I.

Introduction

Electrical Units, units used to express quantitative measurements of all types of electrostatic and electromagnetic phenomena and of the electrical characteristics of components of electrical circuits. The basic electrical units are part of the centimeter-gram-second 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.

II.

Electrostatic Units

The elemental unit of electricity is the absolute charge on a single electron or proton. 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 centimeter away with a force of 1 dyne. The esu equals the aggregate charge carried by 2,082,000,000 electrons or protons.

The basic unit of electrical current, or flow, is the statampere, which is defined as a current of 1 electrostatic unit per second. 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 is required to force 1 electrostatic unit of electricity between those two points.

III.

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 centimeter 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 centimeter 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 second 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 × 10^-11 abcoulombs; 1 statampere equals 3.3356 × 10^-11 abamperes; and one statvolt equals 29,979,245,800 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 British physicist James Clerk Maxwell. See Electromagnetic Radiation.

IV.

International System Units

The International System of Units is a system of units that are practical to use in the laboratory. They are commonly referred to as SI units, from the initials of the French words Système International. The SI unit of electrical current is the ampere (amp), which is defined as 0.1 abamperes. The SI unit of electrical quantity is the coulomb, the amount of electricity passing a given point in a circuit in 1 second when a current of 1 ampere is flowing. The volt (V) is the SI 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 coulomb of electricity from one of the points to the other. The SI unit of electrical work is the watt. It represents the generation or use of electrical energy at the rate of 1 joule per second. The kilowatt is equal to 1000 watts.

V.

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 volt causes a current flow of 1 ampere. The capacitance of a condenser is measured in farads. A condenser of 1 farad capacitance will exhibit a change in potential difference of 1 volt between its plates when 1 coulomb of electricity is transferred from one plate to the other. The henry (H) is the unit of inductance. A coil has a self-inductance of 1 henry when a change in current of 1 ampere per second produces a countervoltage of 1/V. In a transformer, or in any two magnetically coupled circuits, a mutual induction of 1 henry is that inductance which will induce a voltage of 1 volt in the secondary when there is a change of 1 ampere per second in the primary.

VI.

Unit Standards

The standards for electrical units are maintained by national standards laboratories. Originally, the volt was 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 was defined as 0.98203 of the potential of this standard cell at 20° C (68° F). This definition is still used by laboratories in daily measurements. For more accurate standards, the Josephson effect, a phenomenon involving discrete voltage steps, is used to define the volt. The ohm was originally defined by using a collection of standard resistors. Today, the quantum Hall effect, which involves a constant resistance that is independent of experimental conditions, is used to define the ohm. The other electrical units are defined based on these more accurate values for the volt and the ohm.

In all the SI 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 watts, and a megohm is 1 million ohms.

See also Battery; Electrochemistry.