3. Electrical and Magnetic Instruments

These instruments measure electrical quantities such as current, voltage, resistance, and power, and are extensively used in physics laboratories, electrical workshops, and everyday electrical maintenance.

Most analog electrical measuring instruments in this category trace their design back to the moving-coil galvanometer, invented on the principle discovered by Hans Christian Ørsted (that an electric current produces a magnetic effect) and developed further using the results of André-Marie Ampère and Michael Faraday's work on electromagnetism. A single galvanometer movement, by adding an appropriate shunt or series resistor, can be converted into an ammeter, a voltmeter, or an ohmmeter — which is exactly why exam questions often ask candidates to identify how a galvanometer must be modified to serve each of these roles.

3.1 Ammeter

Ammeter. An instrument used to measure the strength of electric current flowing in a circuit, in amperes. It is always connected in series in the circuit so that the same current that flows through the circuit also flows through the ammeter. An ideal ammeter has a very low (ideally zero) resistance so that it does not alter the current it is measuring. Most analog ammeters work on the principle that a current-carrying coil placed in a magnetic field experiences a torque (the moving-coil galvanometer principle), which deflects a pointer proportional to the current.

3.2 Voltmeter

Voltmeter. An instrument used to measure the potential difference (voltage) between two points in an electrical circuit. Unlike the ammeter, it is always connected in parallel (across the component whose voltage is to be measured), and an ideal voltmeter has very high (ideally infinite) resistance so that it draws negligible current from the circuit and does not disturb the measurement.

3.3 Galvanometer

Galvanometer. A sensitive instrument used to detect the presence, direction, and relative magnitude of small electric currents in a circuit; it is not usually calibrated to give a numerical reading of current directly. The common moving-coil galvanometer works on the principle that a current-carrying coil suspended in a magnetic field experiences a torque, causing it to deflect; the angle of deflection is proportional to the current. Both ammeters and voltmeters are essentially galvanometers modified by adding a low-resistance shunt (for ammeters) or a high-resistance series resistor (for voltmeters).

3.4 Ohmmeter

Ohmmeter. An instrument used to directly measure electrical resistance, in ohms. It works by applying a small known voltage from an internal battery across the component under test and measuring the resulting current, then using Ohm's Law (V = IR) internally to calculate and display the resistance. Ohmmeters must only be used on de-energised (power-off) circuits/components.

3.5 Wattmeter

Wattmeter. An instrument used to measure electric power (the rate of consumption of electrical energy) in a circuit, in watts. It combines the functions of an ammeter and a voltmeter internally: it has a fixed current coil (connected in series, like an ammeter) and a moving voltage/pressure coil (connected in parallel, like a voltmeter), and the deflection produced is proportional to the product of current and voltage, i.e., power.

Household electricity meters, which record the total energy consumed over a period (in kilowatt-hours, the unit billed by electricity providers) rather than the instantaneous rate of power consumption, work on a closely related principle to the wattmeter, effectively integrating power over time to give total energy used.

3.6 Multimeter

Multimeter. A versatile, multi-purpose electrical instrument that can measure several quantities — typically voltage (AC and DC), current, and resistance — combined in a single unit, selectable using a dial or switch. Digital multimeters (DMMs), which display readings numerically, have largely replaced older analog (moving-needle) multimeters and are standard equipment for electricians and technicians.

Many modern digital multimeters also include additional functions beyond the basic three, such as testing diodes and transistors, checking circuit continuity (useful for finding a broken wire or a blown fuse, usually indicated with an audible beep), and measuring capacitance or frequency — making the multimeter one of the single most useful general-purpose tools in electrical troubleshooting.

3.7 Potentiometer

Potentiometer. An instrument used to accurately measure or compare potential differences (EMF) without drawing any current from the source being measured, which makes it more accurate than a simple voltmeter for such comparisons. It consists of a long, uniform resistance wire across which a steady current is maintained; the unknown EMF is balanced against the potential drop across a measured length of this wire at the point where a galvanometer shows zero deflection (the null-deflection or balance point method). It is also widely used to compare EMFs of cells and to measure internal resistance of a cell.

A useful memory aid: current always takes the 'series' path (an ammeter must be inserted directly into the flow, like a toll booth on a single road), while voltage is always a 'parallel' comparison (a voltmeter measures the difference between two points, like comparing the water levels on either side of a dam, without diverting the flow itself). This is why an ammeter is built with extremely low resistance and a voltmeter with extremely high resistance — connecting them the wrong way around in a real circuit can damage the meter or given a badly incorrect reading, and is a frequently tested conceptual question.

Quick Revision Table — Electrical/Magnetic Instruments