Capacitors

Part of Basic Electrical Circuits

How capacitors store electrical charge, how to make primitive versions, and where to use them in rebuilt circuits.

Why This Matters

When rebuilding electrical infrastructure after collapse, you need more than just wires and batteries. Capacitors—devices that store electrical charge temporarily—appear in virtually every useful electrical circuit, from radio receivers to motor starters to smoothing power supplies. Without capacitors, many technologies simply cannot function.

Unlike batteries, capacitors charge and discharge almost instantly. This makes them essential for filtering noise from power supplies, timing circuits, coupling signals between stages, and delivering short bursts of energy. A community that understands and can fabricate capacitors has access to a far wider range of electrical technology than one that cannot.

Even crude, handmade capacitors made from available materials can serve critical functions. A homemade capacitor can stabilize a charging system, help start a motor, or form part of a radio receiver. The principles are simple and the materials—metal foil, waxed cloth, glass—have been available for centuries.

The Physics of Capacitance

A capacitor consists of two conductive plates separated by an insulating material called a dielectric. When voltage is applied, electrons accumulate on one plate while the opposite plate loses electrons, creating an electric field across the gap. Energy is stored in this field.

The capacitance (measured in farads, F) depends on three factors:

• Plate area: Larger plates store more charge
• Plate separation: Closer plates store more charge
• Dielectric material: Different materials allow more or less field concentration

The relationship is expressed as: C = ε × A / d

Where ε is the permittivity of the dielectric, A is plate area, and d is the separation distance.

Common dielectric materials and their relative permittivity:

Material	Relative Permittivity
Air/vacuum	1.0
Waxed paper	2.5–3.5
Glass	4–10
Mica	5–8
Beeswax	2.4
Linseed oil	3.4

For primitive construction, the goal is maximum plate area, minimum separation, and the best available dielectric.

Building a Leyden Jar Capacitor

The Leyden jar—invented in the 1740s—was history’s first practical capacitor. You can build one with a glass jar, metal foil, and wire.

Materials needed:

• Glass jar with non-conductive lid (wood, cork, ceramic)
• Metal foil (tin, aluminum, or copper hammered thin)
• Metal rod or wire
• Beeswax or pitch for sealing

Construction steps:

1. Cut metal foil to fit both the outside and inside of the jar, leaving 3–4 cm clearance from the top rim
2. Attach inner foil: coat the inside lower two-thirds with foil, pressing firmly to eliminate air pockets
3. Attach outer foil: wrap the outside lower two-thirds similarly
4. Pass a metal rod through the lid so it hangs down and touches the inner foil
5. Seal the lid with beeswax to prevent moisture ingress

Using the Leyden jar:

• Charge it by connecting the rod to a voltage source (or rubbing it with static electricity)
• The outer foil connects to ground
• Discharge by briefly connecting outer foil to the rod through a resistor

Safety

A charged Leyden jar can deliver a painful or dangerous shock. Discharge through a resistor (100–1000 ohms) before handling. Never short the terminals directly.

Estimated capacitance: A 500 mL jar achieves roughly 1–2 nanofarads—small by modern standards, but useful for radio work and demonstrations.

Building a Rolled Paper Capacitor

For higher capacitance, the rolled construction multiplies plate area while keeping the device compact.

Materials:

• Thin metal foil (hammered copper, tin from cans, or salvaged aluminum)
• Waxed paper or oiled cloth as dielectric
• Thin wire for terminals
• Shellac or beeswax for sealing

Construction:

1. Cut four strips: two metal foil strips (~20 cm × 5 cm) and two dielectric strips (~22 cm × 6 cm)
2. Stack layers: dielectric / foil-1 / dielectric / foil-2
3. Offset the foil strips slightly so foil-1 extends left and foil-2 extends right
4. Roll the stack tightly into a cylinder
5. Attach wire leads to the exposed edges of each foil layer
6. Coat the entire roll in melted beeswax or shellac to waterproof it

Increasing capacitance:

• Double the foil width (doubles capacitance)
• Use thinner dielectric (increases capacitance but risks breakdown)
• Use multiple rolls connected in parallel

This construction yields capacitances in the range of 5–50 nanofarads depending on dimensions and dielectric quality.

Glass Plate Capacitors

For more predictable, stable capacitance, flat glass plate capacitors are more reliable than rolled paper.

Construction:

1. Take two panes of window glass, identical in size
2. Cut metal foil slightly smaller than the glass
3. Sandwich: glass / foil / glass / foil / glass
4. Clamp tightly with non-conductive clamps
5. Connect leads to the two foil layers

Multiple glass/foil sandwiches stacked and connected in parallel increase total capacitance. This design was used in early radio transmitters and is dimensionally stable over time.

Practical Applications

Power supply filtering: Connect a capacitor across a battery or generator output to absorb voltage spikes and smooth fluctuations. Even a small capacitor (10–100 nF) dramatically improves the stability of sensitive circuits.

Motor starting: Single-phase AC motors need a capacitor to create the phase shift required for starting torque. A capacitor in the range of 10–50 microfarads is typical—achievable with a large rolled design.

Timing circuits: Capacitors charge through resistors at predictable rates. This RC timing behavior enables simple timers, oscillators, and automatic controls.

Radio tuning: Variable capacitors with adjustable plate separation allow tuning a radio receiver to different frequencies. A simple variable capacitor can be made from two sets of interlocking metal plates separated by air.

Testing and Measuring Capacitors

Without commercial test equipment, you can test capacitors with a battery and galvanometer:

1. Connect the capacitor in series with a galvanometer and battery
2. On connection, current briefly flows as the capacitor charges—the galvanometer deflects then returns to zero
3. Disconnect the battery; reconnect leads reversed—current flows again briefly as the cap discharges then recharges in the other direction
4. A dead (open) capacitor shows no deflection; a shorted capacitor shows sustained current flow

Estimating capacitance: Using a known resistor and timing how long charge/discharge takes gives a rough capacitance estimate. The time constant τ = RC, where R is resistance and C is capacitance. At time τ, the voltage reaches 63% of its final value.

Storage and Maintenance

Homemade capacitors degrade over time. Key maintenance practices:

• Keep capacitors dry—moisture in the dielectric causes leakage and reduced performance
• Recoat waxed paper types annually with fresh beeswax
• Store glass plate types clamped to prevent warping
• Never exceed the voltage rating—dielectric breakdown is permanent

A well-maintained handmade capacitor can serve for years in low-voltage applications. The construction techniques here represent the state of the art from the 1800s through the early 1900s—more than adequate for rebuilding essential electrical technology.