In the three systems already mentioned spark discharges are used to set up oscillatory currents in the aerial, which in turn set up waves in the ether. Each discharge sets in motion a certain number of waves, forming what is known as a train of waves. The discharges follow one another very rapidly, but still there is a minute interval between them, and consequently there is a corresponding interval between the wave-trains. In the “Goldschmidt” system the waves are not sent out in groups of this kind, but in one long continuous stream. The oscillatory currents which produce ether waves are really alternating currents which flow backwards and forwards at an enormous speed. The alternating current produced at an ordinary power station is of no use for wireless purposes, because its “frequency,” or rate of flow backwards and forwards, is far too low. It has been found possible however to construct special dynamos capable of producing alternating current of the necessary high frequency, and such dynamos are used in the Goldschmidt system. The dynamos are connected directly to the aerial, so that the oscillatory currents in the latter are continuous, and the ether waves produced are continuous also. The “Poulsen” system produces continuous waves in an altogether different manner, by means of the electric arc. The arc is formed between a fixed copper electrode and a carbon electrode kept in constant rotation, and it is enclosed in a kind of box filled with methylated spirit vapour, hydrogen, or coal gas. A powerful electro-magnet is placed close to the arc, so that the latter is surrounded by a strong magnetic field. Connected with the terminals of the arc is a circuit consisting of a condenser and a coil of wire, and the arc sets up in this circuit oscillatory currents which are communicated to the aerial. These currents are continuous, and so also are the resulting waves. The method of signalling employed in these two continuous-wave systems is quite different from that used in the Marconi and other spark systems. It is practically impossible to signal by starting and stopping the alternating-current dynamos or the arc at long or short intervals to represent dashes or dots. In one case the sudden changes from full load to zero would cause the dynamo to vary its speed, and consequently the wave-length would be irregular; besides which the dynamo would be injured by the sudden strains. In the other case it would be extremely difficult to persuade the arc to start promptly each time. On this account the dynamo and the arc are kept going continuously while a message is being transmitted, and the signals are given by altering the wave-length. In other words, the transmitting aerial is thrown in and out of tune alternately at the required long or short intervals, and the receiving aerial responds only during the “in-tune” intervals.