An instrument used for measuring potential difference or electromotive force between points in a circuit. Most voltmeters consist of an ammeter connected in line with a high resistance, and calibrated in volts. The current flowing is proportional to the potential difference, although the presence of the measuring instrument reduces the potential difference.
A voltmeter is an instrument used for measuring the voltage between two points in an electric circuit.
The voltage can be measured by allowing it to pass a current through a resistance;
Potentiometer
A voltmeter may also be realized using a potentiometer, which is a length of uniform resistance material (wire or carbon film, for instance) and a "wiper" that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer. The unknown voltage source may be connected to a current detector, which is in turn connected to the potentiometer's wiper, while the known voltage source is connected to an end terminal of the potentiometer. Now replace the unknown voltage supply with the known voltage supply and repeat the procedure. The unknown voltage is then given by the product of the known voltage and the recorded used length of wire corresponding to the unknown voltage, divided by the recorded length of wire corresponding to the reference voltage.
One may also measure voltage using a potentiometer in the null-balance method.
where
Vt: Voltage across test points
Vk: Known voltage
Re: Potentiometer resistance from one end terminal to the other end terminal
Rw: Potentiometer resistance from wiper to end terminal
There are many implementations for null detectors, including nanovolt-sensitive integrated circuits, simple audio circuits that click to indicate voltage difference, and transformed ammeters, as discussed at the top of this article.
Vacuum Tube Voltmeter (VTVM)
Another popular form of voltmeter is the electronic voltmeter, or vacuum tube voltmeter, frequently referred to as a VTVM. This kind of voltmeter uses a tube (or valve in British English) or transistor circuit to amplify the input voltage, which facilitates two objectives: increased sensitivity, and/or increased input impedance (this equipment usually has an input resistance of 10 to 20 megohms).
Oscilloscope
The oscilloscope method of measuring voltage employs the deflection of the ray in a cathode ray tube (CRT). By comparing the deflection caused by an unknown voltage with that caused by a known reference voltage the unknown voltage can easily be deduced.
Digital voltmeters usually employ an electronic circuit that acts as an integrator, linearly ramping output voltage when input voltage is constant (this can be easily realized with an opamp). The dual-slope integrator method applies a known reference voltage to the integrator for a fixed time to ramp the integrator's output voltage up, then the unknown voltage is applied to ramp it back down, and the time to ramp output voltage down to zero is recorded (realized in an ADC implementation). The unknown voltage being measured is the product of the voltage reference and the ramp-up time divided by the ramp-down time. The voltage reference must remain constant during the ramp-up time, which may be difficult due to supply voltage and temperature variations.
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