This is the e-mail reply that I sent to someone when asked what units are used to measure magnetic field:

back to Leif's Home PageFrom kirschen Thu Sep 5 21:19:15 1996 To: xxx@xxx.xxx.xxx Subject: Re: Hello There are four units commonly used to measure the strength of magnets or magnetic fields. Usage varies depending on whom you're talking to and what situations you're referring to. There are two unit systems in use in physics to measure things: the CGS (centimeter-gram-second) system and the MKS (meter-kilogram-second) system, also called SI (for Systeme Internationale, a french term). In CGS one may measure magnetic fields in Gauss or Oersted (after the physicists Gauss and Oersted). The Earth's magnetic field is a few Gauss. In MKS (which is becoming more preferred) one measures magnetic fields in amps per meter (A/m) or Tesla. 10,000 Guass = 1 Tesla 10,000 Oersted = 1 Tesla 1 A/m = 4*pi Gauss. 1 Gauss = 1 Oersted in a vacuum or in non-magnetic substances (like air), so people mix up Gauss and Oersteds. How do you relate magnetic fields to other things? It's tough -- and it's why I really like A/m (or kA/m = 1000 A/m) or Tesla. There is a law of physics which describes something called the Lorentz force (after another physicist), which goes like this (in MKS): F=q(vB + E) the force in Newtons equals the charge of something (like an electron) times the velocity times the magnetic field plus the charge times the electric field. If E=0: F=qvB for various reasons, the force is perpendicular to the direction the particle is moving. If q = 1 Coulomb (1.9 x 10^16, or 2 followed by 16 zeros number of electrons), and v= 1 meter per second, and B = 1 Tesla, then F = 1 Newton. (gravity exerts a force of 9.8 Newtons on 1 kilogram of mass -- you weigh about 600 Newtons on Earth). That's in units I can understand -- I have no understanding of Gauss, except that they're 1/10,000 of a Tesla, and the Earth's field (moving compasses) is a few Gauss. Why are A/m easier than Gauss/Oersted? If you have a wire, and you put 1 amp of current down it, at a distance of 1 meter, the magnetic field is 1/ (2*pi) A/m {The 2*pi comes in because if you add up the entire magnetic field in a circle of radius 1 meter around the wire, you have to multiply by 2*pi, so the sum of the field around a 1 meter circle around this 1 amp carrying wire is 1 A/m). Summary: Tesla and A/m are used by scientists as a more rigorous magnetic unit. Gauss (or Oersted) are used to estimate things because Gauss were used more often historically, and are a more convenient size. The Earth's magnetic field is a few Gauss. The little bits on a computer disk or on a cassette tape are 20-50 Gauss in magnitude _at_the_read_head_. The fields used in hospital MRI machines are 2-7 Tesla (20,000-70,000 Gauss).

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