The voltmeter is a device used to measure the tension (or electric potential difference) between two points, including the unit of measure is the volt (V) 1.
The vast majority of current measuring devices is built around a digital Voltmeter, the physical quantity to measure being converted to voltage using a suitable sensor. This is the case of the DMM, which, besides offering the voltmeter function, has at least a converter voltage to operate in ammeter and a current generator continued to operate in the ohmmeter.

They are becoming extinct, as still used as quick flags of the order of magnitude or the variation of the measured voltage. They are usually made up of a milliammeter in series with a high resistance. However this resistance, of the order of a few kΩ, is substantially lower than the internal resistance of the digital voltmeters, usually equal to 10 MΩ.
For this reason, analog voltmeters introduce a more significant disruption in the circuit in which they are introduced as digital voltmeters. To limit this disturbance, we went to use galvanometers to a sensitivity of 15 microamps for the full scale on high-end universal controllers (voltmeter-microampere-ohmmeter-capacitance combination). (Metrix MX 205 A)
Magnetoelectriques voltmeters

A magnetoelectrique voltmeter consists of a galvanometer, so a very sensitive magnetoelectrique milliammeter in series with an additional resistance of high value (of a few kΩ to hundreds of kΩ). We realize a voltmeter to several calibers of measure by changing the value of the additional resistance. For measures running alternative, a bridge rectifier diodes is intercalated, but this method allows to measure that sinusoidal voltages. However, they have a number of advantages : they require a battery to operate.
Furthermore, equivalent price, their bandwidth is much wider, allowing AC measures on hundreds of kilohertz here where a standard digital model is confined to a few hundred Hertz.
For this reason, they are still widely used in test on electronic equipment operating at high frequencies (HI-FI)
Ferroelectriques voltmeters

A ferroelectric voltmeter consists of a ferroelectric milliammeter in series with an additional resistance of high value (of a few hundred to hundreds of kΩ Ω). As ammeters of the same type do for currents, they can measure the effective value of voltages form any (but frequency low < 1 kHz).

They are usually made of an analog-to-digital converter double ramp, a system of treatment and a display system.
Measurement of the mean values of continuous tension

To measure voltage is applied at the entrance of the analog-to-digital converter through a resistance whose value depends on the chosen caliber, then body treatment, taking account of this caliber, to display the average value of this voltage.
Measurement of the effective values of alternative tensions
Low range voltmeter

It is used for the measurement of sinusoidal voltages in the field of frequency of electric distribution networks. The voltage to be measured is rectified by a diode bridge and then treated as tension continues. The voltmeter then displays a value of 1.11 times the average value of the rectified voltage. If the voltage is sinusoidal, the displayed result is the effective value of the voltage; If it is not, it is meaningless.
Effective true voltmeter

The majority of the marketed devices perform this measure in three steps :

The voltage is squared by an analog multiplier of precision.
The unit produces the analog-to-digital conversion of the average of the square of the voltage
The square root of this value is then performed digitally.

The analog multiplier of precision being a costly component, these voltmeters are three to four times more expensive than the previous ones. Almost complete scanning of the calculation allows to reduce costs while improving accuracy.

Other methods of measurement are also used, for example :

Analog-to-digital conversion of the voltage to be measured, then fully digital processing of the calculation of the \square root of the mean square.
Equalization of thermal effect caused by the variable voltage and that generated by a tension continues which is then measured.

There are two types of \true effective : voltmeters

Powermeter (from English True Root Mean Square meaning 'real average root square') - it measures the true effective value of a variable voltage.
RMS (Root Mean Square English meaning \average square root\)-the RMS value is obtained through a filter that eliminates the continuous component (average value) voltage, and lets get the effective value of the ripple (RMS AC or RMS ) of the voltage.

The first digital Voltmeter was designed and built by Andy Kay in 1953
The measure with a voltmeter is done by plugging it in parallel on the section of track which we desire to know the potential difference. So in theory, to keep the presence of the device changes to the distribution of potentials and currents within the circuit, no current should flow in its sensor. Which implies that the resistance is an internal sensor said infinite, or at least the largest possible compared to the resistance of the circuit to be measured.