short signals of the Morse code according to the manipulation of the Morse key at the sending station. Except for winding up at intervals the clockwork mechanism which drives the moving band, this detector requires no attention, and it is always ready for work. Another form of detector makes use of the peculiar power possessed by certain crystals to rectify the oscillatory currents received from the aerial, converting them into uni-directional currents. At every discharge of the condenser at the sending station a number of complete waves, forming what is called a “train” of waves, is set in motion. From each train of waves the crystal detector produces one uni-directional pulsation of current, and this causes a click in the telephone receiver. If these single pulsations follow one another rapidly and regularly, a musical note is heard in the receiver. Various combinations of crystals, and crystals and metal points, are used, but all work in the same way. Some combinations work without assistance, but others require to have a small current passed through them from a local battery. The crystals are held in small cups of brass or copper, mounted so that they can be adjusted by means of set-screws. Crystal detectors are extremely sensitive, but they require very accurate adjustment, and any vibration quickly throws them out of order. The “electrolytic” detector rectifies the oscillating currents in a different manner. One form consists of a thin platinum wire passing down into a vessel made of lead, and containing a weak solution of sulphuric acid. The two terminals of a battery are connected to the wire and the vessel respectively. As long as no oscillations are received from the aerial the current is unable to flow between the wire and the vessel, but when the oscillations reach the detector the current at once passes, and operates the telephone receiver. The action of this detector is not thoroughly understood, and the way in which the point of the platinum wire prevents the passing of the current until the oscillations arrive from the aerial is something of a mystery. The last detector that need be described is the Fleming valve receiver. This consists of an electric incandescent lamp, with either carbon or tungsten filament, into which is sealed a plate of platinum connected with a terminal outside the lamp. The plate and the filament do not touch one another, but when the lamp is lighted up a current can be passed from the plate to the filament, but not from filament to plate. This receiver acts in a similar way to the crystal detector, making the oscillating currents into uni-directional currents. It has proved a great success for transatlantic wireless communication between the Marconi stations at Clifden and Glace Bay, and is extensively used.