The most common electrical characteristics measured are current or amperage, electromotive force or voltage, and resistance.
An ammeter measures the amount of electrical current in amperes. Current flows through a circuit, so to be measured it must flow through the ammeter. That means connecting the ammeter in series by breaking into the circuit. The low resistance of the ammeter ensures that the circuit will operate normally when the ammeter is connected to the circuit.
A voltmeter measures potential difference in volts across two points, so it is connected in parallel. It has a high internal resistance, so as not to affect the potential difference across the component or circuit being tested.
An ohmmeter measures resistance in ohms. The item to be tested must first be disconnected from its circuit, so any pressure in the circuit will not affect the readings of the meter.
Although these can all be separate measuring devices, they are now commonly combined into one digital measuring device, known as a digital multi-meter (DMM) or a digital volt ohm meter (DVOM).
Other devices measure other characteristics. For instance, a capacitance meter measures capacitance, an electrometer measures electrical charge, and an oscilloscope measures how electrical characteristics change over time.
