The Telegraph Sounder

The Telegraph Sounder

The telegraph sounder was invented by Alfred Vail after 1850 to aid the rapid reception of Morse Code by ear, replacing the slow and cumbersome Morse register. The register hung on for many years, since it did not demand a highly trained operator. The sounder is simply an electromagnet attracting and releasing a movable armature that makes audible clicks when it strikes its upper and lower stops. The simplicity is somewhat misleading, because the device is arranged to do its job as faultlessly as possible. Practically all sounders were of a very similar classic form. The sounder that I shall describe here was made by the Signal Electric Manufacturing Company of Menominee, Michigan, and is typical.

The sounder is mounted on a finished wooden base 5-1/4 x 3 x 3/8 inches, and is just under 4 in high overall. It consists of a magnetic circuit and a hammer and anvil assembly, screwed to an iron baseplate. The magnetic circuit of the sounder consists of a pair of bobbins on which lacquer-insulated #24 AWG copper wire is wound around an iron core, and top and bottom pieces completing the magnetic circuit. The upper piece is attached to the pivoted hammer bar. The coils are protected by a fabric covering, and by black fiber washers at top and bottom. Connections are made to two screw terminals mounted on the wooden base. The windings have a resistance of about 3.8 W , which is suitable for a local circuit energized by a battery of 1 or 2 V. At 1.5 V, the windings take about 0.4 A. A sounder of this type would be operated by a relay in the line that responded to normal line currents of 20 mA or so, and wound with fine wire, such as #30 AWG. The local battery would be one or two jars of gravity. Special sounders with higher resistance (and lower operating current) were also available for direct connection in the line in emergencies, and these were often mounted together with a key in a set, called a pocket relay, protected by a case. The sounder does not care about the polarity of the current operating it, responding identically to either direction.

The brass hammer bar, 5/16 x 3/8 x 3-3/16 inches, is pivoted 9/16 in. from one end on an iron arbor that mates with brass cup screws held in a horseshoe-shaped iron yoke. The yoke also has an adjusting screw for the phosphor-bronze helical spring that fits in a 1/4 in. hole in the top of the bar. The other end of the bar is received in a rectangular opening in the anvil. The anvil is brass, 5/16 thick, and nearly 2 in. high, which, like the yoke, is fastened to the 2-3/16 x 3-3/8 x 3/32 inch iron base. The anvil holds an adjusting screw for the upper stop, and a similar screw is held in the hammer bar for the lower stop. Some earlier sounders had both limit screws in the yoke, and a different form of spring. The iron plate is fixed to the wooden base by three machine screws in threaded holes in the plate. All brass is bright, all iron is blacked. The name of the manufacturer is on the side of the hammer bar. The wooden base has a single hole with a metal bushing for screwing the sounder to the desktop.

The stops and the spring are adjusted until the sounder gives clicks of the same strength at top and bottom limits. The travel of the hammer bar must not be too small nor too large. The proper adjustment depends on the strength of the current to some degree. This adjustment is not difficult, and once made, is relatively permanent. Each of the adjustment screws has a stop nut that maintains the adjustment. In quiet surroundings, the sounder itself makes a satisfactory sound. It was common to mount the sounder in an open wooden enclosure on a jointed arm, so that it could be brought closer to the operator's ear, and even to wedge an empty Prince Albert can so that it touched the metal baseplate, to give it a distinctive voice, as well as for added amplification. This was especially valuable if other sounders in the vicinity were also busy.

The sounder (to be precise, its relay) was connected to the desired circuit by means of a switchboard using brass plugs. Normally, the circuit was closed, and the sounder hammer was on the lower stop. When someone wished to transmit, he or she opened the switch across the key contacts, breaking the circuit and causing the sounder to click as the hammer hit the upper stop. This, of course, attracted attention. When the transmission was over, the key switch was again closed, and the sounder clicked again. This signaled that you could now break the circuit for a transmission, if you wished. The circuit was normally closed, and you could tell that it was good by looking at the sounder (or relay). The circuit was a party line, and everyone could hear everything that went on.

The American manual telegraph was a remarkable system of simple, rugged apparatus extremely well adapted to its purpose, combined with skilled operators. A typical experienced operator could send and receive at 25 to 30 words per minute. The corresponding bit rate of no more than 25 bps was well handled by wire lines on poles that naturally had low capacitance, and by the small inductance of the relays and sounders. In fact, ground return circuits were universally used for telegraphy, and were quite adequate. The gravity cells later used were excellent on closed circuit; in fact, they had to be so operated to keep the solutions apart. Since they were in fixed, protected locations, they were reliable and easily maintained. Because the telegraph used skilled workers in a world of unskilled labor, as well as giving the first real opportunities for women outside menial labor, its social aspects are as interesting as the technical ones.

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Composed by J. B. Calvert
Last revised 18 July 1999