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_. Furthermore, lest the friction of
the wire, _b_, in the guiding apertures of the frame should impair its
velocity as it moves from left to right, it is connected with a weight
pan by a cord passing over the pulley, _g_, which is so loaded that by
the added velocity with which it strives to fall, the retardation
already alluded to is overcome, so that the frame moves from left to
right with even speed.

In front of the frame, _a a_, is the tuning fork, _f_, which as
estimated makes 184 vibrations in a second. By the stylus, _y_, on the
upper limb of the fork these oscillations are marked upon the sliding
plate of glass as a wave line. Lest, after the first impulses of the
fork have been registered, they should soon die away, in front of it
is an electro-magnet, H, whose pole-faces near the arms of the tuning
fork pass over them. The latter, to be more strongly affected by the
magnet, are provided with faces of soft iron. To the lower face of the
lower arm of the fork a small sharp stylus is fastened, which, with
each beat of the fork, comes into contact with the mercury in the
little cup, _n_, or a spring used instead of it. This closes an
electric circuit, which passes around the magnet, thence going through
the tuning fork by the binding screw, _k_, and thence by connections
not shown in the cut back to the battery. In consequence of the
magnetism thus excited, the arms of the tuning fork are attracted by
the poles of the magnet, and forced to beat with increased amplitude.
In a short time a constant amplitude of oscillation is reached, when
the magnetic impulses are of equal influence with the atmospheric
r