Against D presses a
brass brush, B1 connected with the binding post, T1; while under W is a
long strip of springy brass against which W presses during part of every
revolution. T2 is connected to one end of the coil winding, and T1 through
a 4-volt accumulator or three dry cells, with the other end of the coil.
When W touches B2 the circuit is completed, and the coil draws in the
plunger, the contact being broken before the plunger gets home. The crank
rotates at a very high speed if there is plenty of battery power, all the
moving parts appearing mere blurs.
CONSTRUCTION.
The coil is made by winding 4 oz. of No. 32 cotton-covered wire (price 6d.
to 8d.) on a boxwood reel 2 inches long and 1-1/2 inches in diameter, with
a 9/16-inch central hole. Before winding, bore a hole for the wire through
one end of the reel, near the central part, and mount the reel on a lathe
or an improvised spindle provided with a handle of some kind. The wire
should be uncoiled and wound on some circular object, to ensure its paying
out regularly without kinking; which makes neat winding almost impossible.
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