24 PRESSURE OF GASES. 



then equal to the force of calibration corresponding to the 

 crushing observed. It is given by formula (1). 



(5) The development of the pressure is so rapid that the 

 displacement of the piston taking place during the development 

 of the maximum pressure may be disregarded, the piston 

 having, besides, a sufficient mass; in this case the movement 

 of the piston may be regarded as effected under constant 

 pressure from the start, and throughout the whole of its 

 duration. The calculation shows that the value of this pressure 

 is equal to a force of calibration corresponding to half the 

 crushing. 



8. In practice, and for a given explosive, it has to be ascer- 

 tained whether the instrument works at one or other of these 

 limits, then to estimate the maximum pressure applicable to the 

 intermediate cases. Hence, it is necessary to register the 

 duration of the crushing, as well as the law of the movement of 

 the piston, and to compare the latter with the results given by 

 calculation for the movement of the piston crushing the cylinder 

 under the action of a force which would be a function of the 

 time. Theory shows that the phenomenon is ruled by the 

 ratio existing between the effective duration, T, of the crushing 

 taking place under variable pressure and the duration, r , of this 

 crushing caused by a constant force acting on the piston without 

 initial velocity. 



However, it is preferable to substitute for a correction which 

 is always somewhat doubtful, data obtained under experimental 

 conditions near one or other limit. We will give some results. 



The authors have found for gunpowder, that the crushing 



remains the same when varies from 4'8 to 251, variations 



TO 



which depend upon the degree of aggregation of the powder 

 (dust, grain, cake, compressed blocks). We are, therefore, always 

 in the neighbourhood of the first limit ; that is to say, formula 

 (1) is applicable in all cases. 



The maximum pressure of powder gases at the density of 

 charge 0'70 has thus been found equal to 3574 kgms. per 

 sq. cm. Powdered potassium picrate, on the contrary, was 

 so rapidly decomposed that no appreciable value could be 

 observed for T (expressed in ten-thousandths of a second). 

 The maximum pressure was found equal to 1985 kgms. under a 

 density of charge 0*30. The same salt in compressed blocks 

 gave a more appreciable duration of combustion ; or 0*0005 sees. 

 to 0*0006 sees, and a less amount of crushing. It is, therefore, 

 the other limit which must be applied, and this has been ex- 

 perimentally verified. 



