Another scheme, proposed by Gravier, employed the unsymmetrical form shown in fig. 226. In this pole piece the forward horn is elongated. The action due to this arrangement is such that when the machine is working at small loads, the field in the gap is nearly uniform, but at heavy loads with distorting reactions which have a tendency to drive the flux into the forward horn, the small section of the latter causes it to become saturated, thus reducing the distortion to a minimum. =Eddy Currents; Laminated Fields.=--The field magnet cores and pole pieces, as well as the armature of a dynamo are subject to _eddy currents_, that is, induced electric currents occurring when a solid metallic mass is rotated in a magnetic field. These currents consume a large amount of energy and often occasion harmful rise in temperature. This loss may be almost entirely avoided by laminating the pole piece, or both pole piece and core; in the latter case, both form one part without any joint. [Illustration: FIG. 232.--Illustrating the alteration of magnetic field due to movement of mass of iron in the armature. If the masses of iron in the armature are so disposed that as it rotates, the distribution of the lines of force in the narrow field between the armature and the pole piece is being continually altered, then, even though the total amount of magnetism of the field magnet remain unchanged, eddy currents will be set up in the pole piece and will heat it. This is shown in the above figures, which represent the effect of a projecting tooth, such as that of a Pacinotti ring, in changing the distribution of magnetism in the pole piece.] [Illustration: FIG. 233.--Eddy currents induced in pole pieces by movement of masses of iron. These diagrams, which correspond to those of fig. 232, show the eddy currents grouped in pairs of vortices. The strongest current flows between the vortices and is situated just below the projecting tooth, where the magnetism is most intense; it moves onward following the tooth. At C is shown what occurs during the final retreat of the tooth from the pole piece. These eddy currents penetrate into the interior of the iron, although to no great depth. Clearly the greatest amount of such eddy currents will be generated at that part of the pole piece where the magnetic perturbations are greatest and most sudden. A glance at the figures shows that this should be at the forward horn of the pole piece. However, when a dynamo, with horned pole pieces, has been running for some time as a motor the forward horns are cool and the hindward horns hot.]