A cannon may be fired from a distance in
this way, and as there is no danger of any spark remaining after
the current is shut off, it is safer than the ordinary cannon
which is fired by means of a fuse.
--Contributed by Henry Peck, Big Rapids, Mich.
** Simple Electric Lock [60]
The illustration shows an automatic lock operated by electricity,
requiring a strong magnet, but no weights or strings, which
greatly simplifies the device over many others of the kind.
[Illustration: Lock Operated by a Magnet]
The weight of the long arm, L, is just a trifle greater than the
combined weights of the short arms, A and S. The fulcrum of the
lever is at C, where there is a staple. The lever swings on one
arm of the staple and the other arm is so placed that when the
lever is in an upright position, with the long arm at L', it will
not fall because of its greater weight but stays in the position
shown. The purpose of this is to leave the short arm, A, when in
position at A', within the reach of the magnet. Arm L rests on an
L-shaped hook, H; in this position the door is locked.
To unlock the door, press the button, B.
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