507 
(a) Electrostatic Sensitivity of the oscillograph. 
1t can easily be shown on theoretical grounds that the deflection 8 of a stream of cathode rays 
of velocity v, charge e and mass m,* after passing between a pair of electrostatic deflecting plates of 
length A and distance apart d, is given by the expression 
r 
(1 +—) (1) 
2 
where E is the p.d. applied across the deflecting plates and 1 is the subsequent length of path of the rays 
from the end of the deflecting plates to the photographic film. The velocity v can be expressed in terms 
of voltage V generating the rays, 
v = §,95 V Vx 107 cms/sec. V measured in volts. 
whence we find 
Ses meee te « A (iad (2) 
2V d 2 
where K is the osci llograph constant and — and V are expressed in volts. 
It will be seen from (2) that the deflection d is directly propartiona) to the voltage applied to 
the deflecting plates and inversely proportional to the voltage generating the rays. These theoretical 
results have been verified experimentally. It is important to note, however, that the vacuum conditions in 
the oscillograph must be good for equation (2) to be strictly correct. (f gas is present, the cathode rays 
are reduced in velocity by collision and consequently the sensitivity is Increased, § being larger than the 
value determined by (2)**. (f therefore, the vacuum tube is soft when a record is made (this condition of 
course being avoided in general) it is necessary to make an independent voltage calibration of the oscillo— 
graph. Generally speaking, however, when the vacuum is good, relation (2) is strictly app) icable and there 
is no need to make this special voltage calibration. Under these conditions a standard value of V is taken 
in practice, viz. 3000 volts, and all observations of deflections 6 are reduced to this standard, 8 being 
taken as inversely proportional to V. 
The time axis of the oscillograph record is calibrated according to the method outlined in section 1} 
iv.b above. The circuit of the deflecting magnet is in this case identical with that used in making the 
explosion record, In certain cases the time calibration was recorded on the same plate as the actual 
explosion record but any slight advantage of this method over that in which a second pjate is used, is lost 
in the increased risk of fogging and possibility of spoiting the explosion record itself. Consequently the 
general practice has been to expose a separate plate after the explosion has been recorded, Attempts were 
made to ‘dot’ the p/t record at regular time intervals but the results obtained were not very satisfactory. 
A relay system was devised also whereby a voltage calibration of the oscillograph was recorded at the end 
of the p/t record - see Record h Sheet |} but this was only employed in a few cases. Various experimental 
devices of this nature were designed and tested but on the whole it was found more satisfactory to make 
Calibrations om separate plates, rather than run the risk of spoiling a valuable explosion record by additiona? 
complications. Typical time scales are shown on Sheet | Records a, b, c and d. 
(b) Pressure Calibration of Piezo-Electric Gauges 
The calibration of the gauge is a matter of fundamental) importance to the method and involves certain 
difficulties of a theoretical nature which must first be dealt with. When used to indicate explosion 
pressures, the gauge is subjected to a rapidly varying pressure and the conditions are essentially adiabatic 
in character, A static calibration of the gauge, under isothermal conditions, is therefore at least open 
to question, keys # has dealt with this point thermodynamically and has arrived at the conclusion that 
the eeseee 
mM = 09316 X 107 &.5, units for cathode rays. 
ae Variations due to this cause have occasionally been noticed when the tube is very ‘soft’ - 
Changes of sensitivity of a few per cent being observed. under these conditions, however, 
the indications of softness of the tube are obvious. 
+ Keys = Phi}. Mag. Vol. xiii, p.4u73 (1921) (Adiabatic and |sothermal piezo Electric Constants 
of Tourmaline). 
