What a circuit diagram shows

1. A circuit diagram replaces a drawing of the real apparatus with an agreed symbol for each component, joined by straight lines that stand for the connecting wires.
2. Every component has one standard symbol, so the same diagram can be read by anyone without a written explanation alongside it.
3. A circuit diagram records how the components are connected to one another. It does not record where they sit on the bench, how long the leads are, or how large any object is.
4. The diagram is never drawn to scale, so a small symbol may stand for a large piece of apparatus and a short straight line may stand for a long coiled lead.

Drawing conventions

1. Draw every connecting wire as a straight line with a ruler, and turn every corner as a right angle rather than a curve.
2. Bring each line squarely into the end of a symbol, so that it is obvious which two points of that symbol the wires are joined to.
3. The circuit must form a complete loop with no gaps anywhere in the line, unless a switch has deliberately been drawn in its open position.
4. A meeting point where three or more wires are joined is called a junction, and it is marked with a filled dot so that the join is unmistakable.
5. Two lines that cross without a filled dot are read as not connected, so unnecessary crossings are best removed by redrawing the layout.
6. No symbol is left with a line dangling from it, and no real component is given two symbols.
7. Where labels are wanted, the name of the component is written beside its symbol and not inside it.

Cells and batteries

1. The symbol for a cell is a pair of parallel lines drawn across a break in the wire: one long thin line and one short thick line.
2. The long thin line stands for the positive terminal and the short thick line stands for the negative terminal.
3. A battery is two or more cells joined end to end, so its symbol is simply the cell symbol repeated in a row with a small gap between each pair of lines.
4. Every cell in a battery symbol faces the same way, with all of the long thin lines on the same side, because the cells are meant to push in the same direction round the loop.
5. One cell drawn the wrong way round describes a different circuit from the one intended, so the orientation of the lines is part of the answer and not decoration.

Symbols to recognise

1. A switch is drawn as two contacts in a break in the line: it is open when the moving contact is lifted away from the second contact, and closed when it lies flat and bridges the gap.
2. A filament lamp is a circle with a cross inside it, and a fixed resistor is a plain rectangle with nothing inside it.
3. A variable resistor is the same rectangle with an arrow drawn diagonally across it, and a fuse is a rectangle with a line running through it from one side to the other.
4. An ammeter is a circle containing the letter $A$ and a voltmeter is a circle containing the letter $V$.
5. A diode is a solid triangle pointing at a short bar, with the point of the triangle showing the single direction in which the diode allows charge through, and a light-emitting diode is that same symbol with two small arrows pointing away from it.
6. A light-dependent resistor is a resistor rectangle inside a circle with two arrows pointing in towards it, and a thermistor is a resistor rectangle with a line drawn across it that bends upwards at one end.
7. A motor is a circle containing the letter $M$ and a generator is a circle containing the letter $G$.
8. A loudspeaker is a small rectangle with a cone opening outwards from one side, and a thermocouple is drawn as two lines of different metals meeting at a single point.

Switches and meters

1. A switch drawn in the single loop of a circuit controls the whole loop, because opening it breaks the only path the wires provide.
2. An ammeter is always drawn as part of the loop itself, so the line runs into one side of the circle and out of the other side.
3. A voltmeter is always drawn on its own short branch, with the line leaving the loop just before the component and rejoining it just after, so that the meter sits across that component.
4. Drawn in that way, the voltmeter and the component enclose a small loop of their own on the page, which is how a reader recognises the connection at a glance.

From apparatus to diagram

1. Begin by listing every real object shown in the photograph or named in the description, and write the symbol that stands for each one beside it.
2. Choose one terminal of the supply and trace the leads by hand all the way round to the other terminal, noting each component in the order it is met.
3. Draw the loop as a plain rectangle first, then place the symbols along its sides in that same order, spacing them out so each one is clear.
4. Represent every lead, crocodile clip and terminal block as a plain straight line, since none of them has a symbol of its own.
5. Finish by checking the finished drawing back against the list, so that no component has been left out and none has been invented.