It will thus be seen that the primary current passes through the transmitter, and the secondary traverses the line. The most familiar forms of the telephone are those seen in Figs. 61 and 62, but the ideal form is rigged in a cabinet or little room, which excludes all extraneous interfering sounds. [Illustration: FIG. 61.--WALL TELEPHONE.] [Illustration: FIG. 62.--DESK TELEPHONE.] With the Bell receiver and the Blake transmitter a good practical telephone system may be constructed, but the improvements which have been made in the short life of the telephone are beyond adequate description, or even mention. They relate to the call bell, the battery, the switchboard, meters for registering calls, conductors, conduits, connections, lightning arresters, switches, anti-induction devices, repeaters, and systems. Among those most prominently identified with its development are Bell, Edison, Berliner, Hughes, Gray, Dolbear and Phelps. The activity in this field is best illustrated by the fact that the art of telephony, begun practically in 1876, has at the end of the Nineteenth Century grown into some 3,000 United States patents on the subject. [Illustration: FIG. 63.--TELEPHONE EXCHANGE.] That which has given the telephone its greatest commercial value is the “exchange” system, by which at a central office any member of a telephonic community may be instantly put into communication with any other member of that community. For this purpose, see Fig. 63, a continuous switchboard is arranged along the side of a large room and occupies most of that side of the wall. It comprises a great array of annunciator drops, spring jacks with plug seats, and connecting cords with metal plugs at their opposite ends. Each subscriber is connected to his own spring jack and annunciator drop, and his call to central office (from his magneto-bell) throws down the annunciator drop which bears the number of his telephone, and announces to the attendant his desire to communicate with another. To insure the attention of the attendant, a tiny electric lamp is by the same action lighted directly in front of her, which acts as a pilot signal to call her attention to the drop. The attendant now puts a plug in that spring jack, which automatically restores the drop, and she then asks the number which the subscriber wants, and, upon ascertaining this, puts the plug at the other end of the connecting cord into the spring jack of the subscriber wanted, and by this action disconnects her own telephone. As every telephone subscriber has in the central office an apparatus exclusively his own, it will be seen that a telephone community of several thousands of subscribers involves an imposing array of multiple connections, and a great expense in construction. Girls are chosen as exchange attendants because their voices are clearer. Every telephone jack, however, does not have its Jill, for each girl has charge of a hundred or more jacks,