13G THE farmers' handbook. 



from the ordinary Q type, where a handle is revolved at a high speed and 

 at the same time a button pressed to break the current and flood the 

 circuit with a full, fat spark, to the Rackbar type, where a long handle-bar 

 with a fine-toothed rack attached, is pushed down briskly and a cut-out 

 plate struck at the bottom to flood the circuit. This scheme of liberating 

 the current generated at the very last moment is to ensure that the whole 

 amount will speed round the circuit .in one energetic mass and so provide 

 sufficient to fire all the charges concerned; otherwise the firing would be 

 irregular and some charges would be unexploded. The Rackbar type is by 

 far the better machine, and a 10-shot capacity is the most convenient size. 



The Galvanometer is a very weak, chemical dry-cell battery, and is only 

 useful for testing the electric detonators before inserting them in the various 

 charges, and then after connecting up these charges in series and coupling up 

 to the firing cable, to make sure that all connections are secure and no 

 " shorts " exist. With care these dry cells will last for months, and when 

 they run down thev can be renewed for a few shillings. It would be extremely 

 risky for anyone who is not throroughly conversant with galvanometer testing 

 to attempt to renew these dry cells. Too strong a cell would explode and 

 not test the electric detonator. 



The Firing Cable is, or should be, 100 yards long and composed of two 

 wires or strands of wires, separately insulated and waterproofed to prevent 

 " shorting," and then further insulated and waterproofed to prevent damp 

 or wet penetrating them. They are highly flexible, and should be quite 

 moisture-proof under all circumstances ; with care they will last for years. 



No. 6 Detonators (ordinary).— These consist of small copper cylinders, 

 about I inch diameter, open at one end only, and charged with about 16 

 grains of fulminate of mercury, a very sensitive compound with a high 

 detonating force. Their function is to explode the charges of gelignite by 

 detonation, and they should be handled with all care and respect, since 

 their explosive force is in the neighbourhood of 60 lb. per square inch, which 

 is more than sufficient to blow one's hand off. 



No. 6 Detonators (Electric or E.D. fuses).— These are similar to the ordinary 

 No. 6 detonators as to the copper cylinder and charge of fulminate of mercury, 

 but they have an addition to enable them to be fired electrically instead of 

 by ordinary safety fuse. Attached to each electric detonator are two wires, 

 and on one end of each wire is a small metal pole., one slightly shorter than 

 the other. Uniting the ends of these poles of varying length is an extremely 

 fine platinum wire, over which fits a gun-cotton priming, very similar in 

 appearance to a fusee match head. This priming rests right up against 

 the fulminate of mercury in the copper cylinder, the whole attachment 

 being kept firmly in place by a waterproofed insulation compound on its 

 open end, performing the dual purpose of keeping the contrivance in its 

 place and preventing any moisture reaching the fulminate. In operation, 

 the spark generated in the firing battery flies down one wire and across the 

 fine platinum wire bridge, which instantly becomes incandescent, just as 

 an ordinary electric lamp does, thus setting fire to the gun-cotton priming 

 which explodes the fulminate of mercury and detonates the charge of 

 gelignite. The current having completed its function in the first E.D. fuse, 

 rushes up the other wire and along to the next fuse, repeating the same 

 operation, up to number of charges in the circuit or the capacity of the firing 

 battery. 



