Basic electricity

Three quantities describe a circuit: voltage (E, in volts), the push; current (I, in amps), the flow; and resistance (R, in ohms), what limits it. Ohm's law ties them: E = I × R. Power is P = E × I, in watts.

28 volts across 4 ohms: I = E ÷ R = 28 ÷ 4 = 7 amps
Power: P = E × I = 28 × 7 = 196 watts

In a series circuit the current is the same everywhere and resistances add: 10 + 20 + 30 = 60 ohms. In a parallel circuit every branch has the same voltage and the total resistance is less than the smallest branch: 1/R = 1/R1 + 1/R2 + … Two 12-ohm resistors in parallel make 6 ohms.

Light aircraft run on DC (14 or 28 volts). Larger aircraft also make AC, usually at 400 Hz, which lets transformers and motors be smaller.

Measuring

An ammeter goes in series, so the current flows through it. A voltmeter goes in parallel, across the part. An ohmmeter is used only with the circuit's power off, because it has its own battery.

Wires and protection

Wire sizes follow the American Wire Gauge: the bigger the number, the thinner the wire. A wire is chosen for the current it must carry and the voltage drop allowed over its length. A circuit breaker or fuse protects the wire, so it's sized to the wire, not the device.

Batteries

• Lead-acid: about 2 volts a cell (a 12-volt battery has 6 cells, a 24-volt one 12). The state of charge shows in the electrolyte's specific gravity, read with a hydrometer. Spilled acid is neutralized with baking soda.
• Nickel-cadmium: about 1.2 volts a cell. Specific gravity doesn't show the charge, and the electrolyte level is checked only when fully charged (it sinks into the plates as the battery discharges). Watch for thermal runaway. Spilled electrolyte is alkaline, neutralized with boric acid or vinegar.
• Keep the two kinds, and their tools and charging areas, apart: acid fumes ruin a nickel-cadmium battery.