On the opposite side, I C is the induction-coil; K is the telegraphic key, or switch, which makes and breaks the current; S B is the storage-batteries, or source of electric energy; and S G the spark-gap between the brass balls on the terminal rods. By closing the circuit at K the current flows through the primary of the induction-coil, affects the secondary coil, and causes a spark to leap across the gap between the brass balls. This instantly sets the ether in motion from A on the right, and the impulse is picked up by A on the left. This oscillation breaks down the resistance of the filings at C, and the current from battery, or dry cell (D C), flows through the filings and operates the sounder, or relay (R). This operation takes place instantly, and the particles of metal are seen to cohere, or shift, so that better contact is established. But as soon as the spark has jumped across the gap the action of cohesion ceases until the key (K) is again operated to close the circuit and cause another spark to leap across the gap. The shifting of the metal particles on the board (C) is what takes place in the glass tube of the coherer, Fig. 23, but in this confined space the particles will not drop apart again as on the flat surface, but will continue to cohere. A de-coherer is necessary, therefore, to knock the particles apart, so that the next oscillatory impulse will have a strong and individual effect. There are several forms of de-coherers in use, but for the amateur telegrapher an electric-bell movement without the bell, or, in other words, a buzzer with a knocker on the armature, will answer every purpose. (See description of buzzer on page 64.) It must be properly mounted, so that on its back stroke, or rebound, the knocker will strike the glass tube and shake the particles of metal apart. For this purpose the vibrations of the armature should be so regulated as to obtain the greatest possible speed, in order that the dots and dashes (or short and long periods) will be accurately recorded through the coherer and made audible by the sounder or telephone receiver.