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.
Kibble balance
A conducting wire of length "L" that carries an electric current "I" perpendicular to a magnetic field of strength "B" will experience a Lorentz force equal to "BLI". In the Kibble balance, the current is varied so that this force exactly counteracts the weight "w" of a standard mass "m". This is also the principle behind the ampere balance. "w" is given by the mass "m" multiplied by the local gravitational acceleration "g". Thus
The Kibble balance avoids the problems of measuring "B" and "L" with a second calibration step. The same wire (in practice, a coil of wire) is moved through the same magnetic field at a known speed "v". By Faraday's law of induction, a potential difference "U" is generated across the ends of the wire, which equals "BLv". Thus 
The unknown product "BL" can be eliminated from the equations to give
With "U", "I", "g", and "v" accurately measured, this gives an accurate value for "m". 
Both sides of the equation have the dimensions of power, measured in watts in the International System of Units; hence the original name "watt balance".