One of the most sensitive of these self-restoring kumascopes is the carbon-steel-mercury coherer, the invention of which has been attributed to Castelli, a signalman in the Italian Navy,[46] but also stated on good authority to have been the invention of officers in the Royal Italian Navy, and has, therefore, been called the Italian Navy coherer.[47] This instrument has been arranged in several forms, but in the simplest of these it consists of a glass tube, having in it a plug of iron and a plug of arc-lamp carbon, or two plugs of iron with a plug of carbon between them. The plugs of iron, or of iron and carbon, are separated by an exceedingly small globule of mercury, the size of which should be between one and a-half and three millimetres. The plugs closing the tube must be capable of movement, one of them by means of a screw, as shown in the diagram (Fig. 17), taken from a patent specification communicated to Mr. Marconi by the Marchese Luigi Solari, of the Royal Italian Navy. One of the plugs of this tube is connected to the aerial and the other to the earth, and they are also connected through another circuit composed of a single dry cell and a telephone. The arrangement then forms an extremely sensitive detector of electric waves or of small electromotive forces, or, if a wave falls on the aerial, the electromotive force at once improves the contact between the mercury and the plugs, and therefore causes a sudden increase in the current through the telephone, giving rise to a sound; but when the wave ceases, or the electromotive force is withdrawn, the resistance falls back again to its origin value, and the arrangement is, therefore, self-acting, requiring no tapping or other device for restoring it to receptivity.