Bismuth Coil

a device for measuring magnetic induction whose action is based on the increased electrical resistance of bismuth in a magnetic field. Bismuth coils are made in the form of a flat bifilar (to reduce the induced currents) spiral of chemically pure bismuth wire about 1 mm in diameter. The thinness of the bismuth coil makes it convenient to measure magnetic fields in narrow gaps. The resistance of the bismuth coil is measured twice: outside the field and then in the field being investigated. Magnetic induction is determined according to the change in resistance? R, which is about 5 percent for 10^-1 ml (1,000 gauss), and according to the graduated curve of the bismuth coil. The bismuth coil has low accuracy (about 2 percent) and limited sensitivity (—50 gauss = 5 x 10^-3 ml), since the resistance of bismuth depends to a great degree on temperature; in addition there is resistance hysteresis in a magnetic field.