[Illustration: Fig. 20d.]

[Illustration: Fig. 20e.]

[Illustration: Fig. 20f.]

[Illustration: Fig. 20g.]

[Illustration: Fig. 20h.]

Fig. 20h shows a view of 20f when the wire is horizontal and the
plane of the loop vertical. It is like 4i.

To verify these facts, suspend a loop from Ampere's stand, Fig. 9, and
bring a straight wire near.

A small rectangle or circle may be hung in a similar manner. When the
circuit is closed, it tends to place itself with its axis in a N and S
direction through the earth's influence. The supposition of an E and W
horizontal earth current will explain this action.

To exemplify rotation of a vertical wire by a horizontal loop, Fig. 21
may be shown.

A circular copper vessel with a glass bottom (Fig. 21) has wound
around its rim several turns of insulated wire. In the center of the
vessel is a metallic upright upon the top of which is balanced in a
mercury cup a light copper [inverted U] shaped strip. The ends of the
inverted U dip into the dilute sulphuric acid contained in the