Volt
SI derived unit · quantity: electric potential
01 · Definition
Volta stacked zinc, copper and wet cardboard — and obtained the first source of steady current. Before him, electricity could only be stored up and let off as a spark.
One volt is the potential difference at which a current of one ampere carries a power of one watt. Put another way, every coulomb of charge that passes through such a section gives up one joule of work. Voltage is not a property of a place but a difference between two points: the “potential of a wire” means nothing until the second point is named, which is why every circuit has a common ground. A mains socket gives 230 V, an AA cell 1.5 V, a neuron in the brain works on tens of millivolts, and a transmission line holds 750 kV. A bird sits unharmed on a line at hundreds of kilovolts precisely because both its feet are at the same potential. From this it also follows that the danger of electricity is not measured in volts: what kills is the current through the body, and the volts only decide whether it will flow through the resistance of the skin. A dry palm offers about a hundred kilohms, a wet one a thousand, and the very same voltage turns out to be either harmless or lethal.
The first source of steady current was a little column of alternating zinc and copper discs interleaved with cloth soaked in brine. Each pair gave about a tenth of a volt, and Volta piled up dozens of pairs until the discharge began to be felt in the fingers. The same principle holds today: in a battery of three lithium cells the voltages add because the cells are connected in series. It was this experiment that refuted the galvanic theory of “animal electricity”: the current came from the dissimilar metals, and the frog’s leg was merely a sensitive instrument.
02 · Conversion
Enter a voltage and the passport will lay it out in every unit
The prefixes cover the whole range from the nanovolt signals of a thermocouple to the megavolts of transmission lines. The electronvolt in the table is not a voltage but the energy an electron gains on passing through the given potential difference.
The mains voltage alternates, and the figure 230 means the root-mean-square value — the steady voltage that would release the same power in the same resistance. The peak of the sine wave is larger by the square root of two and reaches 325 volts, while the swing between crests is 650. Insulation is rated precisely for the peak, which is why a cheap device built “for 230” breaks down on the very first cycle.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| V | volt, the SI unit | 230 | the unit of this sheet |
| mV | millivolt | 230000 | medicine, instrumentation |
| µV | microvolt | 2.3·10⁸ | the sensitivity limit of instruments |
| kV | kilovolt | 0.23 | high-voltage engineering |
| MV | megavolt | 2.3·10⁻⁴ | high-energy physics |
| eV per charge | energy of an electron, eV | 230 | numerically equal to the voltage |
| statV | volt of the CGSE system | 0.7672 | the factor is a tenth of the speed of light |
| abV | abvolt of the CGSM system | 2.3·10¹⁰ | the smallest of the historical units |
| V (int.) | international volt of 1908 | 229.92 | defined through a mercury column and the deposition of silver |
| V₉₀ | practical volt of 1990 | 230 | differed by four parts in ten million |
| Weston cells | in units of the standard cell | 225.79 | a cadmium-mercury cell in a thermostat |
| V | volt | 230 | the unit of this sheet |
The prefixes run from nano- to mega-: the range of voltages engineering deals with spans fifteen orders of magnitude. The electronvolt line is a reminder that the energy of an electron is numerically equal to the voltage it has passed through.
03 · Orders of magnitude
from the nanovolt to a lightning dischargeA mains socket in Europe: 230 volts
04 · Measuring instruments
What voltage is measured with
It is connected across the section and must have as high a resistance as possible so as not to draw current. In moving-coil instruments it is of the order of tens of kilohms, in digital ones ten megohms and above; in high-resistance circuits even that distorts the reading noticeably.
The voltage being measured is balanced against a fraction of the voltage of a standard cell, and the moment is caught when the galvanometer reads zero. At that moment the instrument draws no current at all, so the method gives the true electromotive force of the source rather than the voltage under load.
Two resistors divide the voltage in a known ratio, letting kilovolts be measured with an instrument rated for a few volts. On high-voltage lines a voltage transformer plays the same part, separating the measuring circuit from the power circuit galvanically.
A superconducting contact irradiated with microwaves yields a voltage strictly proportional to the frequency: the Josephson constant turns hertz into volts. An array of thousands of junctions gives ten volts with a reproducibility of parts per billion — and this is the present-day standard.
05 · Writing rules
The symbol V is Latin — the Russian “В” will not do here
The Russian “В” and the Latin V look different but stand for the same thing; in international documents only V is admissible. The name of the unit is lowercase — “two hundred and thirty volts” — while the symbol is capital. A millivolt is mV, whereas MV is a megavolt: a difference of a billion times. In alternating current 230 V is the root-mean-square value; the peak of the sine wave is higher by a factor of √2, that is 325 V. Insulation is rated for the peak.
The symbol for the quantity is an italic U or V, for the unit an upright V: in U = 230 V the italic and the upright type carry the distinction. The Russian symbol is В. The prefix kilo- takes a lowercase k: kV, not KV. Millivolt and microvolt are set with the ordinary prefixes, but the “volt-ampere” is a different unit altogether — apparent power — and has nothing to do with voltage. The electronvolt is written closed up, eV: it is a unit of energy.
06 · Neighbouring units
The volt stands in the middle of Ohm’s law: current, voltage and resistance are bound so that any two of them fix the third.
Of the Simetrium passports sit next to the volt ampere and ohm from Ohm’s law, watt as their product, coulomb s the farad from the definition of capacitance, and electronvolt — the energy named after this unit.
07 · Historical section
From a frog’s leg to a quantum step
Alessandro Volta argued with Galvani over the nature of “animal electricity” and, to prove his case, built a source with no biology in it at all. A pile of dissimilar metals gave a continuous current — the first in history.
The International Congress of Electricians defined the volt through the ampere and the ohm, naming the unit after Volta. Its practical realisation was left to a chemical cell, whose accuracy fell far short of what telegraphy demanded.
Weston’s cadmium-mercury cell held its voltage to within microvolts and became the standard for ninety years. It was kept in a thermostat, never allowed to deliver current, and carried about as gingerly as a fragile instrument.
Brian Josephson, a graduate student of twenty-two, predicted voltage steps across a superconducting contact. Since 1990 the volt has been realised in just this way, and the chemical cells have gone off to the museums of metrology.
A volt that does not agree with the volt
From 1990 to 2019 metrology lived with two volts at once. The definition in the International System remained as before, through the watt and the ampere, but the unit was realised by the Josephson effect using a conventionally adopted value of the constant — and this practical unit was written V₉₀. The discrepancy between it and the SI volt was reckoned at tenths of a microvolt per volt: a quantity negligible for engineering and decisive for comparisons between standards. The redefinition of 2019 removed the split: the electron charge and the Planck constant became exact numbers, the Josephson constant with them, and the practical scale coincided with the definition. At the same time thirty years of accumulated measurements in V₉₀ had to be given a correction after the fact — a rare case of past results being recomputed because the definition of a unit had changed.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI units that neither engineering nor daily life can do without. Each gets its own sheet: definition, conversion, instruments, writing rules. In 11 languages.
SI derived units with names of their own
the volt passport is openSI base units
in terracotta — the four units the volt is built fromVoltage in other systems
Passport language
11 languages. The symbol V is international, but the mains voltage is not: 230 volts in Europe, 120 in North America, 100 in Japan — the passport fills in the local figure.