Telegraphone

Poulsen is not aiming at music. He wants a telephone that takes messages while you are out — the answering machine, patented in 1899.

Stroke a steel needle with a magnet and check it picks up filings hours later. Steel remembers a field — that memory is the storage medium.

Wind hard steel wire in a single close helix along a wooden cylinder, exactly as the patent describes. Turns must not touch or the recording bleeds between them.

Fit it on an axle that rotates at a constant rate. Speed changes during recording becomes pitch wobble on playback, so consistency matters more than speed.

Wind several hundred turns of fine enamelled wire on a small soft-iron core, leaving a narrow gap that will ride against the steel wire.

Fix the head so it tracks along the helix as the cylinder turns, staying in contact. Poulsen's head follows the wire; it does not sit still.

Measure the coil. A few ohms to a few tens of ohms is workable; open circuit means a broken enamel joint, which is the usual fault.

Run a permanent magnet along the whole helix to erase it. Always start from a blank medium — old magnetisation prints through as a ghost.

Feed a low-level audio signal through the head while the cylinder turns once. The wire is now magnetised in a pattern matching the waveform.

Disconnect the source, connect the head to an amplifier and speaker, and turn the cylinder again at the same speed. The tone returns.

Connect the head straight to an earpiece, as Poulsen had to. It is barely audible. This faintness is why the invention failed for thirty years.

Replay repeatedly and listen for degradation. There is essentially none — nothing is being scraped away. Compare with a stylus in a groove.

Wipe with the magnet and record again on the same wire. The medium is reusable — no other recording method of 1900 could do this.

Record at half speed and play back at full. Everything shifts up in pitch, and the top end gets worse — how much wire passes per second sets the fidelity ceiling.

The patent. US 661,619, "Method of recording and reproducing sounds or signals", filed 8 July 1899 and granted 13 November 1900 to Valdemar Poulsen, a Danish engineer at the Copenhagen Telephone Company.

It worked, and it won prizes. The Telegraphone took the Grand Prix at the 1900 Paris Exposition, where Poulsen recorded the voice of Emperor Franz Joseph of Austria — believed to be the oldest surviving magnetic recording. The principle was correct in 1899 and is still correct: it is what a cassette, a hard disk platter and a credit-card stripe all do.

🔴 And it failed commercially for three decades, for one reason. The signal a magnetised wire induces back into a coil is tiny — step 11 is not a defect in your build, it is the historical condition. In 1900 there was no way to make a small electrical signal larger. The Fleming valve arrives in 1905 and De Forest's amplifying Audion in 1906, both elsewhere in this collection, and only then does magnetic recording become listenable in a room rather than through a pressed-to-the-ear receiver. Poulsen is the third inventor in this programme whose idea was complete and whose supporting technology did not exist yet — Nipkow needed the same amplifier for television, and Seely's electric iron needed a daytime power grid. Being right early is a distinct failure mode, and it is common.

Why magnetism beats cutting, once you can hear it. Steps 12 and 13 are the two properties no mechanical format has. Playback is non-destructive because the head reads a field rather than being dragged by a groove wall, so a recording can be played indefinitely. And the medium is erasable, which means a reusable, editable, correctable record — a completely different relationship with recorded sound from a wax cylinder you have ruined if you make a mistake. Broadcasting, dictation and eventually multitrack music production all depend on being able to record over something.

What came after the gap closed. Once amplification existed, magnetic recording moved from wire to coated tape — Fritz Pfleumer's paper tape dusted with iron oxide in the late 1920s, then German Magnetophon machines in the 1930s, whose captured examples startled Allied engineers after the war with how good they sounded. Wire recorders lingered into the 1950s and lost, because tape can be cut and spliced and wire cannot. Poulsen himself moved on to the arc transmitter for radio, and is better remembered in Denmark for that.