A tuning-fork in vibration disturbs the surrounding air, and sets up air waves which produce the effect called sound when they strike against the drums of our ears. In a similar way the discharge of a Leyden jar disturbs the surrounding ether, and sets up electric ether waves; but these waves produce no effect upon us in the shape of sight, sound, or feeling. There is however a very simple piece of apparatus which acts as a sort of electric eye or ear, and detects the waves for us. This consists of a glass tube loosely filled with metal filings, and having a cork at each end. A wire is passed through each cork so as to project well into the tube, but so that the two ends do not touch one another, and the outer ends of these wires are connected to a battery of one or two cells, and to some kind of electrically worked apparatus, such as an electric bell. So long as the filings lie quite loosely in the tube they offer a very high resistance, and no current passes. If now electric waves are set up by the discharge of a Leyden jar, these waves fall upon the tube and cause the resistance of the filings to decrease greatly. The filings now form a conducting path through which the current passes, and so the bell rings. If no further discharge takes place the electric waves cease, but the filings do not return to their original highly resistant condition, but retain their conductivity, and the current continues to pass, and the bell goes on ringing. To stop the bell it is only necessary to tap the tube gently, when the filings immediately fall back into their first state, so that the current cannot pass through them. Now let us see how the “coherer,” as the filings tube is called, is used in actual wireless telegraphy. Fig. 33_a_ shows a simple arrangement for the purpose. A is an induction coil, and B the battery supplying the current. The coil is fitted with a spark gap, consisting of two highly polished brass balls CC, one of these balls being connected to a vertical wire supported by a pole, and the other to earth. D is a Morse key for starting and stopping the current. When the key is pressed down, current flows from the battery to the coil, and in passing through the coil it is raised to a very high voltage, as described in Chapter VIII. This high tension current is sent into the aerial wire, which quickly becomes charged up to its utmost limits. But more current continues to arrive, and so the electricity in the aerial, unable to bear any longer the enormous pressure, takes the only path of escape and bursts violently across the air gap separating the brass balls. Surging oscillations are then produced in the aerial, the ether is violently disturbed, and electric waves are set in motion. This is the transmitting part of the apparatus. [Illustration: _a._ Transmitting. _b._ Receiving. FIG. 33.--Diagram of simple Wireless Transmitting and Receiving Apparatus.]