EXPERIMENTS ON SOLID AND GASEOUS EXPLOSIVES. 365 



illuminated by a straight-filament incandescent lamp, the image of the filament being 

 focused on to the slit of the chronograph camera, forming a straight streak of light 

 perpendicular to the axis of rotation. The beam of light is deflected to an amount 

 proportional at each instant to the pressure in the explosion chamber and, travelling 

 along the slit of the camera in a direction parallel to the axis of rotation, traces 

 out a curve on the photographic film. The ordinates of this curve represent the 

 instantaneous pressures, the abscissae the times at which the said pressures existed. 



A low-voltage high-candle-power lamp is used to illuminate the mirror, the 

 comparatively thick filament of such a lamp giving correspondingly more intense 

 illumination. At the moment of firing, the lamp is switched for a few seconds on to 

 twice its normal voltage, and thus the strongly actinic light required is produced. 



The recorder is calibrated by hydrostatic pressure before and after each set of 

 experiments. 



Explosion Chambers. . 



It is well known that the shape of the enclosure has a considerable effect on the 

 behaviour of the explosive during combustion. On the other hand, the ratio of the 

 internal surface to the total volume of the chamber determines to a large extent the 

 rate at which the pressure will subsequently fall. 



With a view of obtaining some further information on these questions, two 

 explosion chambers were constructed having approximately the same volume, but 

 differing largely in shape. The first, a sphere, offers the least possible cooling surface ; 

 whereas the second, a long narrow cylinder, has a surface more than twice as great. 



One of the subjects of the research was to study the oscillations of pressure which 

 are set up under certain conditions. In a long cylinder such oscillations are easily 

 started, but in a small sphere the symmetrical shape and the short distance from wall 

 to wall tend to equalise the pressure existing at each instant throughout the 

 enclosure. Thus, in a spherical enclosure, the pressure rises usually without vibration 

 and forms a smooth curve, the shape of which depends exclusively on the nature of 

 the explosive used. In a long cylinder, however, the normal curve is modified by the 

 distribution of the explosive, the method of firing, and various other factors. 



Before designing these chambers, the relative advantages of solid metal and wire 

 winding were fully considered. The latter construction, if properly carried out, adds 

 considerably to the ultimate strength. A system of winding suitable for a spherical 

 enclosure is, however, not easy to devise, and this fact, together with the ever 

 important consideration of cost, led to the adoption of solid walls. 



Mild steel was chosen as the material best suited to withstand the sudden impact 

 of an explosion. The limit of elasticity, ultimate strength, and elongation of test 

 pieces cut perpendicular to the direction of rolling were carefully determined before 

 the forgings were machined. 



