Telephony

Why This Matters

The telegraph proved that electricity can carry information over long distances, but it requires trained operators and encodes messages one character at a time. The telephone transmits the human voice directly, enabling instant two-way conversation between anyone. Rebuilding telephony gives your community real-time coordination over distances that would otherwise require hours of travel.

What You Need

For a basic two-station telephone:

• Copper wire, 18-22 AWG, enough to span the distance between stations
• Thin iron or steel diaphragm (tin can lid, 0.1-0.3 mm thickness)
• Permanent magnets (salvaged speaker magnets or hand-wound electromagnets)
• Insulated copper magnet wire, 28-36 AWG (several hundred meters)
• Carbon granules (crushed charcoal or carbon rod from batteries)
• Small containers for microphone capsules (metal or wooden)
• Dry cell batteries (1.5V-6V) or any stable DC source
• Hand-crank magneto or AC source for ringing
• Small bell or buzzer for incoming call alert
• Insulators (glass, ceramic, or wooden knobs) for line construction
• Basic hand tools: pliers, wire cutters, screwdriver, soldering iron

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How Sound Becomes Electricity

The fundamental challenge of telephony is this: you need to convert air pressure waves (sound) into proportional electrical signals, transmit those signals over wire, and convert them back into air pressure waves at the other end. Every part of this chain must preserve the waveform faithfully enough for the human ear to recognize speech.

The Human Voice

Human speech occupies a frequency range of roughly 300 Hz to 3,400 Hz. This is a narrow band compared to full audio (20 Hz to 20,000 Hz), but it contains enough information for intelligible conversation. A practical telephone system only needs to reproduce this band.

Parameter	Value
Speech frequency range	300-3,400 Hz
Typical speech level	60-70 dB SPL at 1 meter
Minimum for intelligibility	About 500-2,500 Hz
Male fundamental frequency	85-180 Hz
Female fundamental frequency	165-255 Hz

Tip

You do not need to reproduce the full range of human hearing. A bandwidth of 300-3,400 Hz is sufficient for clear speech. This makes the engineering much simpler --- your microphone and speaker do not need to handle bass or treble frequencies.

The Carbon Microphone

The simplest and most historically important telephone microphone uses carbon granules. This is what made Bell’s telephone practical, and it remains the easiest to build from scratch.

How it works:

1. A thin metal diaphragm sits against a chamber filled with carbon granules
2. A steady DC current flows through the granules
3. When sound waves hit the diaphragm, it compresses and releases the granules
4. Compressed granules have lower resistance (more contact points between particles)
5. Released granules have higher resistance (fewer contact points)
6. The changing resistance modulates the DC current in proportion to the sound wave

Building a carbon microphone:

1. Prepare the carbon granules. Crush carbon battery rods or hardwood charcoal into granules roughly 0.5-1 mm in diameter. Sieve to remove dust (too fine) and chunks (too coarse). The granules must be uniform in size.

2. Build the capsule. Take a small metal container roughly 30-40 mm in diameter and 10-15 mm deep. Drill a small hole in the back plate and insert a metal contact (a bolt works). Fill with carbon granules to about 80% capacity.

3. Mount the diaphragm. Cut a thin metal disc (tin can steel, 0.1-0.2 mm thick) to fit over the open end. It must make electrical contact with the granules but be free to vibrate. Clamp it in place with a ring or crimped edge.

4. Wire it up. One terminal connects to the back contact (through the granules), the other to the diaphragm edge. When DC flows through, sound waves on the diaphragm modulate the current.

    Sound waves
        |
        v
  [Diaphragm (thin metal)]
  [Carbon granules :::::::::]
  [Back plate + contact     ]
        |           |
      Wire 1      Wire 2
        |           |
    Battery -----> Line

Typical carbon microphone resistance: 50-200 ohms, varying by 10-50 ohms with speech. With a 3V battery, this produces current variations of roughly 5-30 milliamps, which is plenty to drive a receiver at the other end.

The Electromagnetic Microphone

An alternative that requires no battery: a coil of fine wire wound around a permanent magnet, with a thin iron diaphragm mounted close to the magnet pole.

1. Sound waves vibrate the diaphragm
2. The moving iron changes the magnetic flux through the coil
3. By Faraday’s law, the changing flux induces a voltage in the coil
4. The output voltage is proportional to the sound pressure

This is essentially a small loudspeaker used in reverse. The output voltage is much lower than a carbon microphone (millivolts vs tens of millivolts), so it works best over short distances or with amplification.

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Converting Electricity Back to Sound

The Electromagnetic Receiver

The receiver (earpiece) is the reverse of the electromagnetic microphone and is remarkably simple.

