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.
Centimetre–gram–second system of units
The practical cgs system is a hybrid system that uses the volt and the ampere as the unit of voltage and current respectively. Doing this avoids the inconveniently large and small quantities that arise for electromagnetic units in the esu and emu systems. This system was at one time widely used by electrical engineers because the volt and amp had been adopted as international standard units by the International Electrical Congress of 1881. As well as the volt and amp, the farad (capacitance), ohm (resistance), coulomb (electric charge), and henry are consequently also used in the practical system and are the same as the SI units. However, intensive properties (that is, anything that is per unit length, area, or volume) will not be the same as SI since the cgs unit of distance is the centimetre. For instance electric field strength is in units of volts per centimetre, magnetic field strength is in oersteds and resistivity is in ohm-cm.