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Polar Direct-Current Telegraph Systems

In a polar, or double-current, system2 the signal impulses are as shown in Fig. 6 if the capacitive and inductive effects are neglected. A dot or a dash is caused by current in one direction, whereas a space is produced by current flow in the opposite direction. This accounts for the term "double-current."

signal in double-current system
Figure 6. Shape of signal current in polar, or double-current, system. Inductance and capacitance are assumed negligible.

The use of double current requires a relay which is polarized,2 that is, which will operate in only one direction for current flow in a specified direction. A relay may be polarized by using a permanent magnet, or by the use of an auxiliary direct-current winding which produces a polarizing magnetic flux. A diagram of a magnetically polarized relay is shown in Fig. 7.

polarized telegraph relay
Figure 7. Diagram of a polarized, or polar, telegraph relay. The magnetic flux produced by the permanent magnet is as shown by solid arrows, and that produced by a given direction of current in the line, or operating, windings is shown by the dotted arrows. The armature, which deflects in one direction or the other in accordance with the telegraph signals in the operating windings, is located at the center. The contacts at the top operate the local circuit to the telegraph office equipment that receives the telegraph signal. (Courtesy Bell System.)

The polarized, or polar, relay consists of yokes of good magnetic flux-conducting material, such as Permalloy, and one or more operating windings that are connected to the line. A permanent magnet provides the polarizing effect. As indicated in Fig. 7, an armature (also of good flux-conducting material) operating in the air gap carries the contacts for closing the sounder circuit. When a current in one direction (for instance, corresponding to a space) flows through the circuit, the steady magnetic flux provided by the permanent magnet will be decreased in one yoke and the flux in the other yoke will be strengthened. The armature will then be drawn to the spacing position. When current in the other direction flows, the action is reversed and the armature moves to the marking position, closing the sounder circuit and causing an audible sound corresponding to the signal being sent.

A simplified polar, or double-current, system giving single operation2 or simplex operation (that is, transmission in only one direction at a time) is shown in Fig. 8. With the switches as indicated, transmission is from the west (left) to the east station. The relays RW and RE are polarized relays actuating the connected sounders for current in one direction only.

double-current telegraph system
Figure 8. Double-current, or polar, system for providing single operation. Relays RW and RE operate telegraph sounders at the west and east stations, respectively.

Last Update: 2011-05-30