Mathematically, the electronic spins are equal to the angular momentum the rotational velocity times the moment of inertia of the rotating electrons. The spins in a ferromagnetic or a ferrimagnetic single crystal undergo spontaneous alignment to form a macroscopic large scale magnetized object. Most magnetic solids, however, are not single crystals, but consist of single crystal domains separated by domain walls. The spins align within a domain below the Curie temperature, independently of any external magnetic field, but the domains have to be aligned in a magnetic field in order to produce a macroscopic magnetized object.
This process is effected by the rotation of the direction of the spins in the domain wall under the influence of the magnetic field, resulting in a displacement of the wall and the eventual creation of a single large domain with the same spin orientation.
Paramagnetism is a weak form of magnetism observed in substances which display a positive response to an applied magnetic field. This response is described by its magnetic susceptibility per unit volume, which is a dimensionless quantity defined by the ratio of the magnetic moment to the magnetic field intensity. Paramagnetism is observed, for example, in atoms and molecules with an odd number of electrons, since here the net magnetic moment cannot be zero.
Diamagnetism is associated with materials that have a negative magnetic susceptibility. It occurs in nonmagnetic substances like graphite, copper, silver and gold, and in the superconducting state of certain elemental and compound metals. The negative magnetic susceptibility in these materials is the result of a current induced in the electron orbits of the atoms by the applied magnetic field. The electron current then induces a magnetic moment of opposite sign to that of the applied field.
The net result of these interactions is that the material is shielded from penetration by the applied magnetic field. The magnetic field or flux density is measured in metric units of a gauss G and the corresponding international system unit of a tesla T.
Instruments called gaussmeters and magnetometers are used to measure the magnitude of magnetic fields. One form of the gaussmeter that is used commonly in the laboratory consists of a current carrying semiconducting element called the Hall probe, which is placed perpendicular to the magnetic field being measured.
As a consequence of the so-called Hall effect, a voltage perpendicular to the field and to the current is generated in the probe. This induced voltage is proportional to the magnetic field being measured and can be simply measured using a voltmeter. Magnetometers are extremely sensitive magnetic field detectors.
In one commonly used form the magnetic force is detected by means of a sensitive electronic balance. In this instrument the magnetic substance is placed on one arm of a balance, which in turn is placed in a magnetic field. The magnetic force on the sample is then determined by the weight required to balance the force generated by the magnetic field. A SQUID consists of an extremely thin electrically resistive junction called a Josephson junction between two superconductors.
Superconductors are materials which undergo a transition at low temperatures to a state of zero electrical resistance and nearly complete exclusion of magnetic fields. In its direct current mode of operation, a SQUID is first cooled down to its superconducting state, and then a current is passed through it while the voltage across the junction is monitored. When the junction senses a magnetic field, the flow of current is altered due to an interference phenomenon at the quantum level between two electron wave fronts through the junction, resulting in a change in voltage.
Whether on land or sea, an explorer could plot a compass course to not only get somewhere but find the way back. In a bit of Irony, the early users of the compass had no clue as to why or how it did what it did. Even the Greeks assigned magnetism a certain human quality because it affected the things around it without touching them. It was the scientific revolution that gave us the process and tools to truly study magnetism.
Gilbert was the first to Identify the magnetic field, the first to understand earth as a giant magnet, and his use of the Latin word electricus became our electricity. What Gilbert did not do was find the relationship between electricity and magnetism. It was when the Danish Physicist Oersted put an electric current near a compass and found the compass pointed that way.
After this, the understanding of electromagnetism was a key enabler of the industrial revolution. The world today would be a different place without our understanding of magnetism.
By different minerals and combinations like Cobalt and Alnico were joined to greatly increase the strength of magnets. I remember being taught that the Greeks discovered naturally occurring magnets of magnetite in Turkey. Magnetite occurs all over the world, but there are especially large deposits in Scandinavia.
The Vikings invented the first practical magnetic compass and used it extensively in their travels to colonize or in war. This enabled them to cross oceans to reach the new world and to invade England at will, even in the dense fog. The Vikings kept the existence of the magnetic compass a secret. In Greece Roughly 4, years ago, a Greek shepherd named Magnes is said to have been tending his sheep in a region of northern Greece called Magnesia.
In Rome In the early A. In Scandinavia With a large lodestone deposit in Scandinavia and not enough light to navigate ships by in the winter, the Vikings had plenty of incentive to put the magnetic properties of lodestone to practical use. In France In the s, French scholar Petrus Peregrinus penned one of the first written accounts of the scientific properties of magnets. In Denmark Two hundred years later, in , Hans Christian Oersted began to explore the relationship between electricity and magnetism.
Tags: Chinese compass , compass , dry compass , first magnet , Hands Christian Oersted , history of magnets , lodestone , Magnes , magnet discovery , magnet history , magnet origins , magnetite , magnets , Petrus Peregrinus , Piny the Elder , Viking navigation , who discovered magnets , William Gilbert Share This:. Roger Owens says:. April 6, at pm.
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