1903.] Hertzian Waves, adapted for Quantitative Work. 399 



surrounding it. The amount of demagnetisation was detected by 

 means of a magnetometer. In this form, it has been much used in 

 experimental work, but it was not a telegraphic receiver.* 



In the sensitive telegraph receiver invented by Mr. Marconi the 

 change in magnetisation of the iron, due to the temporary abolition of 

 hysteresis, is detected by the production of a sound in a telephone 

 connected to a secondary coil surrounding the iron. 



After trying various forms, the writer has found that a convenient 

 magnetic detector for Hertzian waves can be constructed in the follow- 

 ing manner : — 



On a pasteboard tube, about f of an inch in diameter and 5 or 

 6 inches long, are placed six bobbins of hard fibre, each of which 

 contains about 6000 turns of No. 40 silk covered copper wire. These 

 bobbins are joined in series, and form a well-insulated secondary coil, 

 having a resistance of about 6000 ohms. In the interior of this tube 

 are placed seven or eight small bundles of iron wire, each about 

 6 inches in length, each bundle being composed of eight wires, No. 26 

 S.W.G. in size, previously well paraffined or painted with shellac 

 varnish. Each little bundle of iron is wound over uniformly with a 

 magnetising coil formed of No. 36 silk-covered copper wire in one 

 layer, and over this, but separated from it by one or two layers of 

 gutta-percha tissue, is wound a single layer of No. 26 wire, forming a 

 demagnetising coil. This last coil is in turn covered over with one or 

 two layers of gutta-percha tissue. 



The magnetising or inner coils are connected in series with one 

 another, so that when a current passes through the whole of them, it 

 magnetises the whole of the wires in such a manner that contiguous 

 ends have the same polarity. The outer or demagnetising coils are 

 joined in parallel. Associated with this induction coil is a rotating 

 commutator, consisting of a number of hard fibre discs secured on a 

 steel shaft, which is rotated by an electric motor about 500 times a 

 minute. There are four of these fibre discs, and each disc has let in 

 its periphery a strip of brass, occupying a certain angle of the circum- 

 ference. These wheels may be distinguished as Nos. 1, 2, 3, and 4. 

 The brass sector of No. 1 occupies 95° of its circumference ; the brass 

 sectors of Nos. 2 and 3 occupy 135° of their circumference; and that 

 •of No. 4 disc 140° of its circumference. Four little springy brass 

 brushes make contact with the circumference of these wheels, and 

 therefore serve to interrupt or make electric circuits as the disc 

 revolves. The function of the disc No. 1 is to make and break the 



* Note added Marcli 7th. A general term seems to be required to include all 

 forms of wave-detecting devices. The author suggests the word Jcumascope (from 

 KOfia, a wave) for this purpose. Uncouth phrases, such as. anticoherer or self* 

 decohering -coherer, which have crept into use in connection with Hertzian ware 

 telegraphy, would be rendered unnecessary. 



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