sample has a magnetic induction much weaker than the applied field, and a respective negative magnetic susceptibility. The following mathematical equation relates the apparent change in mass to the volume susceptibility of the sample: mb – ma = apparent difference in mass g = gravitational acceleration K1 = volume susceptibility of medium, usually air and of negligible value K2 = volume susceptibility of sample H = applied magnetic field A = area of the sample tube Instrument In a practical device, the whole assembly of balance and magnet is enclosed in a glass box to ensure that the weight measurement is not affected by air currents. The sample can also be enclosed in a thermostat in order to make measurements at different temperatures. Since it requires a large and powerful electromagnet, the Gouy balance is a stationary instrument permanently set up on a bench. The apparatus is often placed on a marble balance table in a non-ventilated room to minimize the vibrations and disruption from the environment. The stationary magnetic of a Gouy balance is often an electromagnet connected to a power source, since balance recordings with and without the applied magnetic field are required of the procedure. See also Evans balance Faraday balance Kibble balance References Magnetic devices Measuring instruments