not convenient, and in some 
cases it is impossible, to generate a time axis by mechanical movement (translation or rotation) of the 
photographic plate in a direction at right angles to the pressure axis of the record. The same result is 
obtained much more simply by passing a current which varies in a known manner through the electro-magnet. 
Thus if a.c, is applied to the magnet winding the cathode ray spot oscillates sinusoidally on the plate. 
|t is, however, preferable that the time scale should be linear, since the photographic record is then the 
true p/t record and no further analysis is necessary. To generate a linear time axis the magnet is wound 
with an auxiliary coil which carries a steady current sufficient to deflect the spot from a position in the 
middle of the plate to a point just off the left-hand edge. 1f now a more powerful current is switched on 
the primary winding, and in opposition to that in the auxiliary winding, the spot will be swept across the 
plate from left to right. The speed of traverse depends of course on the magnitudes of inductance, 
resistance and voltage in the circuit of the primary winding. Sy reducing inductance and increasing 
voltage the speed can be increased as desired. In the early stages the growth of current is very nearly 
linear, and the deflection of the cathode rays will thus be a linear function of time. 
On certain occasions two cathode ray oscillographs were employed simultaneously. 
(b) The Piezo-Electric Gauge. 
In selecting a suitable piezo-electric crystal for use in measuring explosion pressures it is 
necessary to bear in mind the experimental conditions. Tourmaline, quartz and rochelle salt all give 
strong piezo-electric effects but on general grounds tourmaline was considered to be the most suitable crystal 
to use for the investigation of explosion pressures. Its piezo electric sensitivity is greater for 
hydrostatic pressures than for lateral pressures 4, it is strong mechanically and is not attacked by sea water. 
For use in explosion gauges the crystals are cut into slabs, usually 1 cm. thick x. The piezo-electric 
gauge, as used in the explosion research, consists essentially of a mosaic of such slabs of tourmaline 
(Varying eevee 
Originally invented by Schumann for photography of the ultra-violet spectrum. 
¢ The crystalline structure of tourmaline differs from that of quartz. 
x The charge produced by a given pressure is independent of the thickness of the crystal slice - 
the choice of thickness therefore is decided by other conditions to be fulfilled in the design 
e.g. the greater the thickness the smaller the capacity etc. 
