An electromagnetic earpiece converts varying line current into sound with a permanent magnet, a coil and a thin soft-iron diaphragm 0.5 to 1.5 mm from the pole face. The magnet matters twice: it makes the diaphragm follow the direction of the current instead of vibrating at double frequency, and it supplies most of the pull. A coil of several thousand turns of fine wire reaches a telephone-grade 1,000 to 2,000 ohms; a few dozen turns needs a far larger current.
Part of Telecommunications
Turning varying telephone current back into sound with a magnetic driver and a diaphragm, the reverse of the microphone’s job.
The Receiver Reverses The Microphone’s Job With A Magnet Pulling On Iron
Current in the coil adds to and subtracts from the permanent magnet’s field, so the pull on the iron diaphragm rises and falls with the current. The diaphragm moves the air in front of it. That is the same chain as a moving-iron microphone read backwards, and it is why a small loudspeaker element can serve as either (see Electromagnetic Microphone and Electricity to Sound). A carbon microphone cannot be run backwards: it is a resistance, not a generator.
Without The Magnet The Pitch Doubles
Magnetic pull on iron is attractive whichever way the current flows, and it grows with the square of the flux. Write the total flux as a steady part B₀ from the magnet plus a small signal part b from the coil. The force varies as (B₀ + b)² = B₀² + 2·B₀·b + b².
- The 2·B₀·b term follows the signal and is the sound you want
- The b² term is always positive, so it rises and falls twice per signal cycle: the double-frequency distortion
- With no magnet B₀ = 0 and only the b² term remains, so the earpiece sings at twice the input pitch
When the signal is small compared with B₀, the wanted term dominates. A strong magnet also means a small signal current gives a large 2·B₀·b force, which is why a stronger magnet and a smaller gap raise sensitivity, as Electromagnetic Receiver describes. A smaller gap also raises the risk of buzzing against the pole face.
Ampere-Turns Set The Drive, So The Winding Must Match The Current
The magnetic drive of a coil is the number of turns times the current. The corpus gives two very different windings, and they suit different currents.
- Carrier article: 50 to 100 turns of 28 to 32 gauge wire
- Electromagnetic Receiver: several thousand turns of 40 to 44 AWG, 400 to 2,000 Ω
Using the 2.05 mA signal-loop current worked out in Microphone:
- 5,000 turns × 0.00205 A = 10.3 ampere-turns
- 75 turns × 0.00205 A = 0.15 ampere-turns, about 67 times less
On a battery line with milliamps, only the fine, many-turn coil drives the diaphragm hard. A 75-turn coil needs tens of milliamps, which is what a short, low-resistance line with a bigger battery supplies. The two articles are not contradicting each other so much as assuming different line currents, and neither has been tested here.
Worked Example: The Resistance Of A Many-Turn Coil
Electromagnetic Receiver gives a 5 mm winding radius and 0.051 mm wire (44 AWG). Resistivity of copper is 1.68 µΩ·cm, or 1.68 × 10⁻⁸ Ω·m, from Wire Selection for Telephony.
- Mean turn length = 2π × 5 mm = 31.4 mm
- 5,000 turns × 0.0314 m = 157 m of wire
- Cross-section = π/4 × (0.051 × 10⁻³ m)² = 2.04 × 10⁻⁹ m²
- R = 1.68 × 10⁻⁸ × 157 / 2.04 × 10⁻⁹ ≈ 1,290 Ω
That is inside the 400 to 2,000 Ω quoted for telephone-grade receivers. It also loads the loop: it is the same order as the 1,000 Ω receiver assumed in the microphone article’s 1,460 Ω example.
What You Need
- A magnet: neodymium from a hard drive, a loudspeaker magnet or a ferrite magnet
- A coil wound on the pole or a soft-iron core, as many turns of fine enamelled wire as fit, sealed with shellac against moisture
- A diaphragm of soft iron, not hardened steel, which does not follow 3,000 Hz well
- A housing that holds a gap of 0.5 to 1.5 mm and a small front cavity of roughly 1 to 3 cc
Method: Build And Test
Step 1: Wind the coil. Wind in even layers and count turns, then read the DC resistance. 1,000 to 2,000 Ω is the target.
Step 2: Set the gap with a shim. Start at 1 mm. Playing a tone through the earpiece, reduce the gap until the sound is loudest without buzzing, then back off slightly.
Step 3: Click test. Touch a 1.5 V cell through a 500 Ω resistor to the coil. You should hear a click each time.
Step 4: Try a scavenged element. Any headphone or handset element works as an earpiece; its resistance sets what it draws from the line.
Common Mistakes
- Hardened steel for the diaphragm. Its magnetic response is too slow; use soft iron.
- Diaphragm touching the pole. Buzzing or no sound; open the gap.
- A 600 Ω coil on a 50 Ω source. Most of the signal is lost; see Impedance Matching for a transformer fix.
Safety
Ringing voltage and magnets
A magneto ring is about 60 to 90 V AC (see Magneto Generator). It must never reach an earpiece held to the ear, which is what the hook switch prevents by disconnecting the receiver from the line on-hook. Neodymium magnets pinch skin and can shatter if allowed to snap together, so keep them apart and wear eye protection.
What’s Next
- Learn next: Diaphragm Design, resonance and material of the plate
- Combine with: Microphone, the other half of the instrument