Magnetism

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). The magnetic field strength is measured in metric units of oersteds (Oe) and international units of amperes per meter (A/m). 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 A computer graphic of a horseshoe magnet with iron filings aligned around it. Photograph by Alfred Pasieka. Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. 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. The most sensitive magnetometer in a modern physics laboratory utilizes a magnetic sensing element called the SQUID (which stands for Superconducting QUantum Interference Device). 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. Interference is a phenomenon that occurs generally due to the mixing of two wave fronts; the waves add up in some regions and cancel out in others depending on the location of the crest and trough of each wave in space. For example, the interference between the sound waves from two simultaneously played musical instruments tuned at somewhat different frequencies results in the occurrence of beats or modulations in the sound intensity.

A variation of the SQUID magnetometer is the SQUID gradiometer which measures differences in magnetic fields at different positions. Using this type of instrument magnetic field variations in the femtotesla (10^-15 tesla) range can be detected. Devices of this type have been used to map the tiny magnetic signals from the human brain.