[Illustration: FIG. 1.]
To avoid the errors which sometimes occur in a pharmacy or in a
laboratory, where one bottle is taken for another, especially in the
case of those containing highly poisonous or dangerous substances, a
simple arrangement, shown in the cuts, has been proposed. The
apparatus, in principle, is a species of electrical alarm, in circuit
with an ordinary house telegraph line. It consists essentially, as
shown in Fig. 1, of a battery, bell, and pedestal, provided with an
electric contact on which the flask rests. Fig. 2 shows this contact
or break piece. On a series of pedestals thus arranged and
intercalated in the same circuit the flasks containing poisonous or
dangerous substances, whose inadvertent handling might cause trouble,
are placed. In removing one of these flasks the circuit is closed, and
the electric bell notifies the pharmacist of the danger attendant on
the use of the substances contained in the flask referred to, thus
guarding against the errors due to carelessness, and quite too
frequent, especially in pharmacies.--_Chronica Cientifica._
[Illustration: FIG. 2.]
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APPARATUS FOR DETERMINING MECHANICALLY THE REACTION PERIOD OF
HEARING.
The following apparatus, constructed after the designs of Dr. Loeb,
assistant in the Physiological Institute at Wurzburg, is for the
purpose of measuring the reaction period of hearing, that is, the
period which elapses between the time when a sound wave affects the
auditory nerve and is thence transferred to the brain, then affecting
the consciousness, and the moment when the motor nerves can be thrown
into action by the will. It is, therefore, necessary to fix both
instants--when the sound is produced and when the observer has, from
its warning, received the impulse so as to press down a key. The great
advantage of this instrument over others adapted for the same end
consists in this, that the determination in its essentials is effected
entirely by mechanism, and, therefore, the graphic results attained by
it are free from all sources of error, which errors other methods
always introduce to a greater or less extent. Thus its results are
quite unexceptionable.
[Illustration: REACTION PERIOD OF HEARING.]
The apparatus shown in the cut rests on three feet, two of them
consisting of strong screws, so that by aid of the circular level,
_l_, on the base plate, it can be adjusted perfectly level. On a
little shelf attached to a square rod, seen on the left of the
instrument, rising from the base plate, and near its top, is a
horizontal tube, through which, by a bulb not shown in the cut, a
blast of air can be blown. In front of the other opening of the tube
is a horizontal fork of ebonite, whose arms carry on the side opposite
the tube a metallic ball. Through the arms of the fork pass the wires
of the circuit of an electric battery. These terminate in two rounded
ends, which, when the arms approach each other, are touched by the
metallic ball, so that the latter also closes the metallic circuit. By
the blast of air a wooden wedge contained in the tube is driven
between the arms of the fork, the ball falls from them, and the
electric stream is cut off. 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_. 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 the two breaks produced by the successive movements of
the rod.
To make the glass plate carried by the frame available for more
observations, which plate can be used as a photographic negative, the
frame, T, is adjustable up and down upon the pillars, N. This frame
carries the tuning fork, mercury cup, _n_, and the electro-magnet, M.
The spring, _s_, can also be moved up and down along the rod, i.--_H.
Heele in Zeitschrift fur Instrumentenkunde._
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