I have said, in effect, a little above, that, while the current is
running, _the entire circuit is one complete magnet_, which extends
from the inner or positive sides of the zinc plates, where the current
commences, all the way around to the outer or negative aides of the zinc
plates, to which it returns. Viewed in this light its negative pole or
end is the battery fluid, next to the positive surfaces of the zinc
plates, and its positive pole or end is the brass clamp which, holding
the metals together, is in contact with the outer and negative surfaces
of the zincs.
But, for practical purposes, it is sufficiently exact to consider the
_magnetic circuit_ as extending only from the positive _post_ around
through the conducting cords, the electrodes and the person of the
patient to the _negative_ post. The negative end or pole of this magnet
is the wire end of the cord placed in the positive post, and the
positive end or pole is the wire end of the cord placed in the negative
post.
But any magnet may be viewed either as one whole, or be conceived as
composed of a succession of shorter magnets placed end to end. If we
view it as one entire magnet, we call the end in which the magnetic
essence is in greatest quantity the _positive_ end, and the end where it
is in least quantity the _negative_ end. But if we imagine the one
whole magnet as being divided up into several sections, then we conceive
of each section as a distinct magnet, having its own positive and
negative poles. And, all the way through, these sectional magnets will
be arranged with the positive pole of the one joined to the negative
pole of the next in advance of it.
It is just so in respect to the magnetic circuit of a moving current.
The whole circuit, as before remarked, is in reality one long magnet.
But in applying the terms _positive_ and _negative_ in our practice we
often view the whole circuit--the one long magnet--as composed of a
series of shorter ones, arranged with positive and negative ends in
contact; and all the way the current in each section is supposed to be
running from the positive pole of the magnet behind to the negative pole
of the magnet before.
We consider the circuit, from the positive post around to the negative
post, as composed of three magnets, as follows: Magnet No. 1, which
extends from the positive post, along the cord and electrode, to the
body of the patient, where the positive electrode is placed. The
_negative pole_ of this magnet is the _wire end of the cord_ placed in
the positive post, and its _positive_ pole in the _positive electrode_
placed upon the person of the patient. No. 2, which is composed of the
parts of the patient traversed by the current between the two
electrodes. Its negative end or pole is the part in contact with the
positive pole of magnet No. 1, and its positive pole is the part in
contact with the negative pole of magnet No. 3. No. 3 extends from the
positive pole of No. 2, through the electrode and along the cord, to the
negative post. Its negative pole is the _negative electrode_ in contact
with the positive end or pole of magnet No. 2, and its positive pole is
the _wire end of the cord_ in the negative post.
Since in every magnet the magnetic fluid is supposed to be regularly
graduated from minimum quantity in the negative end to maximum quantity
in the positive end, this is true in respect to the one magnet,
consisting of the whole magnetic circuit, as well as in respect to each
one of the sectional series. Consequently there must be the same
quantity of magnetism in each negative pole of the sections as there is
in the positive pole of the section immediately behind it. And the
magnetism of the whole circuit between the positive and the negative
posts is in its _least_ volume next to the _positive post_, and in
_fullest_ volume next to the _negative post_. If we consider the circuit
as divided into two equal halves, the _negative half_ is plainly that
which joins the _positive post_, and the _positive half_ that which
joins the _negative post_.
From this it will be seen that what in practice are designated as the
positive and negative _posts_, and also positive and negative _poles_ or
_electrodes_ are _not_ such _in relation to each other_, but the
_reverse_ of it; that is to say, the positive _post_ is not _positive_
in relation to the _negative post_, but is _negative_ to it; and the
positive _electrode or pole_ is not positive in relation to the
_negative_ electrode, but _negative_ to it. The positive _post_, like
the positive _electrode_, is called _positive_, because it is the
positive end of the sectional magnet next _behind_ it. And the
_negative_ post, as also the negative electrode, is _called negative_
because it is the negative end of the sectional magnet next _in advance_
of it.