. Le manuel téléphonique pratique et le guide de l'échange téléphonique . Figure 280 boucles, avec condensateurs, comme illustré sur la Fig. 280. Ces relais hav-ing lourds armares à mouvement lent sont actionnés par des courants alternatifs ordinaryenvoyés à travers la boucle, et tout en étant ainsi ener-gaisé, Et les cloches qui y sont reliées par les contacts 4>, le courant pulsatorydirectionnel J) approprié est superposé sur la jambe appropriée de la ligne pour faire sonner la cloche du sous-scriber requis, par le contact de relais et la connexion de terre. Figure 280 montre également la mise en garde de la sonnerie par l'opérateur forsonnant
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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
