[Footnote 2:  Electricity and Magnetism, Maxwell, p. 137, Sec.Sec. 489,
   490.]

If the horizontal loop is used (Fig. 14a), the needle tries to assume a vertical position, with the N or S end down, according to the direction of the current.

If it is desired to show that if the magnet is fixed and the loop free, the loop will be attracted or repelled, a special support is needed.

[Illustration:  Fig. 15]

A strip (Fig. 15) of brass, J, having two iron mercury cups, K_{1} K_{2}, screwed near the ends, one insulated from the strip, is fastened upon the horizontal arm of the ring support, Fig. 9, already described.  The cups may be given a slight vertical motion for accurate adjustment.  Small conductors (Figs. 16, 17, 18), which are circles, rectangles, solenoids, etc., may be suspended from the top of the plate by unspun silk, with the ends dipping into the mercury.  The apparatus is therefore an Ampere’s stand, with the weight of the movable circuit supported by silk and with means of adjusting the contacts.  The rectangles or circles are about two inches in their extreme dimension.  Horizontal and vertical astatic system are also used—­Figs. 18, 18a.  The apparatus may be used with either the horizontal or vertical lantern.

[Illustration:  Fig. 16.  Fig. 17.]

[Illustration:  Fig. 18.  Fig. 18a.]

If the rectangle or circle is suspended and a magnet brought near it when the current passes, the loop will be attracted or repelled, as the law requires.  The experiments usually performed with De la Rive’s floating battery may be exhibited.

The great similarity between the loop and the magnet may be shown by comparing the fields above (Figs. 14b, 14c) with the actual fields of two bar magnets, Figs. 19, 19a.

It will be noticed that the lines in Fig. 19, where unlike poles are opposite, are gathered together as in Fig. 14b,—­where the N end of the magnet faces the S side of the magnetic shell; and that in 19a, where two norths face, the line of repulsion has the same general character as in 14c, in which the N end of the magnet faces the N side of the shell.

[Illustration:  Fig. 19.]

[Illustration:  Fig. 19a.]

Instead of placing the magnet perpendicular to the plane of the loop, it may be placed parallel to its plane.  Fig. 14d shows the magnet and loop both vertical.

The field shows that the magnet will be rotated, and will finally take for stable equilibrium an axial position, with the N end pointing as determined by the rule already given.

[Illustration:  Fig. 14d.]

If two loops are placed with their axes in the same straight line as follows, Figs. 14f, 14g, a reproduction of Figs. 14b and 14c will become evident.

It is obvious from these spectra that the two loops attract or repel each other according to the direction of the current, which fact may be shown by bringing a loop near to another loop suspended from the ring stand, Fig. 9, or by using the ordinary apparatus for that purpose—­De la Rive’s battery and Ampere’s stand.