Method of Measuring Explosion Pressures. 



479 



To obtain a relation between the displacement of the 

 cathode ray spot and the pressure exerted on the crystals in 

 the detector vessel, the instrument is calibrated by applying 

 known pressures to it and photographing the resulting dis- 

 placements. Two methods were used. In the first, which 

 was only an approximate calibration, a triple-action compres- 

 sion pump was attached to the tap N of the explosion vessel. 

 A pressure gauge, which had been calibrated by Professor 

 W. M. Thornton of Armstrong College, was screwed in the 

 opening where the spark-plug was usually inserted. The 

 apparatus was attached to the oscillograph exactly as if an 

 explosion pressure was to be measured, and the displacement 

 caused by various pressures found from the tracings on the 

 photographic plate. From these results a curve showing the 

 relation between displacement of the cathode ray beam and 

 pressure is obtained. This method was only used where 

 pressures of about 180 Ib./sq. in. were attained. It was 

 noted that if, after the pressure is applied to the crystals, the 

 charge is removed by earthing the wire F for a moment, 

 on releasing the pressure in the explosion vessel, an equal 

 displacement in the opposite direction is obtained. In the 

 second method of calibrating the explosion vessel, this fact 



Fig. 4. 



£ -80 



.«; 



so 



v? -20 



•60 



40 



Q 



100 200 300 400 500 600 700 800 

 Pressure in /6s./a" 



Calibration Curve of Explosion- Vessel. 

 Cathode Potential = 3700 volts. 



was made use of. The apparatus was connected up exactly 

 in the position in which the vessel is to be used as a detector 

 of pressures, with the capacity of the leads, etc. in circuit. 

 Instead of an air pump, a high-pressure pump using pure 

 paraffin oil was employed, the vessel and connecting tubes 



