For this reason the plan actually
shown at N is preferable, since the work done by the moving pin,
_i.e._, by the descending bell, is always the same. P represents a
carbide-feed effected by a spiral screw or conveyor, which, revolved
periodically by a moving bell, draws carbide out of a hopper of any
desired size and finally drops it into a shoot communicating with a
generating chamber such as that shown in L. Here the work done by the
bell is large, as the friction against the blades of the screw and the
walls of the horizontal tube is heavy; but that amount of work must
always be essentially identical. The carbide-feed may similarly be
effected by means of some other type of conveyor instead of the spiral
screw, such as an endless band, and the friction in these cases may be
somewhat less than with the screw, but the work to be done by the bell
will always remain large, whatever type of conveyor may be adopted. A
further plan for securing a carbide-feed consists in employing some
extraneous driving power to propel a charge of carbide out of a reservoir
into the generator. Sometimes the propulsive effort is obtained from a
train of clockwork, sometimes from a separate supply of water under high
pressure.
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