The ball drops upon the inclined metallic
plate, _p_, bounces off it, and is received in a little sack, S. When
the observer hears the ball strike the plate, he presses on the key,
_t_, and the interval between the two instants, namely, the falling of
the ball upon the plate and the pressing of the key, _t_, is what is
to be mechanically fixed and measured.
The electric current, which is closed by the ball as long as it lies
on the jaws of the fork, flows around the arms of the electro-magnet,
_m_, which continually attracts an armature fastened to a lever arm,
and coming over the poles of the magnet. If the circuit is broken by
the fall of the ball, the armature at once rises upward. By this a
spring contained in the tube, _g_, and hitherto kept compressed, is
released, which gives a shock to the right angled frame, _a a_,
containing a blackened or smoked plate of glass, so that, following
the wire, _b_, acting as a guide, the plate flies from left to right
of the apparatus. To prevent the plate from recoiling, a catch, _d_,
is fastened to the side bar, _c_.
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