The Earth's magnetic field has the properties of direction and intensity. In navigation, the direction of the horizontal component of this field is required to be known. The customary method is to use a pivoted-needle magnetic compass, but the modern need for stability and remote indication makes other methods desirable. Inductive devices, such as rotating coils, may be adapted to measure the intensity of the earth's field and hence its direction, and the development of simple instruments for this purpose is described, together with some systems of special interest.The need for a compact instrument has led to the development of detector units, or magnetometers, which have no moving parts, and static inductive devices—in which alternating exciting fields, combining with the earth's field, act upon iron-cored elements—now replace rotating coils in modern instruments.It is essential that any such device be stabilized against acceleration effects, and the merits and use of both vertical and azimuthal stabilization are discussed.Data transmission of heading is, of course, an essential feature of such compasses, and present-day methods of this are briefly described.