Electrical Units

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.

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.