Magnetometer
Magnetic fields are vector quantities characterized by both strength and direction. The strength of a magnetic field is measured in units of tesla in the SI units, and in gauss in the cgs system of units. 10,000 gauss are equal to one tesla. Measurements of the Earth's magnetic field are often quoted in units of nanotesla (nT), also called a gamma. The Earth's magnetic field can vary from 20,000 to 80,000 nT depending on location, fluctuations in the Earth's magnetic field are on the order of 100 nT, and magnetic field variations due to magnetic anomalies can be in the picotesla (pT) range. "Gaussmeters" and "teslameters" are magnetometers that measure in units of gauss or tesla, respectively. In some contexts, magnetometer is the term used for an instrument that measures fields of less than 1 millitesla (mT) and gaussmeter is used for those measuring greater than 1 mT.
Spin magnetic moment
Just like the "total spin angular momentum" cannot be measured, neither can the "total spin magnetic moment" be measured. Equations (), (), () give the physical observable, that component of the magnetic moment measured along an axis, relative to or along the applied field direction. Assuming a Cartesian coordinate system, conventionally, the "z"-axis is chosen but the observable values of the component of spin angular momentum along all three axes are each ±"ħ"/2. However, in order to obtain the magnitude of the total spin angular momentum, be replaced by its eigenvalue, , where "s" is the spin quantum number. In turn, calculation of the magnitude of the total spin magnetic moment requires that () be replaced by: