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.