June 23, 192 1] 



NATURE 



525 



the ground. It must be capable of being handled 

 roughly without firing, and must not act when 

 the considerable forces involved in firing it from 

 a gun are impressed upon it and upon all its parts. 

 The magnitude of these forces is illustrated by the 

 fact that a fuze weighing 2\ lb. when fired from 

 an eighteen-pounder gun weighs about ii tons — 

 the stress corresponding to 15,000 times the 

 acceleration due to gravity. These forces are 

 taken advantage of to render the fuze ' ' live ' ' — 

 that is, to put it into a condition when it will act 

 on the slightest provocation. 



In the interior of the fuze is a brass cylinder 

 with an axial hole, on the top of which is placed 

 a capsule containing a highly sensitive flash com- 

 position. To prevent this cylinder from moving 

 forward in handling, a bolt lies athwart its top 

 edge, and this bolt is retained in this position by 

 a small pin placed vertically at the back of the 

 bolt and having its base pressed upward by a 

 spring working in a vertical cylindrical cavity. 

 On firing, this pin, weighing 1-3 grams, is 

 acted on by a force equivalent to 20 kg., 

 overcomes the resistance of its spring, and 

 recedes into its cavity. The force due to the 

 shell's rotation causes the bolt to fly outwards, 

 thus freeing the brass cylinder, which 

 now is prevented from moving forward 

 on to a needle only by the interposition 

 of a light spring. The fuze is now 

 " live," and on the slightest check being 

 given to the forward movement of the 

 shell, as, for example, by grazing on 

 soft earth, the cylinder moves forward by 

 its own inertia on to the needle, which 

 pricks the capsule, causing a jet of flame 

 to pass down the centre of the fuze. 

 The object of all this mechanism is to 

 supply at the proper time a flash for operating 

 the next member, the gaine, where it gives rise 

 to a detonation. 



The Gaine. — This is a tube (from French gaine, 

 a sheath) with steel walls of quarter-inch annulus. 

 In its upper portion is a pellet of gunpowder 

 which is ignited by the flash from the fuze, and 

 sends a larger flash on to an open capsule contain- 

 ing fulminate of mercury situated over pellets of 

 tetryl. The fulminate detonates, and in turn 

 causes the tetryl to detonate, and to deliver from 

 the bottom end of the gaine a very intense blow 

 to a series of explosive intermediaries which com- 

 municate the detonation to the main bursting 

 charge. 



Intermediaries. — The first of these is a bag of 

 T.N.T. crystals situated in a thin steel con- 

 tainer tube which encloses it and the gaine. This 

 T.N.T., on detonation, brings to detonation an 

 annular layer of T.N.T. cast round the container, 

 and this in turn brings about the detonation of 

 the main charge of the shell. The train of detona- 

 tion is thus somewhat complicated, and in its 

 evolution many important principles had to be 

 observed. 



Sensitiveness and Violence. — Thus the sensitive- 

 ness of the various explosives used had to be de- 

 NO. 2695, VOL. 107] 



termined, since, on account of the magnitude of 

 the acceleration imparted to all parts of the shell 

 on firing it from a gun, a column of a sensitive 

 explosive over a certain length and weight will 

 be liable to detonate on account of the sudden 

 force applied. In proportion to their sensitive- 

 ness to mechanical shock, therefore, explosives 

 in shell must be graduated in regard to length 

 of column employed. A general principle is to 

 have next to the detonator a somewhat sensitive 

 explosive, and to reinforce the impulse derived 

 from it by one less sensitive, but still delivering 

 an intense blow. It is important, therefore, to 

 have quantitative values for the sensitiveness of 

 explosives to mechanical shock, and some of the 

 values thus obtained are given in the following 

 table : — 



Fig. I. 



It is important also to know the violence of the 

 various explosives used, both by. themselves and 

 also when assembled in the various components, 

 and it was in this connection that the principle of 

 the pressure bar, enunciated by the late Prof. 

 Bertram Hopkinson in a discourse to the Royal 

 Institution in January of 1912, was of the greatest 

 value. This depends on the experimental reso- 

 lution of the momentum of the blow into pressure 

 and time. When a charge is fired against the 

 end of a cylindrical steel bar ballistically sus- 

 pended, a wave of compression travels along the 

 bar and is reflected at the far end as a wave of 

 tension. To investigate the properties of the 

 wave, a short length of the end of the bar farthest 

 from the end to which the blow is delivered is 

 cut off' and the faces are surfaced, the short piece 

 (known as the time-piece) being caused to adhere 

 closely to the bar, usually by a film of vaseline. 

 The compression wave travels unchanged through 

 the joint into the time-piece, but the reflected ten- 

 sion cannot pass through it. Hence when the 

 amplitude of the reflected tension wave reaching 

 the joint becomes greater than that of the on- 

 coming compression wave, the time-piece is pro- 

 jected from the shaft with a momentum which 

 depends on the pressure exerted by the explosive 



