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.