A voltmeter is an electrical test device that can indicate the level and phase difference between the first input and the second input. It is a dedicated voltmeter capable of measuring vector characteristics. Voltmeters are part of multiple test equipment and are often found in electrical design laboratories and dedicated monitoring panels.

Unitary vectors and scalars are two common representations of the actual number of various entities. Scalar units have no or implied direction. Example: An 11 lb (5 kg) mass scalar will have a 5 kg weight (vector). The direction of 5 kilograms is implied downward, and the weight of the object is also a vector, because the weight is a force pointing downwards.

A scalar has only magnitude, while a vector has magnitude and direction. When using a voltmeter to measure the voltage of a 1.5 volt (V) battery, the result reads "1.5V positive piezoelectric." When another battery is measured in series, the result can be read as "3.0V positive voltage". One of the batteries in the series was reversed and the reading became 0V, which became a vector offset. When two vectors of the same magnitude but opposite directions are added, vector cancellation occurs. Vectors can be completely or partially added to or cancelled from each other. Actual vectors are not exactly in phase or out of phase, and the phase difference ranges from 0 to 360 degrees.

Voltmeter can measure the vector difference between two voltage sources. An example is a three-phase power supply in which the phase difference between the three outputs is 120 degrees. The voltmeter will be able to confirm the phase difference between the outputs and also get the so-called phase voltage. The reactance components in electrical and electronic circuits will produce a phase shift in the incoming time-varying signal. The phase shift can be anywhere between 0 and 360 degrees. Currents and voltages in capacitors and inductors often have a 90-degree phase difference. These reactive components in various circuit combinations want to produce a wide range of possible phase differences at different frequencies.

Another use of voltmeters is to measure the frequency and phase response of electronic amplifiers. The frequency response of a device can be plotted by measuring and plotting the input and output levels at various test frequencies. By measuring the phase relationship of the input and output at various frequencies, the phase response can also be plotted as a guide for implementing other circuit stages to obtain the required overall characteristics.