Method of Measuring Explosion Pressures. 477 



rays. This holder slides in a thin rectangular box placed in 

 the cylinder CC, and may be raised or lowered by a thread 

 attached to a glass cock arranged as a windlass, similar to 

 the arrangement used by Sir J. J. Thomson*. There is a 

 window in the tube CC at Y, and a willemite screen is 

 attached to the rectangular box on the side remote from F. 

 When the plate is drawn up, the cathode ray spot may be 

 observed visually while adjustments are being made. The 

 whole apparatus is exhausted to a high vacuum by means of 

 a Graede mercury pump attached to Z. The top of CC is 

 sealed on with wax. After the plate is exposed, it is neces- 

 sary to allow the air back into the apparatus, unseal the top, 

 and remove the plate for development. 



The electrons liberated by the hotwire F are drawn across 

 to L by the strong electrostatic field set up by the potential 

 difference of T. By turning the stopper A, a beam is directed 

 through the tiny hole 0. It was found that by altering the 

 temperature of the filament and adjusting the potential, one 

 or two very small bright spots could be obtained on the 

 screen. The alternating magnetic field WW draws this spot 

 out into a line, the length of which was adjusted to about 

 3 cm. by means of the current in WW. The wire F from 

 the crystal detector (figure 1) is attached to P, the vessel 

 and other plate being earthed. When the crystals are sub- 

 jected to a pressure, the spot moves in a horizontal direction 

 indicated by the dot and dash line in figure 2. The amount 

 of this displacement is proportional to the charge generated 

 on the crystals, provided the capacity of the system and the 

 cathode potential T remain constant. Since the amount of 

 electricity separated is a measure of the pressure, the hori- 

 zontal displacement gives a measure of the pressure and the 

 vertical displacement a measure of the time. 



To indicate how the apparatus is used, consider the case of 

 an explosion of a charge of electrolytic gas and air. The 

 crystal vessel, figure 1, is exhausted and a charge of the ex- 

 plosive mixture introduced through the tap N. The spark- 

 plug L is connected to a battery and the firing key. This 

 latter (shown at S, figure 2) is a form of double key which 

 is used in all the experiments and was made in such a way 

 that on depressing the key, the cathode potential was first 

 connected to the tube and after a short interval the further 

 motion detonated the charge. The photographic plate is 

 lowered in front of the screen and the magnetic field \YA\ is 



o 



* ' Hays of Positive Electricity,' pp. *2-2-2o (1913), Longmans, Green 

 & Co. 



