Place now a vertical loop opposite to the pole of a short bar magnet
cemented to the glass plate with the N pole facing it. If the current
passes in one direction the field will be as represented by Fig.
14b; if it is reversed by the commutator, Fig. 14c is an image of
the spectrum. Applying Faraday's second principle, it appears that
attraction results in the first case, and repulsion in the second. The
usual method of stating the fact is, that if you face the loop and the
current circulates from left over to right, the N end of the needle
will be drawn into the loop.
[Illustration: Fig. 14b.]
[Illustration: Fig. 14c.]
It thus becomes evident that the loop is equivalent to a flat steel
plate, one surface of which is N and the other S. Facing the loop if
the current is right handed, the S side is toward you.
TO SHOW THE ACTUAL ATTRACTION AND REPULSION OF A MAGNET BY A "MAGNETIC
SHELL."
Produce the field as before (Fig. 14), carry a suspended magnetic
needle over the field. It will tend to place itself parallel to the
lines of force, with the N pole in such a position that, if the
current passes clockwise as you look upon the plane of the loop, it
will be drawn into the loop. Reversing the position of the needle or
of current will show repulsion.
Clerk Maxwell's method of stating the fact is that "every portion of
the circuit is acted on by a force urging it across the lines of
magnetic induction, so as to include a greater number of these lines
within the embrace of the circuit."[2]
[Footnote 2: Electricity and Magnetism, Maxwell, p. 137, §§ 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.
[Illustration: Fig. 14f.]
[Illustration: Fig. 14g.]
If two loops are placed in the same vertical plane, as in Figs. 14h
and 14i, there will be attraction or repulsion, according to the
direction of the adjacent currents. The fields become the same as
Figs. 8 and 8a, as may be seen by comparing them with those figures.
[Illustration: Fig. 14h.]
[Illustration: Fig. 14i.]
Having thus demonstrated the practical identity of a loop and a
magnet, we proceed to examine the effects produced by loops on
straight wires.
If the loop is placed with a straight wire in its plane along one
edge, there will be attraction or repulsion, according to the
direction of the two currents, Figs. 20 and 20a, which are obviously
the same as Figs. 8 and 8a.
[Illustration: Fig. 20.]
[Illustration: Fig. 20a.]
[Illustration: Fig. 20b.]
[Illustration: Fig. 20c.]
If the wire is placed parallel to the plane of the loop and to one
side, Figs. 20b and 20c, there will be rotation (same as Figs.
4b and 4c).
If the loop is horizontal and the wire vertical and on one side, the
Figs. 20d, 20e are the same as 4d and 4e.
If the loop is horizontal and the wire vertical and axial, 20f and
20g, there will be rotation, and the figures are mere duplicates of
4g and 4h.
[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
circular vessel.
The current passes from, the battery, up the pillar, down the legs of
the U to the liquid, thence through the insulated wire back to the
battery.
[Illustration: Fig. 21.]
This is the usual form of apparatus, modified in size for the vertical
or horizontal lantern.
(_To be continued._)
* * * * *