480 Dr. D. A. Keys on a Piezoelectric 



being all filled with oil. A valve operated by an electro- 

 magnetic device was used so that the pressure produced in 

 the vessel by the pump could be released suddenly and the 

 corresponding deflexion photographed. The magnetic re- 

 lease was operated by the firing key, so that a single depres- 

 sion of the key gave a zero line and the displaced line 

 corresponding to the particular pressure applied to the 

 explosion vessel. This method was adopted for calibrating 

 the detector vessels used in measuring pressures in water. 

 Figure 4 gives one of these calibration curves. 



IV. Determination of Time-Pressure Curve for 

 Gas Explosions. 



Among the first results obtained with the apparatus which 

 has been described above were the records of explosions of 

 mixtures of electrolytic gas and air. Fig. 5 (PI. XVI.) repre- 

 sents two typical tracings obtained on the photographic plate 

 by such explosions. The cathode ray beam Contained more 

 than one bright spot in this case, producing jfcwo spots and 

 thus tracing out two paths side by side. In making the 

 readings from the plate, one of these is followed and all 

 measurements taken from it. As a check on the first set, 

 the other tracing is also measured and the two resulting 

 curves are found to be similar. 



A simple calculation is necessary in order to obtain the 

 time-pressure curve from this tracing. Since the cathode 

 ray beam is moving up and down under the influence of the 

 alternating magnetic field, its velocity will be different at 

 various points of its path. An oscillograph record of the 

 current used showed it to be very nearly sinusoidal, so the 

 spot may be supposed to move with simple harmonic motion 

 about the centre. The time-displacement curve is thus 

 easily calculated from the plate, knowing the frequency of 

 the alternating current used. The coordinates of various 

 points on the curve are obtained by placing the plate in a 

 holder similar to that used by Sir J. J. Thomson *. 



Table I. gives the results of an actual experiment on the 

 pressure produced by such a gaseous mixture. The cathode 

 potential is noted when the charge is fired, and from the 

 curve shown in figure 3 the displacements may be reduced 

 to what they would have been had the potential been 

 3700 volts, which was the standard chosen. From figure 4 

 the pressure corresponding to a displacement of one cm. is 



* ' Rays of Positive Electricit} r ,' p. 25, fig 1 . 14. 



