Gauss and his demagnetization circuit

Carl Friedrich Gauss was a brilliant mathematician whose insights and writings laid the groundwork for much of what is known today concerning number theory, algebra, differential geometry, orbital mechanics, matrix theory and numerous other topics.

Electronics students are familiar with the gauss as the cgs unit of magnetic flux density. It is defined as one Maxwell per square centimeter. Additionally, Gauss’ name is associated with the process of degaussing, which consists of removing unwanted magnetic fields that attach to objects as diverse as CRT-based TV picture tubes and computer monitors, oscilloscope current probes and ships at sea.

An important year for Gauss was 1796, right about the time his ideological opposite, Napoleon Bonaparte, was rising rapidly in continental European affairs. Gauss figured out how to construct a heptadecagon using compass and straight edge, made great advances in modular arithmetic, and proved the quadratic reciprocity law. Also in this remarkable year, Gauss envisioned the prime number theorem, which rationalizes the folk notion that prime numbers become less common as the integers grow larger.

Based on research Gauss conducted on newly discovered Ceres, the largest object in the asteroid belt, he studied approximation methods relevant to orbital mechanics. This type of phenomenon can be expressed as a wave function. Pursuing these insights, Gauss refined Jean-Baptiste Fourier’s Transform, anticipating the Fast Fourier Transform and creating a method for trigonometric interpolation.

Degaussing is an electromagnetic technique essential for maintaining the integrity of sensitive instrumentation. Degaussing takes a number of forms, but the basic idea is to demagnetize a magnetically permeable object. Such a body is prone to becoming a permanent magnet if it is in the presence of a moving magnetic field or if it moves relative to a stationary magnetic field. Degaussing or demagnetization counteracts the effects of the earth’s magnetic field or of electromagnetic fields produced by nearby electrical devices, such as motors or appliances. These fields may magnetize metallic portions of the cathode ray tube, such as the shadow mask, degrading the color purity of the tube.

Degaussing consists of using an ac current to create a moving magnetic field that overwrites the unwanted magnetization. The ac field is slowly removed (spatially or electrically) until the residual magnetism is gone. Old-time TV servicemen had a tool that performed this therapeutic task on CRT picture tubes, and an advanced oscilloscope current probe does the same thing at the push of a button.

A common type of degaussing circuit that includes a degaussing coil ordinarily uses a positive temperature coefficient resistor, thermistor, or other temperature sensitive component, to slowly reduce the magnitude of the applied ac degaussing wave. The thermistor resistance rises as it heats due to degaussing current flow. This causes the alternating degaussing current to decay in way that demagnetizes the cathode ray tube metallic components.

Another type of degaussing circuit uses a resonant or ring-down degaussing circuit. The resonant degaussing circuit operates by causing a capacitor connected in parallel with the degaussing coil to resonate with the coil to produce oscillations. The finite Q of the resonant circuit causes the degaussing current to decay. The resonant frequency of the degaussing circuit may be of the order of 2 kHz, so that degaussing is completed in less than 5 msec.