Electromagnetic Microphone
Part of Telephony
A microphone that generates voltage by moving a conductive element in a magnetic field, requiring no external power source.
Why This Matters
Unlike the carbon microphone, which modulates an existing current, the electromagnetic microphone generates its own voltage through the principle of electromagnetic induction. When a conductor moves through a magnetic field, it generates an electromotive force proportional to the rate of change of magnetic flux. This self-generating property makes electromagnetic microphones independent of any external bias current — they produce output voltage even with no power supply connected.
This independence from power supply made electromagnetic microphones attractive for portable and battery-free applications. Early dynamic microphones (a type of electromagnetic microphone) were used in public address systems and broadcasting because they could be moved far from any power source. For telephone construction in resource-constrained situations, an electromagnetic microphone avoids the need for a bias battery or the complexity of a carbon granule cartridge.
The tradeoff is lower output than a carbon microphone — electromagnetic microphones generate millivolts rather than modulating tens of milliamps. They typically require a transformer or amplifier stage to match the level to the telephone line. Understanding both microphone types allows appropriate selection for specific construction constraints.
Induction Principle
Faraday’s Law states that the EMF generated by a conductor moving in a magnetic field equals the rate of change of magnetic flux threading the conductor. For a straight conductor of length L moving at velocity v perpendicular to a field of strength B, the EMF is simply:
EMF = B × L × v
Voice frequencies drive the conductor at velocities of 0.01-1 mm/second for typical sound levels. With achievable field strengths of 0.1-0.5 Tesla and conductor lengths of 10-30 mm, output voltages of 0.1-50 millivolts result. These are small signals requiring careful impedance matching.
Two practical configurations implement this principle in telephone microphones:
Moving iron: A permeable iron diaphragm moves in the fringing field of a permanent magnet with a coil wound nearby. As the diaphragm moves, it changes the flux distribution through the coil. The coil sees changing flux and generates voltage. No conductor actually moves through the field — instead, the moving iron changes the flux path.
Moving coil (dynamic): A small coil of wire is attached to the diaphragm and suspended in the gap of a powerful permanent magnet. When the diaphragm moves, the coil moves with it through the constant magnetic field. The coil conductors cut magnetic flux lines and generate voltage proportional to their velocity.
Moving Iron Microphone Construction
The moving iron microphone is simpler to construct than the moving coil type because it requires no precision voice coil. The coil is stationary and can be wound at leisure; only the iron diaphragm must be positioned correctly.
Mount a small bar permanent magnet vertically with a coil of 2,000-5,000 turns wound around its center. Cut a circular disk of soft iron to serve as the diaphragm, about 40-60 mm in diameter and 0.1-0.2 mm thick. Clamp the disk at its rim over the end of the magnet, with a gap of 0.5-1.5 mm between the disk center and the magnet pole face.
When sound pressure flexes the iron disk, the gap changes and the flux through the coil varies. The coil generates a voltage proportional to the rate of gap change. At voice frequencies, this produces a clear audio output.
The output of a moving iron microphone is on the order of 0.5-5 mV for normal speaking at 30 cm distance. This is too low to drive a telephone line directly but easily matches to an audio transformer that boosts the impedance and voltage to telephone-grade levels.
Moving Coil (Dynamic) Microphone Construction
The dynamic microphone requires a strong permanent magnet with a precision cylindrical gap, and a lightweight voice coil that fits precisely in this gap without touching the walls. The precision requirements make it harder to fabricate than moving iron, but the output quality is excellent.
The gap must be concentric to within 0.1 mm — if the coil rubs against the magnet walls, it will be damaged and produce distorted output. Machine the magnet assembly on a lathe or use a salvaged loudspeaker as the magnet/gap structure (small speaker elements make excellent dynamic microphones with appropriate coupling).
Wind the voice coil on a former (typically aluminum or paper tube) that fits precisely in the magnet gap with 0.3-0.5 mm clearance all around. Use 42-46 AWG enameled wire, 30-80 turns depending on desired impedance. Lower turns give lower impedance (8-50 ohms) suited for direct connection to low-impedance lines or transformers. Higher turns give higher impedance (600+ ohms) for direct line connection.
Attach the voice coil former to the center of the diaphragm with epoxy. The diaphragm must suspend the coil precisely in the gap through its full range of motion. A corrugated surround around the diaphragm rim allows axial movement while maintaining centering.
Amplification and Line Matching
Electromagnetic microphones generate much less signal than carbon microphones at typical voice levels. A carbon microphone directly modulating a 48V telephone circuit creates current swings of ±5-20 mA — equivalent to a signal power of milliwatts. An electromagnetic microphone generates 1-10 mV — equivalent to microwatts.
To make an electromagnetic microphone work in a telephone system, either amplify the microphone output before the line, or use a step-up transformer. A transformer with a turns ratio of 1:50 transforms a 5 mV source at 150 ohms to 250 mV at 375 kohms — this high-voltage, high-impedance signal can now drive a vacuum tube or transistor amplifier for further processing.
Alternatively, if a carbon microphone amplifier circuit is already present in the telephone design (as in some hybrid instruments), the electromagnetic microphone feeds into this amplifier chain and benefits from the existing gain stages. In resource-constrained environments, using a carbon microphone for the transmitter and an electromagnetic receiver (earphone) for the listener — which is the standard telephone arrangement — avoids the need for amplification stages while benefiting from both transducer types.