DYNAMITE 



2750 



DYNAMO 



ments embrace the dynamics and 

 statics of a particle which is acted 

 upon by forces in one plane or 

 in more than one plane ; the plane 

 dynamics of a rigid body ; the 

 three dimensional statics or dyna- 

 mics of a rigid body, with which is 

 associated the theory of screws; 

 and by an extension within 

 modern limits of the meaning 

 of the term dynamics, those 

 problems of analytical dynamics 

 which investigate the action of 

 forces on systems of particles, or 

 the " problem of three bodies," 

 which refers to the mutual at- 

 tractions of three bodies in 

 space, e.g. the Earth, Sun, and 

 Jupiter, a problem which can only 

 be solved in special cases. See 

 Motion. 



Dynamite. Name applied to a 

 variety of high explosives of which 

 the essential feature is a high con- 

 tent of nitroglycerine, absorbed 

 in an active or inert porous base. 

 Dynamite was invented by Nobel in 

 1866, when, owing to many disas- 

 trous explosions nitroglycerine was 

 prohibited in various countries. He 

 found that it could be rendered 

 comparatively safe by absorption 

 in kieselguhr (q.v.), which is cap- 

 able of retaining up to three times 

 its weight of nitroglycerine. This 

 mixture is still known as No. 1 dyn- 

 amite, whilst No. 2 and No. 3, 

 which are more rarely used, con- 

 tain 35 p.c. and 25 p.c. of nitro- 

 glycerine respectively. About 1 p.c. 

 of magnesium carbonate or chalk is 

 usually added to neutralise the 

 acid evolved by nitroglycerine on 

 storage. 



The calcined kieselguhr is 

 weighed out into a rubber bag, 

 after it has been thoroughly mixed 

 with the carbonate and sieved, the 

 nitroglycerine being added in small 

 portions, while the mass is kneaded 

 by hand to form a paste, the opera- 

 tion lasting about 30 minutes. The 

 dynamite is formed into cartridges 

 by pressing it into parchment 

 paper tubes by a wooden plunger 

 working through a funnel. All 

 these operations are dangerous and 

 only small quantities of explosive 

 are dealt with in light buildings 

 protected by mounds. 



In America kieselguhr has been 

 largely displaced as the absorbent 

 by " active " bases, generally con- 

 sisting of a mixture of wood meal, 

 flour, or similar carbohydrate and 

 sodium or potassium nitrate. A 

 whole series of dynamites is made 

 with nitroglycerine contents rising 

 by 5 p.c. from 15 p.c. to 69 p.c., 

 but this range of dynamites has 

 never been popular in Europe, 

 although a variety of explosives of 

 the same type are employed for 

 various purposes. 



- Dynamite is a powerful high ex- 

 plosive of considerable brisance, 

 while its plasticity is a great ad- 

 vantage in filling boreholes and 

 applying it to solid objects for 

 demolition purposes. It is safe to 

 handle and transport, but is too 

 sensitive to shock for employment 

 in shell, and since it is detonated by 

 the impact of a rifle bullet it is not 

 a safe military store. If ignited in 

 small quantities it burns fiercely 

 but does not explode. Black pow- 

 der will cause it to explode, but 

 complete detonation can only be 

 effected by initiation with a de- 

 tonator. 



Dynamite has two marked dis- 

 advantages, one being that expo- 

 sure to moisture displaces the 

 nitroglycerine, causing exudation 

 and dangerous sensitivity, and 

 secondly, it freezes about 10 C. 

 when, whilst it is more insensitive to 

 detonation, causing misfires, it is 

 more sensitive to shock andfriction. 

 Frozen dynamite must be carefully 



thawed in a special oven. Exuda- 

 tion is guarded against by the use 

 of special wrappers, and freezing 

 may be prevented by using nitro- 

 glycerine containing substances 

 which lower its freezing point and 

 storing the dynamite in heated 

 magazines. Dynamite is chiefly 

 used for blasting operations where 

 a powerful shattering effect is 

 required. See Explosives ; Nitro- 

 glycerine ; Safety explosives. 



Dynammon. Safety explosive 

 manufactured by the Austrian 

 government. Two varieties are 

 made, Wetter-dynammon, com- 

 posed of ammonium nitrate 94 p.c., 

 charcoal 4 p.c., potassium nitrate 

 2 p.c. ; and dynammon, composed 

 of ammonium nitrate 87 '5 p.c., 

 charcoal 12*5 p.c. The former is 

 the more suitable for use in coal 

 mines where coal dust or explosive 

 gas may be present. They are pre- 

 pared by milling the dried ingre- 

 dients together in mills such as are 

 used for gunpowder. 



DYNAMO: A GENERATOR OF ELECTRIC 



POWER J. L. Pritchard, Editor of The Aeronautical Journal 



In this article is given in brief outline the principle of the dynamo 



from Faraday'' s initial discovery. Further information on the subject 



will be found under the articles A Iternating Current ; A Iternator ; 



Armature: Commutator; Electricity; Magnetism 



Oersted in 1819 discovered that 

 a wire conveying an electric cur- 

 rent is surrounded by a magnetic 

 field, a magnetic needle tending to 

 set itself at right angles to the wire 

 carrying the current. Following 

 this Michael Faraday experimented 

 to find out if the converse were 

 true, i.e. if a magnetic field could 

 induce an electric current, and 

 eventually in 1831 he succeeded in 

 making this all-important dis- 

 covery the forerunner of the 

 modern dynamo. 



Faraday discovered that if a 

 magnet were passed through a coil 

 of wire a current of electricity was 

 induced in the wire while the mag- 

 net was moving. The same effect 

 was produced if the magnet were 

 kept still and the wire moved. If 

 the magnet be horseshoe in shape 

 the magnetic field is concentrated, 

 and if a ring of copper wire be 

 passed between the poles, cutting 

 the lines of force of the magnetic 

 field at an angle, an electric current 

 passes along the wire. 



This is the essential principle of 

 the dynamo or electric generator, 

 and from this simple fact has been 

 built up the complicated modern 

 dynamo. Fig. 1 shows Faraday's 

 famous experiment. 



Since the strength of the mag- 

 netic field is not constant at all 

 points of the magnet, it follows 

 that as the coil of wire moves 

 across it the strength of the in- 



duced electric current varies. 

 Moreover it is found that decreas- 

 ing the number of lines of force 

 passing through, the conductor pro- 

 duces an induced current in one 

 direction, while increasing the 

 number of 

 lines through 

 the conductor 

 produces an 

 induced cur- 

 rent in a con- 

 trary direc- 

 tion. In other 

 words, cur- 

 rent? alter- 

 nate along the 

 wire as it 

 passes across 

 the magnetic 

 field. 



The strength 

 of the induced 

 current, 

 E.M.F.oreleo- 



Dynamo. Fig. 1. 

 Faraday's experi- 

 ment. N S is a mag- 

 net which is moved 

 into the hollow coil 

 of wire, W, causing 

 a current along the 

 wires w, w' 



tromotive force, is proportional 

 to the strength of the field, or 

 rather to the number oi lines of 

 force cut by the ring in a eiven 

 time, and the direction of" the 

 current is at right angles to the 

 lines of force and to the direction 

 in which th.> ring is moved. If 

 the plane of the moving ring is 

 parallel to the lines of force, 

 equal and opposite currents would 

 be induced and would neutralise one 

 another. By a device known as 

 the commutator the alternating 



