A simple compass indicates the direction of a magnetic field, but the magnetometer, invented by the German mathematician and scientist C. F. Gauss (1777-1855), could also measure its absolute strength. Before Gauss's time, people had compared the fields at two different spots on Earth by observing how long it took a suspended magnetized needle to make a certain number of oscillations. Gauss, and his physics professor friend Wilhelm Weber (1804-1891), gauged the field strength of the magnetized needle by measuring by how much it twisted two fibers from which it was suspended, when held at a right angle to Earth's field.
Gauss and Weber founded the Magnetischer Verein (Magnetic Club), whose members measured not only magnetic fields all over Earth but also their variations with time, caused by slow changes in Earth's liquid iron core and by changes in the electron clouds that surround Earth. Electron clouds are influenced by particles emitted spasmodically by the solar wind.
Modern flux-gate magnetometers are much handier. They measure an ambient magnetic field by noting the way in which it influences the induced electrical currents in a wire coil wrapped round a magnetic core, the field in the core being produced by a second AC driven coil. Magnetometers have been fitted to most interplanetary spacecraft. They also play an important role in geology, especially in prospecting for iron ore deposits, and in archeology, where they can reveal metallic objects buried in the ground.
