The action of a magnetic pole diminishes so rapidly with the increase of distance that it may suffice to remove the armature to a distance relatively small compared with its own dimensions, or with those of the magnet, in order to reduce the action to a negligible value. But if the magnet, N S, and the armature, A, being at a certain distance, we bring between them a piece of iron or nickel, d, then the magnetic force upon A is immediately and very considerably increased. In modern language, the resistance of the magnetic circuit has been reduced by the introduction of a better magnetic conductor, and the number of lines of force passing through A is proportionately increased. The mass of the piece, d, may, moreover, be relatively small compared with that of N S and A. If d be again withdrawn, the magnetic resistance is increased, and the lines through A are again a minimum. Now, it is evident that we can also obtain the same effect by sufficiently heating and cooling the intermediate piece, d; and again, with a broad field we can alter the distribution of the lines at will by heating or cooling one side of this piece or the other. For this reason we will call the piece d the _thermo-magnetic distributor_, or, briefly, the distributor. We will now describe the manner in which this principle has been realized in the practical construction of both a thermo-magnetic generator and motor. [Illustration: Fig. 1.] Fig. 1 shows an elevation and part section of one of the arrangements employed. Fig. 2 is a plan of the same machine (in the latter the ring, _a a_, appearing on a higher plane than it actually occupies). [Illustration: Fig. 2.] N S is an electro-magnet, _a a_ the armature, wound as a Gramme ring, and fixed to a frame with four arms, which can turn freely upon a pivot midway between the poles. The cross arms of the frame are attached at 1, 2, 3, 4, Fig. 2. Between the magnets and the armature is placed the distributor, _d d_, where it occupies an annular space open above and below. Both the magnets and the armature are coated on the sides facing the distributor with mica or some other non-conductor of heat and electricity. The distributor is attached to and supported by the cross arms, so that it turns with the armature. The distributor is composed of a ribbon of iron or nickel, bent into a continuous zigzag. This form has the advantage of presenting, in the cool part of the distributor, an almost direct road for the lines of force between the poles and the armature, thus diminishing the magnetic resistance as far as possible. At the same time the Foucault currents are minimized. To the same end it is useful to slit the ribbon, as in Fig. 3. This also facilitates the folding into zigzags. [Illustration: Fig. 3.]