October 26, 191 ij 



NATURE 



THE SHARMAN SYSTEM OF WIRELESS 

 TELEPHOXY AXD TELEGRAPHY. 

 AX TIRELESS telephony by induction and conduction 

 * * has of late years occupied an insignificant 

 position, owing to the efforts that have been made to 

 obtain satisfactory transmission of speech by means 

 of undamped oscillations, such as are produced with 

 the "singing arc." But provided that a reasonable 

 distance can be overcome, the conduction system is 

 one which demands serious attention on account of 

 the small amount of power required and the simplicity 

 of the apparatus. 



The system on which Mr. A. W. Sharman has been 

 at work for some time is based on the earlier experi- 

 ments of Preece, but its essential feature is the " im- 

 pulse coil," with which the microphonic currents are 

 intensified before transmission through the conductive 

 medium. 



During the past twelve months the writer has had 

 various opportunities of working with Mr. Sharman 's 

 apparatus, both on land and sea, and there has been 

 ample proof of the possibilities of the system, as with 



of twenty feet between the extreme ends; the ends 

 are attached to plates or rods — stair-rods proving 

 eminently practical for placing in the earth. 



The receiving circuit consists of two similar elec- 

 trodes, which are placed in series with the telephone 

 receiver. A change-over switch in the instrument 

 enables one to speak or listen at will. 

 _ There appear to be several factors upon which the 

 limit of clear speech transmission depends. Thus with 

 a wider base line, i.e. a greater distance between the 

 electrodes, longer distances can be covered, while with 

 an increase in the battery power used with the micro- 

 phone a similar result is obtained. There is a serious 

 limit, of course, to increasing the primary energy, as 

 the microphone will not admit of the use of more 

 than eight or ten volts in practice ; various forms of 

 microphone have been tried, and those permitting of 

 the use of the highest voltage and largest amount of 

 current have proved the most successful. 



A number of experiments have been made with 

 the earth as the conducting medium, and distances of 

 a mile have been covered without difficulty ; the nature 



App; 



a primary energy of only a few watts clear telephonic 

 communication has been established at distances rang- 

 ing between half a mile and a mile and a half. 



The speaking apparatus consists of a microphone, 

 battery, and impulse coil, all of which are in series; 

 tappings are taken off the coil, the two wires leading 

 to metal electrodes, which are either stuck in the 

 ground or submerged in the water, as the case may 

 be. The impulse coil consists of a comparatively low 

 number of turns of thick copper wire, wound round 

 a soft iron composite core of special construction, and 

 the result is that with every variation in resistance of 

 the microphone, when someone is talking, a momen- 

 tary current of great intensity is induced. This 

 is " transformed down " by the portion of the 

 coil used for transmitting the impulse to the conduc- 

 tive medium, the coil thus serving additionally as an 

 auto-transformer. The wires leading from the impulse 

 coil may be from ten feet in length upwards; thus with 

 two ten-foot wires there would be a possible distance 



NO. 2 191, VOL. 87] 



of the ground does not appear to be a very important 

 factor, as good speech has been transmitted through 

 chalk, gravel, and various other soils, also from the 

 interior of coal mines at a depth below the surface 

 of nearly one thousand feet ; in the latter case a 

 number of different strata separated the two instru- 

 ments, without any apparent detriment to the speech. 

 The electrodes appear to act as the foci of an 

 elliptical disturbance, which travels chiefly in the direc- 

 tion at right angles to the major axis, and not at 

 all in the line joining the two points. It is thus 

 desirable to have the two base lines parallel, and the 

 necessity for this provides a means of directing the 

 energy, so that with a flexible base line, speech will 

 only be carried in certain desired directions, and cannot 

 readily be "tapped" in other directions. The direc- 

 tional effect was very noticeable in experiments re- 

 cently carried out on the sea at Pegwell Bay, near 

 Kamsgate, even when the distance between the water 

 "plates" was a hundred feet and more. 



