LearnTronics
History
The Telephone
Electricity begins carrying the continuously changing shape of a human voice
The telegraph had shown that electricity could carry symbols over a wire. A switch at one end created on/off current changes, and an electromagnet at the other end turned those changes back into clicks or marks.
The telephone asked for something more difficult: instead of sending only on and off, the circuit had to follow the constantly changing pressure waveform of speech.
telephone: make the electrical signal vary continuously with sound
Alexander Graham Bell — 1876
Alexander Graham Bell received his fundamental United States telephone patent in 1876. His early telephone used the motion of a thin diaphragm near a magnet and coil to convert sound vibration into a changing electrical signal, and a related electromagnetic receiver converted the changing current back into mechanical motion and sound.
The receiver is an electromagnetic transducer
A telephone receiver is a useful new kind of component: a transducer. It converts energy from one form to another.
- The transmitter converts sound pressure and mechanical motion into an electrical signal.
- The receiver converts the electrical signal back into magnetic force and mechanical motion.
- The final moving diaphragm recreates sound pressure in the air.
The signal is no longer merely a command to turn something on. Its instantaneous size matters because that changing size carries information about the original sound.
The carbon transmitter makes the signal stronger
Early electromagnetic transmitters worked, but their output was weak. During the late 1870s, experimenters including Thomas Edison and Emile Berliner developed carbon transmitters in which sound pressure changed the electrical resistance of a carbon contact or carbon granules.
With a battery supplying current, the changing carbon resistance caused a larger changing current to flow in step with the sound. This was an important practical improvement because the voice signal could travel farther and operate receivers more effectively.
The sound does not have to supply the electrical output power. It changes a resistance, and that changing resistance controls current supplied by a battery.
That is not electronic amplification yet, but it points toward the same general idea of one small physical signal controlling a larger flow of electrical energy.
A microphone is a variable electrical component
We can think of a carbon microphone as a resistor whose value changes rapidly with sound pressure. If it is placed in a battery circuit, those resistance changes become current changes.
Later microphones will use many other methods — moving coils, capacitors, piezoelectric materials, electrets, and semiconductors — but the job remains the same: convert sound into an electrical signal.
Measurement trail 15 — a signal has shape as well as size
Until now we have often treated voltage and current as steady values. A telephone makes us care about how they change from moment to moment.
A simple voice signal
If the microphone current rises, falls, reverses direction in some systems, or otherwise changes with the diaphragm motion, the pattern of those changes is the information.
This idea of a waveform will become central to audio, alternating current, radio, oscilloscopes, and every kind of analog electronics.
Why the telephone matters to electronics
The telephone created an enormous demand for better transmitters, receivers, transformers, relays, loading coils, switching systems, and eventually amplifiers. Weak signals had to survive long wires and still reproduce speech intelligibly.
When the triode arrives decades later, long-distance telephony will become one of the strongest reasons to learn how to amplify a small signal without changing its information.