Electronics Measurements: What a Multimeter Can Measure
A meter is a device that measures electrical quantities in your electronics projects. A multimeter, therefore, is a combination of several different types of meters all in one box. At the minimum, a multimeter combines three distinct types of meters (ammeter, voltmeter, and ohmmeter) into a single device.
Ammeter measures current
Current is the flow of electric charge through a conductor. Current is measured in units called amperes. It should come as no surprise, then, that a meter that measures amperage is called an ammeter.
Ammeters usually measure current in milliamperes, also called a milliamp, and abbreviated mA. One mA is one-thousandth of an ampere.
Voltmeter measures voltage
The second fundamental quantity of electricity is voltage, a term that refers to the difference in electric charge between two points. If those two points are connected to a conductor, a current will flow through the conductor. Thus, voltage is the instigator of current. The device that measures voltage is called a voltmeter.
It turns out that, all other things being equal, a change in the amount of voltage between two points results in a corresponding change in current. Thus, if you can keep things equal, you can measure voltage by measuring current, and you already know of a device that can measure current: It's called an ammeter.
The basic difference between an ammeter and a voltmeter is that in an ammeter, you let current run directly through the meter so that you can measure the amount of current. In a voltmeter, the current is first run through a very large resistor and then through the ammeter, and the device makes the necessary calculations.
There's a direct relationship between voltage, resistance, and current in an electrical circuit. In particular, if you know any two of these quantities, the third one is easy to calculate. In a voltmeter, a large fixed resistance is used, and the ammeter measures the current. Because you know the amount of the fixed resistance and the amount of current, you can easily calculate the amount of voltage across the circuit.
Ohmmeter measures resistance
As you know, a resistor is a material that resists the flow of current. How much the current is restricted is a function of the amount of resistance in the resistor, which is measured in units called ohms. The symbol for ohms is the Greek letter omega, Ω. A device that measures resistance is called an ohmmeter.
Like voltage, resistance can also be measured with an ammeter. Remember there's a direct relationship between voltage, resistance, and current in any circuit, and that if you know any two of these quantities you can easily calculate the third? To recap, to measure voltage, a voltmeter provides a fixed resistance, uses an ammeter to measure the current, and then uses the resistance and current to calculate the voltage.
To measure the resistance of a circuit, an ohmmeter provides a fixed amount of voltage across the circuit, uses an ammeter to measure the current that flows through the circuit, and then uses the amount of voltage provided by the meter and the amount of current read by the meter to calculate the resistance.
Other electronic circuit measurements
All multimeters can measure current, voltage, and resistance. Some multimeters can perform other types of measurements as well. For example, some meters can measure the capacitance of capacitors, and some meters can test diodes or transistors. These features are handy, but not essential.
Schematic symbols for meter functions
Ammeter, voltmeter, and ohmmeters are often included in schematic diagrams. When they are, the following symbols are used: