During the time that signals are being transmitted, sparks follow one another across the spark gap in rapid succession, a thousand sparks per second being by no means an uncommon rate. The violence of these rapid discharges raises the brass balls of the gap to a great heat. This has the effect of making the sparking spasmodic and uncertain, with the result that the signals at the receiving station are unsatisfactory. To get over this difficulty Marconi introduced a rotary spark gap. This is a wheel with projecting knobs or studs, mounted on the shaft of the dynamo supplying the current, so that it rotates rapidly. Two stationary knobs are fixed so that the wheel rotates between them, and the sparks are produced between these fixed knobs and those of the wheel, a double spark gap thus being formed. Overheating is prevented by the currents of air set up by the rapid movement of the wheel, and the sparking is always regular. [Illustration: PLATE XIII. _Photo by_ _Daily Mirror_. (_a_) MARCONI OPERATOR RECEIVING A MESSAGE.] [Illustration: _By permission of_ _The Marconi Co. Ltd._ (_b_) MARCONI MAGNETIC DETECTOR.] In the receiving apparatus already described a filings coherer was used to detect the ether waves, and, by means of a local battery, to translate them into audible signals with a sounder, or printed signals with a Morse inker. This coherer however is unsuitable for commercial working. It is not sufficiently sensitive, and it can be used only for comparatively short distances; while its action is so slow that the maximum speed of signalling is not more than about seventeen or eighteen words a minute. A number of different detectors of much greater speed and sensitiveness have been devised. The most reliable of these, though not the most sensitive, is the Marconi magnetic detector, Plate XIII._b_. This consists of a moving band made of several soft iron wires twisted together, and passing close to the poles of two horse-shoe magnets. As the band passes from the influence of one magnet to that of the other its magnetism becomes reversed, but the change takes a certain amount of time to complete owing to the fact that the iron has some magnetic retaining power, so that it resists slightly the efforts of one magnet to reverse the effect of the other. The moving band passes through two small coils of wire, one connected with the aerial, and the other with a specially sensitive telephone receiver. When the electric waves from the transmitting station fall upon the aerial of the receiving station, small, rapidly oscillating currents pass through the first coil, and these have the effect of making the band reverse its magnetism instantly. The sudden moving of the lines of magnetic force induces a current in the second coil, and produces a click in the telephone. As long as the waves continue, the clicks follow one another rapidly, and they are broken up into the long and