Wires, of course, connect the antenna to the earth, thereby forming what is called an "oscillatory circuit." The ordinary electric circuit is a complete path of wire or other good conductor around which the current can flow in a continuous stream. An oscillatory circuit is one which is incomplete, but the ends of which are so formed that they constitute the two "plates" of a condenser. In that way, according to theory, the circuit is completed between the two ends by a strain or distortion in the "Ether" between them. A continuous current will not flow in such a circuit, but an alternating, intermittent or oscillating current will flow in it in many respects as if there were no gap at all but a complete ring of wire. At some convenient point in this oscillatory circuit are inserted the wireless instruments, one set for sending and the other set for receiving, either being brought into circuit at will by the simple movement of a switch. In small installations the central feature of the sending apparatus is an Induction Coil operated by a suitable battery or by current from a dynamo. Connected with it is a suitable spark gap consisting of two or three metal balls well insulated and so arranged that the distance between them can be delicately adjusted. This is generally done by a screw arrangement with insulating handles, so that the operator can safely adjust them while the current is on. The current from the battery or dynamo to the coil is controlled by a key similar to those used in ordinary telegraphy, the action being such that on depressing the key the current flows and the coil pours forth a torrent of sparks between the knobs of the spark-gap, but on letting the key up again the sparks cease. Since the sparks send out etherial waves which in turn affect the distant receiving apparatus it follows that a signal is sent whenever the key is depressed. Moreover, if the key be held down a short time a short signal is sent, but if it be kept depressed for a little longer a long signal is sent, by which means intelligible messages can be transmitted over vast distances.