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POPULAR SCIENCE MONTHLY. 



valuable series of experiments on this subject. He found that if a 

 magnetized steel needle or a very small bundle of extremely thin iron 

 wires is magnetized and placed in the interior of a small coil, the 

 ends of which are connected to two long collecting wires, then an elec- 

 tric wave started from a Hertz oscillator at a distance causes an imme- 

 diate demagnetization of the iron. This demagnetization he detected 

 by means of the movement of the needle of a magnetometer placed near 

 one end of the iron wire. Although Eutherford'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 instru- 

 ment based upon his discovery that the magnetic hysteresis of iron can 

 be annulled by electric oscillations. In one form, Mr. Marconi's mag- 

 netic receiver is constructed as follows''^ (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 wound 

 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 con- 

 nected at one end to the earth and the 

 other to the aerial. Two permanent horse- 

 shoe magnets are placed as shown in Fig. 18, with their similar 

 poles together, and, as the iron band passes through their field, 

 a certain length of it is magnetized, and owing to the hysteresis of the 

 material, it retains this magnetism for a short time after it has passed 

 out of the center 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 magnetized portion of the iron and, so to speak, brings 

 the magnetized 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 electro- 

 motive force which causes a sound to be heard in the telephone con- 

 nected 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 ad- 



* See Proc. Roy. 8oc. Lond., June 12, 1902. 'Note on a Magnetic Detector 

 for Electric Waves whidi can be employed as a Receiver for Space Telegraphy,' 

 by G. Marconi. 



Fig. 18. Marconi Mag- 

 netic Receiver. Wi W.,, 

 wheels; /, iron wire band 

 P, primary coil ; S, secondary 

 coil; T, telephione; A, aerial ; 

 E, earthplate. 



