Patent GB-125452-A (GB)
Title: Screening or Jamming Systems for Telegraph, Telephone and other Electric Signalling Systems.
IPC: H04K1/00
Filed: 1916-08-04
Abstract: 125,452. Western Electric Co., Nash, G. H., and Mack, R. A. Aug. 4, 1916. Tapping earth return currents, preventing; " jamming" earth signal currents .-The signalling currents of telephone and other signalling systems having earth returns are prevented from being intelligibly tapped by introducing variable currents into the earth at points between the signalling circuits and the points at which their earth return currents may be tapped. The variable currents may also be employed as " jamming '' currents for leakage signalling currents between two stations. In the arrangement shown.. in Fig. 1, the signalling circuits 17, provided with earth returns, are screened from intelligible tapping at points above the line sections 11 by earthing the ends of these sections at the points 12 and placing in each section an alternating current source 13. The frequency and the value of the current from each source 13 is continuously changing. Each line section 11 may be separately earthed. When a single line .is screened the earthed screening conductor is placed parallel to the earthed signalling conductor. In the arrangement shown in Fig. 4, two stations 41, 42, earthed respectively at 44, 45 and 47, 48, are arranged to communicate to each other by earth currents. If earthed sections 11 containing sources 13 of alternating- current are laid near these stations as shown, then the alternating-current from the sources 13 will be superposed on the received signalling currents, thereby preventing intelligible signalling currents from being received. The alternating-current source placed in each line section 11 comprises an induction coil 15, Fig. 5, a variable potential source 26, and an electro-magnetic change-over switch 19. The secondary winding 14 of the induction coil 15 is connected in the line section 11, while the primary winding 16 is adapted to be connected to a portion or the whole of a battery 26 through the vibrating switch 34, the changeover switch 19 and key switches 27, 28, 29, and 31. The winding 20 of the switch 19 is energized from a battery 25 through the key switch 27, a contact 24, and the armature 21. When the armature 21 is in its extreme right-hand position and the switch 27 is operated, current is supplied from a portion of the battery 26 to the primary winding 16 of the induction coil 15, and a current of one value and of gradually increasing frequency is supplied to the line section 11. The winding 20 of the switch 19 is also energized to move the armature 21 into engagement with the contact 23 to connect the whole of the battery 26 to the primary winding 16. A current of different value and frequency will then be supplied to the line section 11. To test whether the earthing of the line circuit is maintained, a switch 30 is operated to place a telephone 33 in the line secton 11; while to test whether the two primary circuits are operative the telephone 33 is connected in circuit by the respective operation of the keys 28 and 29. The value of the current flowing in the primary circuit is obtained by switching in a milli-ampere meter 32 by the operation of a key switch 31.