H

the plug cords c, etc., the ends of which are wrapped with brass wire wound spirally. These are bent around in the form of a hook and returned through the upper holes. The wires from the ringing keys are passed through the holes in the opposite ends of the strips and soldered permanently in place. The operator's transmitter t is connected in circuit by the flexible wire cord e, which, by aid of the weight i, balances the transmitter and enables it to be adjusted as to height. The relative positions of the line drops and clearingout drops are shown at 7 and o respectively.

The Useful Life of the Switchboard is

about 20 years. During this time it will require occasional repairs, particularly on the cords and plugs, and perhaps a drop will burn out now and then, necessitating a new one.

FIG. 125.-End Elevation of an Exchange Switchboard, Showing the Relative Positions of the Apparatus on It

Telephone Exchange Service should be supplied under a definite form of contract. This latter

may take the form of a flat-rate contract in which the party supplied agrees to pay a fixed sum per year for unlimited telephone service. Another form is the message-rate contract in which a party agrees to pay for a certain number of messages sent per year. If this number is not reached he is allowed a rebate, but if the number is exceeded. he is charged a fixed sum per message in addition to his contract price. Still another form is the pay-station contract often made with shop- and hotel-keepers. In this, a telephone set is installed for public use, and the party in whose place of business it is located is allowed a percentage of all calls above a certain number sent over his line, he collecting from the public the usual charge per call. In message-rate and pay-station contracts the operator at the exchange must keep account of the number of such calls sent in and the stations to be charged with them.

Lamp Annunciators. The most advanced practice on switchboards is to substitute incandescent lamps for mechanical drops. Eight to twenty volt bulbs of low candlepower are used, one for each calling up connection. A simple low voltage lamp represents the maximum of simplicity and takes the place of the mechanism of the drop, inevitably more or less complicated. Lamps are considerably cheaper than the modern self-restoring drops, as they operate when current passes and cease to give light when the current flow stops. In this manner they present the self

restoring feature of the most improved magnetic drops without complication of the latter. Lamp signals are coming into general use on the large and more important switchboards. At first, very low voltage lamps were used, but these proved unreliable, because they were susceptible to very slight changes in the voltage, were costly because they were hard to make and burnt out very easily. To produce the lighting current a storage battery is installed in the central station, which gives an almost constant voltage. In one system the removal of the receiver from the hook switch throws the lamp in circuit with the station storage battery. The lighting current goes through the whole line and through the transmitter at the subscriber's station.

Several objections have been advanced in reference to this system. The most valid is that if a cross occurs between the lines, a very low resistance circuit is produced through which current may reach the lamp, and perhaps burn it out. The obvious way of disposing of this trouble is to put the lamp on a relay circuit. The calling current closes the relay and the lamp is lighted from the storage battery through the unchanged resistance of the local circuit. While it is apparently more complicated than the straight circuit system, the relay system avoids the necessity of adjusting the resistance of long and short circuits so as to give each of the lamps the proper current. A simple presentation of the lamp signal an

nunciator system is given at Fig. 126. The subscriber's apparatus is shown in the upper part of the cut at B and the central office connections in the lower part at C. At C, g and h indicate choke

coils, the annunciator lamp and rri a battery. The apparatus g, h and I are all in one metallic circuit, and there is one such circuit with a numbered lamp for each subscriber.

When the receiver is on the hook-switch, the circuit, including the annunciator lamp is closed through the high resistance of about 1,000 ohms of the calling bell e. This cuts down the current so the lamp shows no light. When the receiver is

taken off the hook

switch, this springs up and closes the circuit by coming in contact with the two terminals above it, as shown at C. This short-circuits the bell coils. In this short circuit is included the secondary of the induction coil at the subscriber's station. This short circuit aggregates less than 50 ohms for the most part and the lamp is lighted.

This notifies the central station operator, who effects the desired connection when told by the calling subscriber. The local battery at the subscriber's house sometimes is a storage battery which operates on the transmitter when the hook-switch is released. When the receiver is hung up on the hook, the local secondary battery is in closed circuit with the coils of the calling bell magnet and receives a slight current of about one-fiftieth ampere, which keeps the storage battery in good condition for use.

The foregoing description is of the simplest kind of a lamp signalling system and there are many variations, some involving more complicated connections. If the local battery becomes too weak, the subscriber's transmitter will work with current from the central station battery and the local battery will act as a kind of an equalizer. In general practice the calling lamp is on a relay circuit and the relay closes its circuit when the receiver is taken off the hook.

Automatic Telephone Systems.-Automatic telephone systems, in which the central office. operators are dispensed with and their functions performed by ingenious mechanism, are in use in a number of large cities, notably Los Angeles, Cal.; Portland, Ore.; Oakland, Cal.; Omaha, Neb.; Columbus, Ohio; Grand Rapids, Mich., and several others. The automatic telephone switchboard appears to be a very complicated assembly and while it is more complex than the manually