We must next direct attention to those wave-detecting devices which depend upon magnetisation of iron, and here we are able to record recent and most interesting developments. More than seventy years ago Joseph Henry, in the United States, noticed the effect of an electric spark at a distance upon magnetised needles.[50] Of recent times, the subject came back into notice through the researches of Professor E. Rutherford,[51] who carried out at Cambridge, England, in 1896, a valuable series of experiments on this subject. He found that if a magnetised steel needle or a very small bundle of extremely thin iron wires is magnetised and placed in the interior of a small coil, the ends of which are connected to two long collecting wires, then an electric wave started from a Hertz oscillator at a distance causes an immediate demagnetisation of the iron. This demagnetisation he detected by means of the movement of the needle of a magnetometer placed near one end of the iron wire. Although Rutherford's wave detector has been much used in scientific research, it was not, in the form in which he used it, a telegraphic instrument, and could not record alphabetic signals. Not long ago Mr. Marconi invented, however, a telegraphic instrument based upon his discovery that the magnetic hysteresis of iron can be annulled by electric oscillations. In one form, Mr. Marconi's magnetic receiver is constructed as follows[52] (see Fig. 18): An endless band of thin iron wire composed of several iron wires about No. 36 gauge, arranged in parallel, is made to move slowly round on two pulleys, like the driving belt of a machine. In one part of its path the wire passes through a glass tube, on which are found two coils of wire, one a rather short, thick coil, and the other a very fine, long one. The fine, long coil is connected with a telephone, and the shorter coil is connected at one end to the earth and the other to the aerial. Two permanent horse-shoe magnets are placed as shown n Fig. 18, with their similar poles together, and, as the iron band passes through their field, a certain length of it is magnetised, and owing to the hysteresis of the material, it retains this magnetism for a short time after it has passed out of the centre of the field. If then an electric oscillation, coming down from the aerial, is passed through the shorter coil, it changes the position of the magnetised portion of the iron and, so to speak, brings the magnetised portion of iron back into the position it would have occupied if the iron had had no hysteresis. This action, by varying the magnetic flux through the secondary coil, creates in it an electromotive force which causes a sound to be heard in the telephone connected to it. If at a distant place a single wave or train of waves is started and received by the aerial, this will express itself by making an audible tick in the telephone, and if several groups of closely adjacent wave trains are