The vast majority of current measuring devices are built around a digital voltmeter, with the physical quantity to be measured being converted into voltage using a suitable sensor.

This is the case of the digital multimeter which, in addition to offering the voltmeter function, has at least one voltage current converter to operate it as an ammeter and a constant current generator to operate as an ohmmeter

The ohmmeter

An ohmmeter is an instrument that measures the electrical resistance of an electrical component or circuit. The unit of measurement is the ohm, denoted Ω. Two methods can be used to measure the value of a resistance : - Measurement of a voltage with a current generator.
- Measurement of a current with a voltage generator (or D.D.P).

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They are endangered, although still used as rapid indicators of the order of magnitude or variation of the measured voltage. They usually consist of one millimeter in series with high resistance. However, this resistance, of the order of a few kΩ, is significantly lower than the internal resistance of digital voltmeters, usually equal to 10 MΩ.

For this reason, analog voltmeters introduce a greater disturbance into the circuits into which they are introduced than digital voltmeters.
To limit this disturbance, we went so far as to use galvanometers with a sensitivity of 15 micro-amps for full scale on high-end universal controllers (voltmeter-micro-ammeter-ohmmeter

The ohmmeter

An ohmmeter is an instrument that measures the electrical resistance of an electrical component or circuit. The unit of measurement is the ohm, denoted Ω. Two methods can be used to measure the value of a resistance : - Measurement of a voltage with a current generator.
- Measurement of a current with a voltage generator (or D.D.P).

-capacimeter combination). (Metrix MX 205 A for example)

A magnetoelectric voltmeter consists of a galvanometer, therefore a very sensitive magnetoelectric millimetermeter, in series with an additional resistance of high value (from a few kΩ to a few hundred kΩ).
A voltmeter with several measuring gauges is made by changing the value of the additional resistance. For ALTERNATING CURRENT MEASUREMENTS, a diode rectifier bridge is interspersed but this method can only measure sinusoidal voltages. However, they have a number of advantages : they do not require a battery to operate.

In addition, at the same price, their bandwidth is much wider, allowing AC measurements over several hundred kilohertz where a standard digital model is limited to a few hundred hertz.
It is for this reason that they are still widely used in testing on electronic equipment operating at high frequencies (HI-FI)

A ferroelectric voltmeter consists of a ferroelectric millimeter ammeter in series with an additional resistance of high value (from a few hundred Ω to a few hundred kΩ). As ammeters of the same type do for currents, they make it possible to measure the effective value of voltages of any shape (but of low frequency) < 1 kHz).

They usually consist of a dual ramp analog-to-digital converter, a processing system and a display system.

It can only be used for the measurement of sinusoidal voltages in the frequency range of electrical distribution networks. The voltage to be measured is straightened by a diode bridge and then treated as a DC voltage. The voltmeter then displays a value equal to 1.11 times the average value of the rectified voltage. If the voltage is sinusoidal, the result displayed is the effective value of the voltage; if it is not, it makes no sense.

The majority of devices on the market perform this measurement in three steps :

1 - The voltage is raised squared by a precision analog multiplier.
2 - The device performs the analog-to-digital conversion of the average of the square of the voltage
3 - The square root of this value is then performed numerically.

Since the precision analog multiplier is an expensive component, these voltmeters are three to four times more expensive than the previous ones. Near-total digitization of the calculation reduces cost while improving accuracy.

Other measurement methods 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 average square".
- Equalization of the thermal effect generated by the variable voltage and that generated by a DC voltage which is then measured.

There are two types of voltmeters "true effective" :

- TRMS (from English True Root Mean Square meaning "true square root mean") - It measures the true effective value of a variable voltage.
- RMS (from English Root Mean Square meaning "square root average") - The value RMS is obtained through filtering that eliminates the dc component (average value) of the voltage, and allows to obtain the effective value of the voltage ripple.

The first digital voltmeter was designed and built by Andy Kay in 1953.
The measurement with a voltmeter is carried out by connecting it in parallel to the portion of the circuit whose potential difference is desired.
Thus in theory, so that the presence of the device does not change the distribution of potentials and currents within the circuit, no current should flow in its sensor. This implies that the internal resistance of said sensor is infinite, or at least is as great as possible compared to the resistance of the circuit to be measured.