A circuit diagram is a shared code for showing how components are joined

1. A circuit diagram replaces a picture of the real apparatus with standard symbols joined by straight lines.
2. The lines stand for connecting wire of negligible resistance, so their length and shape carry no meaning at all.
3. Draw the wires as straight lines with right-angled corners and use a ruler, because a wobbly line can be misread as a resistor.
4. Two wires that cross without a blob are not joined; a blob at the crossing means they are connected.
5. Every component sits in a gap in the line, and the whole arrangement must form at least one complete loop before any current can flow.

Sources, switches and meters

1. Cell: a long thin line for the positive terminal and a short thick line for the negative, supplying a fixed e.m.f.
2. Battery: two or more cells joined in series, drawn as repeated cell symbols, giving a larger e.m.f.
3. Power supply: a d.c. or a.c. source that can usually be set to a chosen voltage.
4. Generator: a circle marked G, producing an e.m.f. from a rotating coil.
5. Switch: a break in the line with a hinged arm, and opening it stops the current everywhere in that loop.
6. Ammeter: a circle marked A, always drawn in series because it has to carry the current it measures.
7. Voltmeter: a circle marked V, always drawn in parallel across the component being measured.

Resistive components and the things they drive

1. Fixed resistor: a plain rectangle, limiting the current with a resistance that does not change.
2. Variable resistor: a rectangle with an arrow through it, and moving the slider changes the resistance and so changes the current.
3. Potential divider: two resistors in series across the supply, or one resistor with a sliding contact, used to tap off part of the supply voltage.
4. Lamp: a circle with a cross, which glows and whose resistance rises as the filament heats up.
5. Heater: a rectangle that turns almost all of the energy delivered to it into thermal energy.
6. Motor: a circle marked M, which turns while a current flows through it.
7. Bell: a dome-shaped symbol that rings for as long as a current flows.
8. Fuse: a rectangle with a line through it, which melts and breaks the circuit if the current becomes too large.
9. Relay: a coil that closes a switch in a second, separate circuit when a small current flows through it.
10. Magnetising coil: a coil of wire that produces a magnetic field whenever a current passes through it.
11. Transformer: two coils drawn either side of a core, changing an alternating voltage up or down.

Sensing components change their resistance when their surroundings change

1. A thermistor is drawn as a resistor with a diagonal line across it, and the type you need has a resistance that falls as the temperature rises.
2. Put one in a circuit and the current rises as it warms, which is how a thermostat or a fire alarm senses heat.
3. A light-dependent resistor, or LDR, is drawn as a resistor inside a circle with two arrows pointing in towards it.
4. Its resistance is high in the dark and low in bright light, so the current in its branch rises when light falls on it.
5. An LDR is what switches a street lamp on at dusk and turns a phone screen brighter in sunlight.

Diodes and LEDs conduct in one direction only

1. A diode is drawn as a triangle pointing at a bar, and it lets conventional current pass only in the direction the triangle points.
2. In the reverse direction its resistance is enormous, so the current is effectively zero.
3. That makes a diode the component used to turn a.c. into d.c., and to protect a circuit from a battery fitted the wrong way round.
4. A light-emitting diode, or LED, uses the same symbol with two arrows pointing away from it, and it gives out light while conducting in the forward direction.
5. An LED needs a series resistor to limit the current, because its own resistance drops very low once it starts conducting.