. Il pratico manuale telefonico e la guida alla centrale telefonica . Fig. 280 anse, con condensatori, come mostrato in Fig. 280. Questi relè con armature pesanti a movimento lento sono azionati da correnti ordinarialternate inviate attraverso l'anello, e mentre ener-gised, E le campane collegate attraverso i contatti 4>, l'opportuna corrente pulsatoreunidirezionale J) viene sovrapposta sul proprio ramo del lineto ring la campana del sottoscrittore richiesto, attraverso il contatto relè e la connessione. Fig. la figura 280 mostra anche la combinazione di chiavi di squillo utilizzate dall'operatore che si frona
1390 x 1798 px | 23,5 x 30,4 cm | 9,3 x 12 inches | 150dpi
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. The practical telephone handbook and guide to the telephonic exchange . Fig. 280 loops, with condensers, as shown in Fig. 280. These relays hav-ing heavy slowly-moving armatures are operated by ordinaryalternating currents sent through the loop, and while so ener-gised, and the bells therebyconnected through the 4>contacts, the appropriate J)unidirectional pulsatorycurrent is superimposed onthe proper leg of the lineto ring the bell of the sub-scriber required, throughthe relay contact andearth connection. Fig. 280 also shows thecombination of ringing keysused by the operator forringing on a line connectedon the right-hand side. This arrangement, though a great improve-ment for common-batteryworking, still retains manyof the faults of the Hibbard system, with added complication at the sub-station, where itis most objectionable. The Leich System.—This 4-party line system is based. Fig. 281 276 PRACTICAL TELEPHONE HANDBOOK upon the principle that the impedance of a retardation coilvaries directly as the square of the frequency of the currentwaves traversing it, while the impedance of a condenser variesinversely as the square of the frequency. Fig. 281 gives the connections of a 4-party line. Twoordinary polarised bells are joined to the A wire and two to theB wire of the 4-party line and to earth. One bell (1000w) ofeach pair is joined directly in series with a retardation coil(2000w) and a condenser (2mf.), and the other bells (1000w) areshunted by a retardation coil (1000w) and also a condenser(3mf.) in series. Alternating currents of two frequencies are used for ringing—one of about 2400 alternations per minute, and the other ofabout 7200 alternations per minute, or three times as high.The bells first mentioned are rung only by the low-frequencycurrent, which also passes through the other bell connected tothe same leg: but as the condenser is of comparative