Construction:

1. Wind 1,000-3,000 turns of 36 AWG insulated copper wire around a soft iron core (a bolt or nail)
2. Mount a thin iron or steel diaphragm 0.5-1 mm away from the pole of the electromagnet
3. Add a permanent magnet to provide a bias field (this keeps the diaphragm slightly attracted at rest, so it can respond to both positive and negative signal excursions)

How it works:

• The varying electrical signal from the microphone flows through the coil
• This creates a varying magnetic field that adds to or subtracts from the permanent magnet’s field
• The diaphragm is pulled more or less strongly, vibrating in time with the signal
• The vibrating diaphragm pushes air, recreating the original sound

Tip

The permanent magnet bias is critical. Without it, the diaphragm responds only to the magnitude of the signal (always attracted, never pushed), which doubles the frequency and distorts speech beyond recognition. With the bias magnet, the diaphragm tracks the actual waveform.

Diaphragm Selection

Material	Thickness	Properties
Tin plate (tin can)	0.1-0.2 mm	Easy to find, adequate response
Sheet iron	0.2-0.3 mm	Better magnetic response, needs filing thin
Spring steel	0.1 mm	Excellent, if available
Aluminum	Any	Does not work --- not magnetic

The diaphragm should be clamped firmly at its edges and free to vibrate in the center. Tension matters: too loose gives boomy, resonant sound; too tight gives thin, tinny sound. Experiment with clamping pressure until speech sounds natural.

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The Complete Telephone Circuit

Basic Two-Wire Connection

The simplest telephone connects two stations with two wires:

Station A                              Station B
[Microphone]---+                  +---[Microphone]
               |                  |
            [Battery]          [Battery]
               |                  |
[Receiver]-----+------ Line -----+-----[Receiver]
               |                  |
               +------ Line -----+

Each station has its own microphone and receiver wired in series with a battery. When person A speaks, their microphone modulates the current through person B’s receiver. And vice versa.

Problem: In this simple circuit, you hear your own voice at full volume in your own receiver (sidetone). This is disorienting and causes people to speak too quietly.

The Anti-Sidetone Circuit

To reduce sidetone, use a balancing network (a Wheatstone bridge configuration):

1. The microphone connects across one arm of the bridge
2. The receiver connects across the other diagonal
3. A balancing impedance (a resistor matching the line impedance) connects across a third arm
4. The telephone line connects across the fourth arm

When the bridge is balanced, the local microphone signal cancels in the local receiver but passes fully to the line. The incoming signal from the remote station is unbalanced and reaches the local receiver at full strength.

Practical balancing impedance: For lines up to 5 km of 20 AWG copper wire, a 600-ohm resistor provides reasonable balance. Adjust for your specific line by listening to sidetone level and changing resistance until your own voice is just barely audible.

Tip

Some sidetone is actually desirable. If you hear nothing of your own voice, you feel like the phone is dead and start shouting. Aim for sidetone about 10-15 dB below your normal speaking level.

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Signaling and Ringing

The Magneto Ringer

You need a way to alert the distant station that you want to talk. The standard method is a hand-cranked magneto generator that sends AC ringing current down the line.

Building a magneto:

1. Mount a permanent magnet on a hand-crank shaft
2. Wind a coil of 2,000-5,000 turns of fine wire around the magnet’s path
3. Turning the crank spins the magnet past the coil, generating AC voltage
4. A good hand magneto produces 75-90V AC at 16-25 Hz

The bell:

1. An electromagnet with a spring-loaded clapper sits between two gong bells
2. AC ringing current alternately pulls and releases the clapper
3. The clapper strikes alternating gongs, producing the familiar telephone ring

Magneto (hand crank) --> Line --> [Capacitor] --> [Bell coils] --> [Clapper between gongs]

The capacitor (0.5-2 microfarads) blocks DC from the talk battery but passes AC ringing current. This prevents the bell from being activated by the steady DC talk current.

The Hook Switch

When the handset is on the hook (hung up):

• The bell circuit is connected to the line (ready to ring)
• The microphone and receiver are disconnected
• The line draws minimal current

When the handset is lifted off the hook:

• The bell circuit is disconnected
• The microphone and receiver connect to the line
• DC current flows, indicating the line is in use

This is simply a mechanical switch operated by the weight of the handset.

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Switching: Connecting Multiple Subscribers

A point-to-point telephone works for two stations. But a community needs many stations interconnected. This requires switching.

Manual Switchboard

The simplest multi-subscriber system uses a central switchboard with an operator.

Components:

• Each subscriber’s line terminates at a jack (socket) on the switchboard
• Each jack has a lamp or drop indicator that signals an incoming call
• The operator has cord pairs --- two plugs connected by wire with a talk key
• To connect a call: the operator plugs one cord into the calling jack and the other into the called party’s jack

Capacity: A single operator can handle 50-100 lines. Beyond that, you need multiple operators and switchboard sections.

Building a simple switchboard:

1. Mount rows of phone jacks (1/4-inch or 6.35 mm jacks work) on a panel
2. Wire each subscriber’s line pair to a jack
3. Add a simple indicator for each line (a small lamp in series with a relay, or a mechanical drop indicator triggered by line current)
4. Make cord pairs from flexible wire with plugs on each end
5. Train an operator

Step-by-Step (Strowger) Switch

For automatic switching without an operator, the step-by-step switch is the simplest mechanical approach.

