474 Dr. D. A Keys on a Piezoelectric 



determined. The variation of the electrical charge acquired 

 by the crystals under the influence of the pressure is 

 measured by means of the cathode-ray oscillograph. Since 

 the charge separated on to arm aline is directly proportional 

 to the pressure applied *, we have a convenient method of 

 obtaining exceedingly rapid pressure changes in terms of 

 variation in electrical charge*. The amount of the charge is 

 measured by the deflexion of a beam of cathode rays which 

 passes between two parallel condenser plates that receive the 

 charge separated on the crystals. A magnetic field applied 

 parallel to this electrostatic field between the condenser 

 plates is excited by an alternating current of known fre- 

 quency, giving a time displacement of the beam perpen- 

 dicular to that caused by the electric field. The cathode 

 rays fall directly on a photographic plate, and a charge-time 

 record is thus obtained. Since there is a linear relation 

 between the charge and pressure applied to the crystals, the 

 arrangement is a pressure-time measuring device with 

 negligible inertia. 



Tin 



II. The Apparatus. 

 apparatus consists of two parts, — the detector vessel 



containing the crystals and the special form of cathode-ray 

 oscillograph. The final type of crystal vessel which was 

 developed and used is shown in figure 1. It consists of a 



Fiir. 1. 



brass container AB about 6 inches in diameter and 1J inches 

 dsep with walls \ inch thick. This pot is fitted with a 

 t inch steel plate JK, which may be bolted down with twelve 

 | inch steel bolts. An ordinary spark plug L is screwed 

 into the centre of this cover and a fine wire is connected 



* Oeuvres de P. Curie, p. 16. 

 periments, see § III. p. 478. 



Confirmed also by the calibration ex- 



