Scientific American Supplement, No. 611, September 17, 1887 | Page 9

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 resistance and the internal force of the tuning fork

restraining its movements.
Finally, the stylus, s, which touches the glass plate directly above y, is
for registering the moments when by the falling ball the sound is
produced and when the observer presses the key. This is brought about
by the rod, i, to which s is firmly screwed, being jerked upward a short
distance at each of these instants, so that the horizontal lines which the
stylus, s, marks upon the screen passing in front of it are broken at both
places.
The mechanism which jerks the rod, i, upward is thus arranged: The
inclined plate, p, on which the ball drops, is carried by the upper
horizontal arm of an angular lever turning on the axis, x, and
counterpoised by the balancing weight, _x'_. By the falling ball this
arm is pressed downward, and the lower horizontal arm, w, of the lever
is also moved. On a second horizontal axis the lever, v, partly
concealed, moves, restricted as to its length of swing by the screws, n.
As long as the concealed arm is not moved, v is lightly pressed by the
small spring, e, against w. The projection, z, at the upper end of v holds
the rod, i, which the strong spring, h, is continually pressing upward.
When the ball falls upon the plate, p, the arm, w, presses against the
lower end of v, the projection, z, sets free the rod, and it springs upward.
This movement is soon arrested, as the projection, _z'_, engages with a
stud situated on the right side of the rod, i. This projection is situated on
the vertical arm of an angular lever whose other arm is the key, t. When
the observer presses the key, the rod, i, again is jerked upward by the
spring, h. The screw, o, tapped into the rod, i, prevents the rod going
higher than necessary, by striking a plate, which also serves as guide
for i.
To determine the interval between the falling of the ball and pressing of
the key, one has finally to count the waves inscribed by the tuning fork,
which come under the portion of the line inscribed by s, which is
bounded by