How it works:

1. The calling subscriber lifts the handset and hears a dial tone
2. They rotate a dial (or press buttons that generate pulses)
3. Each digit sends a series of electrical pulses down the line
4. Each pulse steps a mechanical selector one position (the “step” in step-by-step)
5. The first digit selects a row, the second digit selects a column, and so on
6. After all digits are dialed, the switch connects to the called line

The rotary dial generates pulses by briefly interrupting the DC current on the line. Dialing “3” generates 3 pulses. Dialing “0” generates 10 pulses. Each pulse is about 60 milliseconds on, 40 milliseconds off.

Tip

You can simulate a rotary dial by rapidly tapping the hook switch. Each tap generates one pulse. This works on any step-by-step exchange --- tap 3 times for “3”, 7 times for “7”, and so on. Timing must be consistent: about 10 pulses per second.

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Building the Physical Line

Wire Selection

Wire Gauge (AWG)	Resistance per km	Max Practical Distance	Notes
14	8.3 ohms	50+ km	Heavy, expensive, best for trunk lines
18	21 ohms	20-30 km	Good general purpose
20	33 ohms	10-20 km	Adequate for short runs
22	53 ohms	5-10 km	Short distances only
24	84 ohms	2-5 km	Within a building or compound

Total loop resistance (both wires) should stay below about 1,000 ohms for carbon microphone circuits. For a 20 AWG line, that limits you to about 15 km.

Pole Line Construction

1. Poles: Straight timber, 5-7 meters tall, treated with creosote or char to resist rot. Set 1-1.5 meters into the ground. Space poles 30-50 meters apart.

2. Crossarms: Horizontal wooden beams bolted to poles, supporting the wire. Use 50x75 mm timber, 1-1.5 meters long.

3. Insulators: Glass or ceramic knobs mounted on crossarm pins. The wire wraps around the insulator groove and is tied with soft wire. Insulators prevent current leakage to the pole (especially in wet weather).

4. Wire stringing: Pull wire taut but allow some sag for thermal expansion. In summer heat, wire expands and sags more. In winter cold, wire contracts and tightens. Allow roughly 0.5-1 meter of sag per 50-meter span.

Tip

Lightning protection is essential for any outdoor telephone line. Install carbon block arrestors at each station entrance --- these are two carbon blocks separated by a thin air gap, connected between each wire and ground. Lightning jumps the gap to ground instead of entering the station. Replace carbon blocks after each lightning strike.

Splicing Wire

When wire runs are longer than your spool, you must splice:

1. Strip 50 mm of insulation from each wire end
2. Cross the bare ends and wrap tightly in a “Western Union” splice: twist each wire around the other at least 5 times
3. Solder the joint (if solder is available)
4. Wrap with insulating tape or rubber
5. A good splice should have less than 0.1 ohms resistance

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Common Mistakes

Mistake	Why It’s Dangerous	What to Do Instead
Using aluminum wire for long runs	60% higher resistance than copper, corrodes at joints	Use copper; if aluminum is all you have, keep runs short and use anti-oxidant compound at joints
Skipping the bias magnet in the receiver	Sound is distorted and unintelligible (frequency doubling)	Always include a permanent magnet in the receiver
Running both wires on the same side of the pole	Increased crosstalk and noise pickup	Transpose wires (swap positions) every 5-10 poles
No lightning protection	A single strike destroys the telephone and can injure the user	Install carbon block arrestors at every station entrance
Carbon granules too fine (powder)	Packs solid, stops working	Sieve granules to 0.5-1 mm uniform size
Carbon granules too coarse	Poor frequency response, scratchy sound	Crush and sieve to proper size
Line wires touching vegetation	Signal loss in wet weather, eventual wire corrosion	Clear vegetation 1 meter on each side of the line
No capacitor before the bell	DC talk current activates the bell constantly	Always use a 0.5-2 uF capacitor in series with the bell

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What’s Next

With telephony established, your community has real-time voice communication. The next steps are:

• Radio --- eliminate the need for physical wire connections entirely by transmitting voice through electromagnetic waves
• Telecommunications --- scale telephony into a full network with multiplexing, long-distance trunking, and modern switching

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Quick Reference Card

Telephony --- At a Glance

Carbon microphone: Granules in capsule, DC battery, resistance varies with sound pressure

Receiver: Electromagnet + permanent bias magnet + thin iron diaphragm

Speech band: 300-3,400 Hz is sufficient for intelligible conversation

Power: 3-6V DC battery for talk circuit, 75-90V AC hand magneto for ringing

Line limits: Keep total loop resistance under 1,000 ohms (about 15 km with 20 AWG copper)

Ringing capacitor: 0.5-2 uF in series with bell, blocks DC, passes AC ring signal

Anti-sidetone: Balance network (approximately 600 ohms) reduces own-voice feedback

Lightning protection: Carbon block arrestors at every station entrance, connected to earth ground

Wire transposition: Swap wire positions every 5-10 poles to reduce noise pickup

Pole spacing: 30-50 meters, poles 5-7 m tall, set 1-1.5 m into ground