Meters for measuring circuits

1. Measuring circuits are built so that the meters record the current and potential difference without changing what they measure.
2. An ammeter is placed in series and has a very low resistance, so adding it to a circuit barely changes the current.
3. A voltmeter is placed in parallel and has a very high resistance, so it draws almost no current from the component it measures.
4. A multimeter combines several meters in one, and a dial selects whether it measures current, potential difference or resistance.
   1. The black lead goes into the socket marked COM, while the red lead goes into the socket for volts and ohms or the socket for amperes, depending on the measurement.
   2. On the resistance setting, the multimeter passes a small current from its own battery, so the component must be disconnected from the circuit first.
5. Each setting has a range, such as $20V$ or $200mA$, and the best range is the smallest one that is still larger than the value being measured.

Accurate readings

1. The resolution of a digital meter is set by its last digit, so a meter reading $1.23V$ has a resolution of $0.01V$.
   1. Choosing a smaller range usually gives more digits after the decimal point, and so a better resolution.
2. Before a meter is used, check for a zero error by reading it with no current or no potential difference applied.
   1. A zero error must be subtracted from every reading, because it shifts all the readings by the same amount.
3. On an analogue meter, read the needle with your eye directly in line with it, so that parallax does not make the reading too high or too low.
4. Readings change if the component warms up, so close the switch only while taking each reading.
5. Take each reading more than once and calculate a mean, ignoring any value that is clearly anomalous.

Testing for faults

1. An open circuit has a break in it, such as a broken wire, a loose connection or a blown filament, so there is no current anywhere in that loop.
2. A short circuit is a low-resistance path that lets the current bypass a component, so that component stops working and the current in the rest of the circuit rises.
3. A continuity tester, made from a cell and a buzzer or lamp, or a multimeter on its resistance setting, shows whether a wire, fuse or switch provides a complete path.
   1. A reading of nearly $0\Omega$, or a sounding buzzer, shows a complete path, while a very large reading shows a break.
4. A voltmeter can find the faulty part of a series circuit without disconnecting anything.
   1. Across a working component in a circuit with a break, the voltmeter reads zero, because no current flows through it.
   2. Across the broken component, the voltmeter reads the full supply potential difference, because the voltmeter completes the circuit around the gap.
5. A component that has been short-circuited has a potential difference of zero across it, even though the supply is working.

Circuits that measure

1. A sensor in series with a fixed resistor turns a change in the surroundings into a change in potential difference, which a voltmeter can read.
2. In a temperature-sensing circuit, an NTC thermistor is connected in series with a fixed resistor and a d.c. supply, with a voltmeter across the fixed resistor.
   1. As the temperature rises, the resistance of the thermistor falls, so the current increases.
   2. The fixed resistor now takes a larger share of the supply potential difference, so the voltmeter reading rises.
3. A light-sensing circuit works in the same way, with an LDR in place of the thermistor, so the voltmeter reading rises as the light gets brighter.
4. The circuit is calibrated by recording the voltmeter reading at several known temperatures or light intensities, so that any later reading can be converted back into a temperature or light intensity.