The London, Edinburgh and Dublin philosophical magazine and journal of science . lence of a looped circuit of this kind to a straight-awaycircuit had been shown, provided the circuit was balanced asall telephone circuits must be balanced in order to eliminatecross talk and noise. This point was also carefully testedduring the investigation. For the loading of this cable 300 coils were cross-section of this loading coil, known as the T-14coil, is shown in fig. 2. On a wooden spool a primary of578 turns of Xo. 20 single cotton-covered wire was wound,and a secondary of ±6d turns of
The London, Edinburgh and Dublin philosophical magazine and journal of science . lence of a looped circuit of this kind to a straight-awaycircuit had been shown, provided the circuit was balanced asall telephone circuits must be balanced in order to eliminatecross talk and noise. This point was also carefully testedduring the investigation. For the loading of this cable 300 coils were cross-section of this loading coil, known as the T-14coil, is shown in fig. 2. On a wooden spool a primary of578 turns of Xo. 20 single cotton-covered wire was wound,and a secondary of ±6d turns of No. 20 single cotton-coveredwire. The turns were so chosen as to give the primary andsecondary the same inductance, and they also had approxi-mately the same resistance. The cable circuit, as has been Lines in Telephonic Transmission. 327 explained, consisted of 300 lengths connected in each length and the succeeding length a T-14 coilwas inserted, the primary in one wire of the pair and thesecondary in the other wire of the pair, the connexions being Fig. ed ia. so made as to put the coil into the cable inductively. Eachcoil added about *11 henry and 12 ohms to the circuit. Toensure the reliability of the test it was necessary to so placethe loading coils that the mutual induction should be negli-gible. Accordingly they were spread over all the spaceavailable, and tests snowed that any effect of mutual inductionbetween coils was quite negligible. The experimental line is shown in the photograph onPlate VI. The three reels of cable are all visible: one is inplain sight, another is at the extreme left, and the third can bedistinguished at some distance to the right, The reel at theextreme right had no connexion with this experiment. Thecables were brought out to pot-heads, and each wire termi-nated in a screw-cup. In this way any combination of con-nexions could be made. Two of the pot-heads of the middlereel of cable are in plain sight in the phot »graph.
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Keywords: ., bookcentury1800, bookdecade1840, bookidlondon, booksubjectscience
