TOEPEDOES. 
181 
wires d e to the points of the nails, and press some wax about 
them, so as to form an insulator, b. Then roll paper round it, 
tying it at the bottom round the wax and Avires, and leaving a 
cup at the top above the cylinder. Fill this with fine-grained 
powder (c), twist the paper and tie it up firmly, and then paint 
and A^arnish the whole. The passage of a current between the 
heads of the nails will fire the powder. There are many other 
fuzes constructed on the same principle ; but there is no need 
of referring to them, as the two we have described will give the 
reader a good idea of the Avay in which they act. 
The firing-key is a \^ery simple apparatus for closing the 
circuit and exploding the charge of the torpedo, by connecting 
the battery with the insulated cable. It is arranged in a 
shallow box or tray, aaaa (fig. 10), and consists of two bind- 
ing-screws, B and c, the former of which holds the end of the 
wire from the cable, while the latter is similarly connected 
with the voltaic battery. A strong steel spring, n, is fastened to 
the binding-screw, b, in such a Avay that its free end, e, stands 
suspended a little above the other one ; and through this end 
of the spring a small metal screw, e, with a broad head of some 
non-conducting material, is driven.* Now, on placing the 
finger on the head of the screw, e, and pressing doAvn the spring, 
it will touch the binding-screw connected with the battery, 
and thus the electrical circuit will be completed. The current 
from the battery will pass up the wire and by the binding- 
screw, B, into the spring, then on to the second binding-screAV 
and into the cable, thence to the first wire of the fuze and on 
to the second (firing the priming on its way), and thence to 
the water, which will conduct it to the ground and back by the 
earth-plate to the battery. The sketch (fig. 11) shows the 
course of the current, the direction of which is indicated by the 
darts. A is the battery, b the firing-key, c the anchored tor- 
pedo coi;inected with it by the cable, e. d is the earth-plate of 
the battery, which receives the return current marked by the 
dotted line. We may here observe that experiment has 
proved that there is no need of an earth-plate being attached 
to the torpedo ; for, on account of the high conducting power 
of sea-water, even a few inches of bare wire is quite sufficient 
for the purpose. With iron-cased torpedoes, the bottom of the 
case may be made to act as an earth-plate, by merely connect- 
ing the return wire of the fuze with it. 
We have all this time spoken only of the structure of a 
* In the English form of this apparatus the usual plan is to place a cover 
on the box, having a hole, across which a piece of india-rubber is stretched, 
just above the screw, e j then, by pressing down the india-rubber, the spring 
is depressed. 
