The first instrument to measure the magnetic field while being flown in an aircraft was the Earth inductor magnetometer. This was done in the USSR in 1936. But the system's sensitivity of 100 nT, called gammas in those days, was hardly suitable for routine surveys of large areas. Hans Lundberg built, after World War II, a helicopter-borne system for mineral prospecting in the Canadian Precambrian Shield. The rotor was spun by compressed air at 1800 Hz between a set of gimbal-mounted Helmholtz coils whose axis was aligned vertically. Thus, the system measured variations of the Earth's vertical field with a sensitivity of about 25 nT.