The removal or diminution of a magnetic field applied to a magnetic substance when the latter has been thermally isolated from its surroundings. The process concerns paramagnetic substances almost exclusively, in which case a drop in temperature of the working substance is produced (magnetic cooling). See also: Paramagnetism
Your search for all content returned 125 results
Save search
You must be logged in as an individual save a search. Log-in/register
Article
Alternating current
Electric current that follows a sine waveform that reverses direction periodically, usually many times per second. Electrical energy is ordinarily generated by an alternating-current generator owned by a public or investor-owned (private) utility organization and provided to a customer, whether industrial, commercial, or residential, as alternating current. See also: Electric power generation
Article
Alternating-current circuit theory
The mathematical theory for the analysis of electric circuits when the currents and voltages are alternating functions of time. In an electric circuit carrying direct current only (Fig. 1), the current I (amperes) flowing through a circuit of total resistance R (ohms) when a steady voltage V (volts) is applied is given by Ohm's law, Eq. (1). See also: Circuit (electricity); Direct-current circuit theory; Ohm's law
(1)
Article
Ammeter
An electrical instrument for measuring electric current. Currents are usually either unidirectional and steady (direct current or dc) or alternating in direction at a relatively low frequency (alternating current or ac). A current that is unidirectional but regularly fluctuating is a superposition of dc and ac. Higher-frequency ac is often referred to as radio-frequency or RF current. At frequencies above about 10 MHz, where the wavelength of the signal becomes comparable with the dimensions of the measuring instrument, current measurements become inaccurate and finally meaningless, since the value obtained depends on the position where the measurement is made. In these circumstances, power measurements are usually used. See also: Current measurement; Microwave measurements
Article
Ampere's law
A law of electromagnetism which expresses the contribution of a current element of length dl to the magnetic induction (flux density) B at a point near the current. Ampère's law, sometimes called Laplace's law, was derived by A. M. Ampère after a series of experiments during 1820–1825. It plays a fundamental role in the International System (SI) definition of the ampere. See also: Physical measurement
Article
Atmospheric electricity
Electrical processes that take place in the lower atmosphere. This activity is ubiquitous and is of two kinds: the intense local electrification accompanying storms, and the much weaker fair-weather electrical activity occurring over the entire globe that is produced by the thousand or so electrified storms continuously in progress. The relative importance of the various mechanisms that cause storms to accumulate electrically charged particles is unknown, and the extent to which atmospheric electricity affects meteorological processes has yet to be determined. It is possible that atmospheric electricity may be important in forming precipitation and in providing energy to severe tornadoes. See also: Precipitation (meteorology); Tornado
Article
Attenuation (electricity)
The exponential decrease with distance in the amplitude of an electrical signal traveling along a very long uniform transmission line, due to conductor and dielectric losses. If the peak voltage at the sending end of the transmission line is denoted by V0, the peak voltage at a distance x from the sending end is given by Eq. (1),
(1)
Article
Ballast resistor
A resistor that has the property of increasing in resistance as current flowing through it increases, and decreasing in resistance as current decreases. Therefore the ballast resistor tends to maintain a constant current flowing through it, despite variations in applied voltage or changes in the rest of the circuit. The ballast resistor acts as a variable load on the system; therefore it differs from load resistors, which have a constant resistance. Ballast resistors are now mainly of historical interest; electronic devices fulfilling the same function have replaced them. See also: Resistor
Article
Biot-Savart law
A law of physics which states that the magnetic flux density (magnetic induction) near a long, straight conductor is directly proportional to the current in the conductor and inversely proportional to the distance from the conductor. There is also a law in fluid dynamics bearing this name; it is concerned with vortex motion, and bears a close analogy to the law discussed in the present article.