Telegraph

Carrier Currents.

In recent years it has become practicable to increase greatly the traffic capacity of certain classes of tele graph circuits by the use of alternating or pulsating currents of different frequencies for each message channel operated. In sys tems employing this principle each transmitter controls the appli cation to the line of current at a certain frequency. At the receiving station the complex line current composed of these numerous components is passed through electrical filters, each of which excludes all other frequencies and delivers to the receiv ing instruments only the signals represented by the particular frequency assigned to that "channel." As many as ro to 1 2 channels are often obtained on one metallic circuit by this means.

Subsidiary Systems.

Most telegraph administrations have established methods of distributing time signals to facilitate set ting clocks and other timepieces in agreement with some recog nized standard of correct time. In the United States one of the telegraph companies distributes over a very extensive network of circuits the "noon signals" furnished by the Naval observatory at Washington. These signals are used to check the operation of master clocks of great accuracy and reliability in all the prin cipal cities. The master clocks in turn control the operation of large numbers of transmitting instruments which send out hourly synchronizing impulses to over I I 5,000 electrically driven clocks.

Another adjunct to commercial telegraph service for which wide use has been found is the provision of means whereby a patron may readily summon to his place of business or residence, a messenger who will carry to the nearest telegraph office any messages which the patron may desire to have transmitted. Over a quarter of a million "call boxes" for this purpose are provided by one system in the United States. Each call box is essentially an automatic telegraph transmitter which, when operated by the turning of a crank, sends to the telegraph office code signals which identify the patron to whom the box is assigned. These signals are received upon a Morse ink-writing register and in response a messenger is promptly started 'for the office or residence indi cated as the source of the call. The circuit arrangements are characterized by ingenious devices which ensure the reception of the signal even under very adverse conditions due to faults upon the line conductors used.

Central Office Equipment.

In any large telegraph system it is necessary to concentrate the operating equipment at certain points in what are called central offices. These act as clearing houses for a definite area, collecting all the business from that area and despatching it to destination. Such offices vary con siderably in size, the variation being dependent on the area served and the volume of traffic that such area produces, ranging from one small operating room to the 155,000 sq. ft. of operating space in the new Western Union building in New York. Special attention must be paid to the design of central office buildings. The operating rooms must be in large units and be adequately provided with light and air. Properly designed illumination is a large factor in increasing the speed and accuracy of operators, and, owing to the inevitable concentration of workers, the con dition of the air is important. The greater part of the central

telegraph office is taken up with operating rooms. In the smaller offices it is usual to collect all the operating equipment in one room, but where the size of the office prohibits this, the equip ment is divided according to its method of operation. Thus those circuits which are manually worked, with a key and sounder, will be found on one floor, multiplex circuits will form a second group and short-line printer or simplex a third. Furthermore, the build ing must provide quarters for switchboard and testing equipment, for the service department—which deals with questions of ac curacy, obtaining better address, etc., after a message has been sent—and for rest rooms and lavatories for the operators.

Power Supply.

The power required for the operation of telegraph circuits is divided into local and main line, the former supplying the current which operates all the local circuits of Morse and automatic sets, and the latter that which goes out over the lines to the distant station. The local power in any one office is a fixed potential, but the main line power must be of two or three different voltages to satisfy the various require ments of the circuits. In a very small office, where the power de mands are slight, and usually of only one voltage, primary bat teries are in some cases used. This. type of battery was widely employed in the past, but is now rapidly becoming obsolete. Much more general is the use of storage batteries, which in some countries are used to supply telegraph power for the largest offices. They have the advantages of complete reliability and steady voltage. In the United States, the general practice is to use motor generators in all but the smaller offices. Power is purchased from the local supply company and is converted to the potentials required. If the local supply is direct current and is sufficiently steady and free from interruption it may be used directly and booster sets installed to give such variations in voltage as are necessary. • If the local supply is subject to fre quent interruptions storage batteries are customarily employed. Three main line voltages are provided in the Western Union sys tem, no, 16o and 240 volts, with separate machines for positive and negative current and a third, reserve, machine at each voltage. These machines have been specially designed to give constant voltage under all load conditions, a result obtained by using com pound-wound machines with a flat-compounded series field. Motor generators have the advantage over storage batteries of requiring less space and costing less to maintain. To guard against tempo rary failures of the main source of supply an emergency supply is always furnished. This may take the 'form of a storage bat tery or may be a generator set driven by a gas or gasolene engine. The emergency engine generator must be sufficiently large to provide all the telegraph power required and also to maintain sufficient lights to enable work to be carried on and, in some cases, to keep running essential services such as message con veyors and pneumatic tubes. In some offices a storage battery has been provided to take care of the interval between the failure of the main power and the starting of the emergency engine.