'Method of Measuring Explosion Pressures. 475 



between the lower terminals as shown. N is a well-ground 

 brass cock through which the-explosive gas mixture may be 

 admitted into the vessel. The tourmaline crystals CO are 

 1 cm. thick and are attached by means of a little wax to the 

 bottom of the vessel and to the iron plate HI, which is 

 5 inches in diameter and y 1 ^ inch thick. They are mounted 

 so that all the positive faces of the lower crystals are in 

 contact with a thin lead plate DE and the positive faces of 

 those attached to HI are downward, i. e., also in contact with 

 DE. The plate HI is screwed down to the bottom by three 

 machine bolts, only two of which, MM, are shown in the 

 diagram. In this way the whole is kept rigid and the 

 crystals are held in contact with the lead plate DE. DE is 

 connected to an insulated copper wire F which passes out 

 through a plug G in the side of the vessel. The lower part 

 of the vessel is filled with vaseline PP in order to prevent 

 the heat from coming in contact with the crystals when an 

 explosive mixture is detonated above HI. The pyroelectricity 

 produced by the sudden change in temperature would counter- 

 act the piezoelectricity generated in the crystals. The 

 vaseline also insulates the plate DE when the apparatus is 

 suspended in water. 



When a pressure is applied above HI, DE becomes 

 charged, the amount of this charge being directly propor- 

 tional to the total pressure exerted on the crystals *. The 

 area of each crystal was about 12 sq. cm. and both layers 

 consisted of five or six such crystals. 



Crystals of tourmaline were used in preference to quartz, 

 since the piezoelectric properties of the former are more 

 suitable for experiments in which hydrostatic pressures are 

 to be measured. It has been shown that the quantity and 

 sign of the charge obtained on any face of a crystal by a 

 given pressure depends upon the direction of the applied 

 force. If a section of quartz is subjected to a steady hydro- 

 static pressure, no resultant charge is detectable on any face, 

 i. <?., when compressed uniformly on all sides, it shows no 

 electrification t- Tourmaline, on the other hand, gives a 

 resultant polarization when subjected to a hydrostatic 

 pressure. Koch J has found the value of this charge to be 

 1*86 xlO -2 e. s. units per kilogram/cm. 2 Hence in those 

 experiments crystals of tourmaline were used. 



A diagram of the cathode ray tube used is shown m 



* P. Curie, /. c. 



t P. P. Koch, Ann. d. Physik, vol. xix. p. 580 (1906), 



X L. c. p. 586. 



2K 2 



