| The photograph "represents an instrument in which the
axial forces of a vertical helix raises an iron rod whose centre is below
that of the coil. This motion is communicated to a hammer, which strikes
a bell. A wire, fixed to the iron rod below, rests on a spring [the flat
strip lying on the base on the left-hand side]. When the rod is lifted
by the current in the coil, its upper end raises the handle of the hammer.
A guiding wire [up the center of the helix] keeps the rod vertical. The
circuit is broken [as the rod is lifted from the end of the flat spring]
by the lifting of the handle, and is renewed when the rod falls."
(from the 1851 edition of the Manual of Magnetism, pg 175)
The bell enables the repetition rate of the system to be monitored. More important, the inertia of the hammer sets a limit to the rate.
This apparatus is in the Smithsonian collection.
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