The earth inductor, in its most complete form, as used for the measurement of the magnetic inclination, consists essentially of a coil (Crpar; of insulated wire which can be rapidly and continuously rotated about an axis (ABrpar;, Fig. 1, which coincides with a diameter of the coil. The axis AB is capable of measured rotation about the two mutually perpendicular axes, one horizontal, DE, and the other vertical, V; by construction AB is perpendicular to DE. Thus the direction of AB can be made to coincide with that of any radius of a sphere having its center at the intersection of the three axes. In the future we shall speak of AB as the axis of rotation, of DE as the inclination axis, and of V as the azimuth axis, and shall call the plane which includes both AB and DE the “inclined plane” of the instrument.