A voltmeter measures the potential difference between positive and negative terminals in a circuit, indicating the direction of current flow. Voltage is a variable measurement, while current flow is measured in amperes. Other devices that measure voltage include oscilloscopes and A/D converter cards. Digital multimeters are commonly used today. Earlier voltmeters were analog and produced a graduated pointer display. A/D converters change the analog signal to a digital value for computer analysis.
Voltage measurement is accomplished in many applications by the use of a voltmeter, which is a hand-held device that measures the potential or voltage difference between positive and negative terminals that contact locations in a conductive electrical circuit. Since voltage is defined as this potential difference between two points in a conducting circuit, there are many circuits that also have a built-in voltage measurement capability. Voltage is defined as the electrical difference between two points in a circuit carrying current of one ampere and work energy of one watt, and is an essential characteristic of any electrical device.
When a voltage measurement is taking place, if the reading indicates a positive value, it means that the voltage measuring device is displaying the true nature of current flow within the circuit, from the positive conductor through the voltage measuring device to the conductor negative and back into the circuit. If the voltmeter indicates a negative value, it means that the positive and negative leads are reversed and current is actually flowing in a reverse direction. Regardless of how the leads of a voltmeter are placed at points in a circuit, measuring voltage is an indicator of only a difference in potential, at two points in a circuit, and is strictly speaking a variable measurement and not of the effective current flow.
Current flows from positive to negative potential in a circuit. Voltmeters indicate this as a positive value on the readings. It is also true that the actual physical flow of electrons in circuits is in the reverse direction from negative to positive, as a counterbalancing effect of current flow. Voltage measurement is often confused with current flow, but current flow is actually measured in amperes, while voltage can be viewed as a snapshot over time of an electrical potential difference at two points in a circuit.
As a voltage measuring device works, it also redirects some of the current in a circuit in the process of measuring through the unit. This changes the effective voltage of the circuit while the voltmeter is connected. However, most voltmeters are constructed to have little effect on circuit performance during such measurements.
Other types of external and integrated circuit devices that measure voltage include the oscilloscope and analog-to-digital (A/D) converter cards in computers. Some devices also measure more than just voltage, such as measuring the combined voltage-current or the resistance of a circuit, and these units are called multimeters. Many modern versions of voltage measuring devices are digital and give a discrete reading and, in the case of multimeters, are commonly referred to as digital multimeters (DMMs).
Early forms of voltmeters were built on the analog principle of a continuous measurement of a voltage value and had a graduated pointer with a needle that fluctuated up and down as the voltage changed. In this respect, they resemble, to some extent, a reading from an oscilloscope, which produces a graphical display of the continuous measurement of voltage over a specified period of time. Computers use A/D converters to measure voltage and change the measurement to a binary signal that the computer understands. These devices also take the true input voltage in analog signal terms and change it to a digital value that the software on your computer can analyze.
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