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.
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 ½frame, 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.