units used to express quantitative measurements of all types of electrostatic and electromagnetic phenomena and of the electrical characteristics of components of electrical circuits (see ELECTRIC CIRCUIT,; ELECTRICITY,; ELECTRIC METERS,). The basic electrical units are part of the centimeter-gram-second system (see CGS SYSTEM,; INTERNATIONAL SYSTEM OF UNITS; METRIC SYSTEM,), but because, in most cases, these units are either too large or too small for convenient measurement, a number of practical units have been adopted for use in engineering.

Basic Units.

The elemental unit of electricity is the absolute charge on a single ELECTRON, or PROTON, (qq.v.). The symbol for this unit is e. The CGS unit of electrical charge is the electrostatic unit (esu), which is defined as the quantity of electricity that when concentrated at a point in a vacuum will repel a like charge 1 cm away with a force of 1 dyne. The esu equals the aggregate charge carried by 2,082,000,000 electrons or protons.

The basic units of electrical current or flow is the statampere, which is defined as a current of 1 esu/sec. The statvolt, the basic unit of electromotive force, or potential difference, is the difference in potential that exists between two points when 1 erg of WORK, (q.v.) is required to force 1 esu of electricity between those two points.

Electromagnetic Units.

Besides the electrostatic units of charge, current, and potential difference, a parallel group of basic electromagnetic units exists. The basic magnetic unit, comparable to the elemental unit of electricity, is the unit magnetic pole, defined as a point magnetic pole that in a vacuum will act on a similar pole 1 cm away with a force of 1 dyne. The unit used to measure the strength of magnetic fields is the oersted. A field that acts on a unit magnetic pole with a force of 1 dyne has a strength of 1 oersted. The electromagnetic unit of electric current is called the abampere. If a current of 1 abampere flows in a wire 1 cm long, the wire is pushed sidewise with a force of 1 dyne by a magnetic field of 1 oersted acting at right angles to the wire. The abcoulomb is the quantity of electricity passing any point in a circuit in 1 sec when a current of 1 abampere is flowing in the circuit. The abvolt, the electromagnetic unit of potential difference, is the potential difference between two points when 1 erg of work is necessary to move 1 abcoulomb of electricity from one point to the other. See also POTENTIAL ENERGY,.

The mathematical relationships between the electrostatic and electromagnetic units are as follows: 1 esu equals 3.3356 x 10^–11 abcoulombs; 1 statampere equals 3.3356 x 10^–11 abamperes; and one statvolt equals 29,979,600,000 abvolts. This last figure is exactly equal to the velocity of light through a vacuum, which is expressed in centimeters per second, as predicted by the electromagnetic-wave theory developed by the British physicist James Clerk Maxwell. See ELECTROMAGNETIC RADIATION,.

Practical Units.

The unit of electrical current in common use is the ampere, which is defined as 0.1 abamperes. The practical unit of electrical quantity is the coulomb, the amount of electricity passing a given point in a circuit in 1 sec when a current of 1 amp is flowing. The volt is the practical unit of potential difference. It is equal to 100 million abvolts and can be defined as the potential difference existing between two points when 1 joule (10 million ergs) of work is required to move 1 coul of electricity from one of the points to the other. The unit of electrical work is the watt. It represents the generation or use of electrical energy at the rate of 1 J/sec. The kilowatt is equal to 1000 W.

Because of the difficulty of making measurements in terms of the absolute units, the practical units are also defined for purposes of practical standardization as follows: The ampere is the amount of current that will deposit 0.001118 g of silver per sec if passed through a silver nitrate solution; the ohm is the resistance of a column of mercury 106.3 cm in length and 1 sq mm in cross section at a temperature of 0° C (32° F); the volt is the electromotive force necessary to produce a current of 1 amp through a resistance of 1 ohm. The volt is also defined in terms of a standard voltaic cell, called the Weston cell, which has poles of cadmium amalgam and mercurous sulfate and an electrolyte of cadmium sulfate. A volt is defined as 0.98203 of the potential of this standard cell at 20° C (68° F).

In all the practical electrical units the conventional prefixes of the metric system are used to indicate fractions and multiples of the basic units. Thus a micromicrofarad is a trillionth of a farad, a microampere is a millionth of an ampere, a millivolt is a thousandth of a volt, a millihenry is a thousandth of a henry, a kilowatt is 1000 W, and a megohm is 1 million ohms.

Resistance, Capacitance, Inductance.

All components in electrical circuits exhibit one or more of the characteristics of resistance, capacitance, and inductance. The commonly used unit of resistance is the ohm, which is the resistance of a conductor in which a potential difference of 1 V causes a current flow of 1 amp. The capacitance of a condenser is measured in farads. A condenser of 1 farad capacitance will exhibit a change in potential difference of 1 V between its plates when 1 coul of electricity is transferred from one plate to the other. The henry is the unit of inductance. A coil has a self-inductance of 1 H when a change in current of 1 amp/sec produces a countervoltage of 1/V. In a transformer, or in any two magnetically coupled circuits, a mutual induction of 1 H is that inductance which will induce a voltage of 1 V in the secondary when there is a change of 1 amp/sec in the primary.

See also BATTERY,; ELECTROCHEMISTRY,.