FIRING MACHINES, blasting batteries, electric blasting machines or exploders are elec tric generators which are used in firing charges of explosives. in blastine. and in military and naval operations. .Broadly speaking, there' are three types: (1) In which the current is in duced in coils of wire surrounding the poles of a permanent magnet, by suddenly detaching a soft-iron armature therefrom; (2) in; winch a coil-wound armature is rotated between the poles of a permanent magnet; (3) in which a coil-wound armature is rotated between the poles of an electromagnet as in an ordinary dynamo. It inn, been found advantageous in practice to construct the dynamo-electric ma chines on the series method of winding, that is, the field-magnet coils, armature and external circuit (which in this case is the firing-line) together iu.series. These machines are operated by cranks or by means of racks and pinions, many mechanical arrangements having been devised by which to secure the continu ously accelerated. motion and automatic switch, off which is desired. The general principle upon which the dynamo-electric machines work is that an armature, rotated between the voles of an electromagnet, has set up in its cods a feeble electric current, due to the magnetiza tion produced by the residual magnetism in the iron. By continuing rotation (with the firing line short-circuited) the weak current thus started passeS round the field-ma et coils, in creases the intensity 'of the and, conse quently, the volume of current in the armature.
This gradual increase or building up of the current goes on, until the maximum capacity of the machine is reached. At the _limit of mag netic saturation, the short circuit previously ex isting is automatically broken and the whole available energy is switched on to the firing line, through the binding posts or terminals on the side or top of the machine, to which the leading wires are attached. Machines are rated in the market at the full number of detonators which a machine of that pattern will fire, but, to ensure freedom from a machine should never be worked 'up to its rated maxi mum capacity. The diameters of the leading wires should vary with their lengths, thus for distance up to 600 feet No. 16 B. W. G. (0.065 inch) wire should be used, for 800 feet No. 14 (0.083 inch) and for 1,000 feet No. 13 (0.095 inch). To prevent accidental explosions, it should be the invariable rule, that the leading wires are not attached to the binding posts of a firing machine, until all other preparations for firing are completed and everyone has got to a safe distance from the blast or mine; and it should be also the invariable rule that the leading wires are detached from 'the' machine as soon as the blast has been fired.