Sounder/Receiver

Part of Telegraph

The telegraph sounder converts electrical pulses from the telegraph line into audible clicks, allowing trained operators to read Morse code by sound rather than by sight.

Why This Matters

The sounder replaced the original Morse writing instrument (which scratched marks on moving paper tape) as the primary telegraph receiver, and the reason is instructive: trained operators could copy code from sound faster than from written marks. The human ear, trained to recognize patterns, processes the dot-dash rhythms of Morse code faster than the eye processes visual marks. A skilled sounder operator could copy 35+ words per minute by ear in an era when typewriting to that speed was a significant skill.

This principle — that the ear is optimized for pattern recognition in time — explains why Morse code, designed as a visual system, works better as an auditory system. It also explains why modern CW radio operators listen to Morse code rather than decode it visually from an oscilloscope. The sounder design itself reflects this: it is optimized not to write or display information, but to make a loud, distinct click at each current pulse, providing clean acoustic signals for the trained ear to interpret.

For a rebuilding community, the sounder is the easiest-to-build receiving instrument and the standard for a simple telegraph system. Once operators are trained to copy by sound, the sounder is preferred over any visual or writing instrument for its speed and simplicity.

How a Sounder Works

The sounder is an electromagnet with a spring-loaded metal armature positioned near the pole face. When current flows through the electromagnet coil, the armature is attracted and strikes a stop (an anvil or backstop). When current stops, the return spring lifts the armature back to its rest position, which also strikes a stop.

Two distinct clicks per code element: the “down click” when current flows (armature attracted), and the “up click” when current stops (armature returns). An experienced operator hears the rhythm of these paired clicks as dots and dashes. A dot sounds like click-click in rapid succession; a dash sounds like click… click (longer pause between clicks).

The tone and quality of the clicks is adjustable and matters for comfortable operation over long periods. A sharp, bright click is tiring to listen to for hours. A muffled, soft click is hard to hear in a noisy environment. The ideal sound is a clean, medium-pitched “tick-tock” that carries above ambient noise but is not harsh. This is achieved through the combination of armature mass, anvil material, stop adjustment, and return spring tension.

Sounder Construction

The electromagnet: soft iron core (annealed iron bar, 10–15 mm diameter, 60–80 mm long), wound with 500–1000 turns of 26–30 AWG enameled wire. The core is bent into an L or U shape so that the pole face points toward the armature from below or the side. Resistance should be 50–100 ohms for typical telegraph current levels.

The armature: a flat strip of soft iron, 2–3 mm thick, 10–12 mm wide, 50–60 mm long. Pivoted at one end on a small pin through the frame. The free end faces the magnet pole face with a gap of 1–3 mm when the magnet is off.

The frame: a small wooden or metal block that holds both the electromagnet and the armature pivot. The frame mounts on a larger base. Hardwood (oak, maple) or thin sheet metal bent into shape both work. The critical dimensional requirement: the armature pivot must be firmly held and the magnet position adjustable so the pole face-to-armature gap can be set precisely.

Stops (anvils): two small steel or brass pins or screws — one above the armature (limits downward movement when energized), one below (limits return travel). Adjust these with the instrument powered at normal current: the down stop should limit movement just enough for clean contact with minimum residual gap; the up stop sets the return position. Metal stops produce sharper clicks; rubber or felt stops produce softer sound.

Return spring: a strip of phosphor bronze or spring steel, 0.2–0.4 mm thick, attached to the armature and anchored to the frame. Sets the return force. Too stiff: the magnet cannot attract the armature at normal current. Too weak: the armature sticks from residual magnetism. Adjust by trimming the spring length or changing its attachment point.

Calibration and Adjustment

Setting the air gap (magnet-to-armature clearance) is the most critical adjustment. Too large a gap: the magnet’s attractive force is too low to pick up the armature at normal current. Too small: the magnet attracts the armature even at small currents (low selectivity — even leakage and noise causes false clicks).

Start with a gap of 1.5 mm. With the sounder connected to the telegraph line and normal operating current flowing, the armature should be firmly attracted (not rattling). When current stops, it should return cleanly to the up stop. If it doesn’t attract: reduce the gap (move the magnet closer), or increase the magnetizing current. If it sticks: increase the gap slightly, or add a small copper strip between the pole face and armature at rest position (copper’s non-magnetic property prevents residual magnetism from holding the armature).

The gap between the armature and the down stop determines the “throw” — how far the armature moves. Short throw gives a sharp click but less visual confirmation of operation. Long throw gives a louder, more distinctive click. For sounder operation, 1–2 mm throw is typical.

The resonance of the armature-spring system should be away from the typical dot frequency. If the natural frequency of the armature-spring system resonates with the dot rate, the armature bounces on dots and produces spurious extra clicks. If this happens, change the spring stiffness or add a small damping pad (felt) against the armature return path.

Operating With a Sounder

An experienced sounder operator copies code by listening to the rhythm, not by consciously counting dots and dashes. The letter E (single dot) sounds like a very quick tick-tick. T (single dash) sounds like tick…tick (perceptible pause). W (dot-dash-dash) sounds like tick-tick, tick…tick, tick…tick.

Open sounder operation — listening with the instrument on a table, not using headphones — was standard in 19th-century telegraph offices. The sounder was loud enough to be heard at the back of the operating room. Operators developed the ability to tune out other sounders and focus on their own.

For quiet operation or weak-signal conditions, a resonating box (a wooden box tuned to amplify the click frequencies) amplifies the sounder acoustically. The sounder sits on or inside the box, which acts as a Helmholtz resonator. Careful shaping of the box interior can increase perceived loudness by 3–6 dB without any electrical changes.

A separate receiver circuit test: connect a battery of known voltage through a known resistance to the sounder coil. Measure the current with an ammeter. The sounder should respond reliably at 15–25 mA and should fail to respond at below 5 mA. These thresholds define the operating range; verify them periodically as the sounder ages and internal resistance may change.