in diameter, with their bare ends inserted in a detonating
cap. For firing, the fuze wires are joined to long leading wires,
connected with some source of electric current. By joining the fuze
wires in series or in groups, any number of holes may be fired
simultaneously, according to the current available. A round of holes
fired in this way, as for driving tunnels, sinking shafts, or in large
surface excavations, produces better results, both in economy of
explosive and effect of the blast, than when the holes are fired
singly or in succession. Also, the miners are enabled to prepare for
the blast with more care and deliberation, and then to reach a place
of safety before the current is transmitted. Another advantage is that
there is no danger of a hole "hanging fire," which sometimes causes
accidents in using ordinary fuzes.

Hanging fire may be due to a cut, broken or damaged powder fuze, which
may smoulder for some time before communicating fire to the charge.
"Miss-fires," which also are of not infrequent occurrence with both
ordinary and electric fuzes, are cases where explosion from any cause
fails to take place. After waiting a sufficient length of time before
approaching the charged hole, the miner carefully removes the tamping
down to within a few inches of the explosives and inserts and fires
another cartridge, the concussion usually detonating the entire
charge. Sometimes another hole is drilled near the one which has
missed. No attempt to remove the old charge should ever be made.

High tension electricity, generated by a frictional machine, provided
with a condenser, was formerly much used for blasting. The bare ends
of the fuze wires in the detonating cap are placed say 1/8 in. apart,
leaving a gap across which a spark is discharged, passing through a
priming charge of some sensitive composition. The priming is not only
combustible but also a conductor of electricity, such as an intimate
mixture of potassium chlorate with copper sulphide and phosphide. By
the combustion of the priming the fulminate mixture in the cap is
detonated. As these fuzes are more apt to deteriorate when exposed to
dampness than fuzes for low-tension current, and the generating
machine is rather clumsy and fragile, low-tension current is more
generally employed. It may be generated by a small, portable dynamo,
operated by hand, or may be derived from a battery or