Absolute measurement ofD by means of a proton magnetometer

In a large coil with vertical axis the current is adjusted so thatZ=0. A smaller coil of a new type with four sets of cylindrical turns is placed with its axis horizontal. ThisD-coil is provided with a telescope pointing at a mark nearly in the magnetic meridian. A proton magnetometer sensor is placed in the common centre of the coils. Two series of readings are taken with theD-coil in the erect and inverted position respectively. Variations ofD, H, F, andD-coil current are recorded. A simple formula gives the mean value ofD.