364 



ME. J. E. PETAVEL ON THE PRESSURE OF EXPLOSIONS. 



Chronograph. 



Owing to the high speed required, the chronograph used for this work had to be 

 specially designed. It is unnecessary to go into all the details of its construction. 

 The ordinary methods were used to measure the velocity of the rotating drum and to 

 ensure the constancy of speed during the course of an experiment. 



When measuring the rise of pressure during an explosion, a linear speed of between 

 100 and 1000 centims. per second was used. For measuring the fall of pressure 

 during the cooling of the products of combustion the driving mechanism could be 

 geared down to give a linear speed of 5 or 10 centims. per second. 



The drum of the chronograph can be easily detached and taken to the dark room, 

 where the photographic film is wound on ; it is then placed in a light-tight box. As 

 explained in connection with fig. 4, this box is so arranged that the drum can be 



SCALE IN INCHCS. 



Fig. 4. Chronograph camera. 



The drum D is shown fixed on the axle A of the chronograph. To remove the drum without exposing the 

 film which is wound round it to the light, the camera is first moved a little to the right, causing the 

 ring E on the camera to fit into the groove F of the drum. The brass tube G is next forced into the 

 groove H ; its cover, C, can then be removed and the nut N unscrewed. The camera, with the drum 

 firmly held in it, can now be detached from the chronograph (by sliding it to the right) and taken to 

 the dark room, where the film is developed and replaced by a fresh one. 



fixed on to the axle of the chronograph in the full daylight without fogging the film. 

 The box surrounding the revolving drum is pierced with a long and very narrow slit ; 

 this, in turn, is covered by a shutter, which is lifted immediately before the explosive 

 is fired and closed again a second later, after the photograph has been taken. 



Thanks to the above arrangement it is not necessary for the room in which the 

 experiments are carried out to be absolutely dark. The mirror of the recorder is 



