Humans do not have a specific sensory organ to directly perceive electric current. Therefore, measuring instruments are needed to detect the presence of current and to quantify its magnitude.
In principle, all effects of electric current can be used for measurement, such as heat generation or chemical reactions.
Robust, but not very sensitive, measuring instruments are so-called hot-wire ammeters. The electrical power converted into heat warms a wire, whose change in length is transferred into a pointer deflection by a special mechanism.
There are also current measuring devices based on the principle of electrolysis, which measure the amount of substance transformed per second.
Common ammeters are based on the magnetic interactions of the current and the resulting forces. Such devices are usually called galvanometers. In a moving-coil galvanometer, a coil carrying the current to be measured is placed between the poles of a permanent magnet. The magnetic field of the current then creates a torque that rotates the coil against a restoring spring, with the magnitude of the deflection indicated by a pointer.
Another type is the soft-iron galvanometer, which consists of two pieces of soft iron magnetized in the same direction by a surrounding coil. The repulsion between the iron pieces is also transmitted mechanically to a pointer. Soft iron is used because it can be easily magnetized and reversed.
Today, electronic measuring instruments are widespread. In these, the voltage drop across a resistor is amplified and then displayed with suitable electronic circuits on a digital display.
In general, measurements of electrical quantities are distinguished between voltage and current measurements, although both rely on the same principle.
- For current measurements, the entire current must pass through the measuring instrument. Therefore, the device must be connected in series with the other components in the same loop, and its internal resistance should be as small as possible.
- For voltage measurements, the instrument must be connected in parallel to the component across which the voltage is to be measured. In this case, the internal resistance should be as large as possible so that not too much current is diverted through the instrument.
In general, the internal resistance must be chosen such that the measured value falls within the scale range while not influencing the circuit too much.
Devices that can measure both current and voltage are called multimeters. These usually include a selector switch that allows switching between different measurement methods and sensitivity ranges.