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advantages, both the series and the bridging systems are widely used in small communities, the cost of such installations being materially less than for direct-line service.

It has been estimated that for small businesshouses, or where 8 or 10 calls are the daily average, but two stations should be connected on a line; for residences, or where 3 or 4 calls per day are usually made, but 4 stations should be connected; and for certain rural districts, up to 10 or 12 stations are practicable.

The bridging system is preferable to the series system in that the path of the talking current is cleared of the impedance offered by the various bell magnets in the latter case, for when the bells are in series their individual impedances are added together. It is thus possible in the bridging system to have a greater number of stations connected on a line (in certain localities as many as 25 are connected), to operate over much longer distances, and to have far better service than in the series system. On the other hand, bridging bells, which on any one line should all be wound to the same resistance, cost from 25 to 50 per cent. more than series bells, and when many stations are connected 4-magnet generators must be used in place of 3-magnet generators; these increased expenses, however, are small in comparison with the advantages mentioned.

Different forms of Telephone Sets.—Telephone sets are made up in a great variety of ways; two of the most common forms are the solid-back wall set and the desk set shown respectively in Figs. 31 and 46. In addition to these a number of other forms are shown in Figs. 54, 55, 56, and 57. Fig. 54 illustrates a cabinet wall set. This set differs from the solid-back wall set, Fig. 31, in having the battery box b extended to the floor so that the set is supported upon the floor rather than by the wall. This is desirable in cases where the wall cannot be used for the purpose. The induction coil is mounted in the magneto box a, and the battery cells are placed one above the other in b instead of side by side as in the solidback wall set. Fig. 55 shows a hotel or residence set, its compactness being its chief characteristic. At B the door of the set is open, showing the box to contain all of the telephone apparatus except the Fig. 54.—Cabinet battery, which must be placed elsewhere. The transmitter is placed upon a knuckle joint, which permits of the usual vertical adjustment provided in most sets for conforming to the height of the user. Fig. 56 shows a swinging

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Wall Set

ch must be placed

arm desk set for office use. This set differs considerably from the desk set, Fig. 46; its adjustable

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arm a enables the apparatus to be kept ready for instant use, yet out of the way and securely fastened in place. Fig. 57 shows a desk cabinet set, intended for use where it is desired to sit down in telephoning; this set is especially suited for hotels, business offices, and telephone booths. The transmitter t is mounted on an iron arm, and the head of the transmitter is hinged so as to be adjustable. The battery is placed in the box b below the writ

FIG. 56. Swinging-Arm Desk Set

ing desk m, and the rest of the apparatus is mounted in the glass-covered compartment n.

Telephone Booths.—These are sound-proof wooden cabinets containing a telephone set and the necessary space for one person in telephoning They are built with double walls, roof, floor, door, and windows, and have an air space of 1 inch between the inner and outer parts. This air space deadens or retards the passage of sound from the outside to the inside, or the reverse. Fig. 58 shows the usual form of a telephone booth.

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