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centres flowing in the same direction. It is evident that upon
the least disturbance from this position, they will be brought
back to it by the mutual attraction of the parts of the circles
on the sides which are near each other. This is therefore their
position of stable equilibrium, and it is evident that the fronts
of the currents in this position are on opposite sides of their
common plane.



2025. Circulating currents have the magnetic properties.
From what has been proved, it is apparent that an heliacal cur-
rent has all the properties of a magnet. Such currents exert
the same mutual attraction and repulsion, have the same po-
larity, submitted to the influence of terrestrial magnetism have
the same directive properties, and exhibit all the phenomena of
variation and dip as are manifested by artificial and natural
magnets. And it is evident that these properties depend on
the circulating and not on the heliacal character of the current,
inasmuch as the effect of the progression of the helix being
neutralized by carrying the current back in a straight direction
along its axis, the phenomena instead of being disturbed are
still more regular and certain.


These properties of circulating currents have been assumed by
Ampere as the basis of his celebrated theory of magnetism, in
which all magnetic phenomena are ascribed to the presence of
currents circulating round the constituent molecules of natural
and artificial magnets, and around the earth itself.

Let a bar magnet be supposed to be cut by a plane at
right angles to its length. Every molecule in its section is
supposed to be invested by a circulating current, all these cur-
rents revolving in the same direction, and consequently their
fronts being presented to the same extremity of the bar. The
forces exerted by all the currents thus prevailing around the
molecules of the same section may be considered as represented
by a single current circulating round the bar, and the same
being true of all the transverse sections of the bar, it may be
regarded as being surrounded by a series of circulating currents
all looking in the same direction, and circulating round the bar.
That end of the bar towards which the fronts of the currents
are presented will have the properties of a south or boreal pole,
and the other end those of a north or austral pole.

2026. Magnetism of the earth may proceed from currents.
In this theory the globe of the earth is considered to be traversed
by electric currents parallel to the magnetic equator. The
forces exerted by the currents circulating in each section of the
earth, like those in the section of an artificial magnet, are con-
sidered as represented by a single current equivalent in its
effect, and which is called the mean current of the earth, at
each place upon its surface. The magnetic phenomena indicate
that the direction of this mean current at each place is in a
plane at right angles to the dipping-needle, and that it is
directed in this plane from east to west, and at right angles to
the magnetic meridian.

2027. Artificial magnets explained on this hypothesis. In
bodies such as iron or steel, which are susceptible of mag-
netism, but which are not magnetized, the currents which
circulate round the constituent molecules are considered to cir-
culate in all possible planes and all possible directions, and
their forces thus neutralize each other. Such bodies, therefore,
exert no forces of attraction or repulsion on each other. But,
when such bodies are magnetized, the fronts of some or all of
these currents are turned in the same direction, and their
forces, instead of being opposed, are combined. The more


perfect the magnetization is, the greater proportion of the cur-
rents will thus be presented in the same direction, and the mag-
netization will be perfect when all the molecular currents are
turned towards the same direction.

2028. Effect of the presence or absence of coercive force.
If the body thus magnetized be destitute of all coercive force,
like soft iron, the currents which are thus temporarily turned
by the magnetizing agent in the same direction will fall into
their original confusion and disorder when the influence of that
agent is suspended or removed, and the body will consequently
lose the magnetic properties which had been temporarily im-
parted to it. If, on the contrary, the body magnetized have
more or less coercive force, the accordance conferred upon the
direction of the molecular currents is maintained with more or
less persistence after the magnetizing agency has ceased ; and
the magnetic properties accordingly remain unimpaired until
the accordance of the currents is deranged by some other cause.

2029. This hypothesis cannot be admitted as established
until the existence of the molecular currents shall be proved.
To establish this theory according to the rigorous principles
of inductive science, it would be necessary that the actual ex-
istence of the molecular voltaic currents, which form the basis
of the theory, should be proved by some other evidence than
the class of effects which they are assumed to explain. Until
such proof shall be obtained, they cannot be admitted to have
the character of a vera causa, and the theory must be regarded
as a mere hypothesis, more or less probable, and more or less
ingenious, which may be accepted provisionally as affording an
explanation of the phenomena, and thus reducing magnetism to
the dominion of electricity.



2030. Instruments to ascertain the presence and to measure
the intensity of currents. It has been shown that when a vol-
taic current passes over a magnetic needle freely suspended, it


will deflect the needle from its position of rest, the quantity of
this deflection depending on the force, and its direction on the
direction of the current.

If the needle be astatic, and consequently have no directive
force, it will rest indifferently in any direction in which it may
be placed. In this case the deflecting force of the current will
have no other resistance to overcome than that of the friction
of the needle on its pivot ; and if the deflecting force of the
current be greater than this resistance, the needle will be de-
flected, and will take a position at right angles to the current,
its north pole being to the left of the current (1918).

If the needle be not astatic it will have a certain directive
force, and, when not deflected by the current, will place itself
in the magnetic meridian. If, in this case, the wire con-
ducting the current be placed over and parallel to the needle,
the poles will be subject at once to two forces ; the directive
force tending to keep them in the magnetic meridian, and the
deflecting force of the current tending to place them at right
angles to that meridian. They will, consequently, take an in-
termediate direction, which will depend on the relation between
the directive and deflecting forces. If the latter exceed the
former, the needle will incline more to the magnetic east and
west ; if the former exceed the latter, it will incline more to the
magnetic north and south. If these forces be equal, it will take
a direction at an angle of 45 with the magnetic meridian.
The north pole of the needle will, in all cases, be deflected to
the left of the current (1918).

If while the directive force of the needle remains unchanged
the intensity of the current vary, the needle will be deflected at
a greater or less angle from the magnetic meridian, according
as the intensity of the current is increased or diminished.

2031. Expedient for augmenting the effect of a feeble cur-
rent, It may happen that the intensity of the current is so-
feeble as to be incapable of producing any sensible deflection
even on the most sensitive needle. The presence of such a
current may, nevertheless, be detected, and its intensity mea-
sured, by carrying the wire conducting it first over and then
under the needle, so that each part of the current shall exer
cise upon the needle a force tending to deflect it in the same
direction. By this expedient the deflecting force exercised by
the current on the needle is doubled.


Such an arrangement is represented vafig. 651. The wire
passes from n to z over, and from
y to x under the needle; and it is
evident from what has been ex-
plained (1918), that the part zn
and the part y x exercise deflecting
forces in the same direction on the
poles of the needle, both tending
Fig. 651. t o deflect the north or austral pole

a to the left of a person who stands at z and looks towards n.
It may be shown in like manner that the vertical parts of the
current g x and y z have the same tendency to deflect the north
pole a to the left of a person viewing it from z.

2032. Method of constructing a reoscope, galvanometer, or
multiplier. The same expedient may be carried further. The
wire upon which the current passes may be carried any number
of times round the needle, and each successive coil will equally
augment its deflecting force. The deflecting force of the
simple current will thus be multiplied by twice the number of
coils. If the needle be surrounded with an hundred coils of
conducting wire, the force which deflects it from its position of
rest will be two hundred times greater than the deflecting
force of the simple current.

The wire conducting the current must in such case be
wrapped with silk or other non-conducting coating, to prevent
the escape of the electricity from coil to coil.

Such an apparatus has been called a multiplier, in con-
sequence of thus multiplying the force of the current. It has
been also denominated a galvanometer, inasmuch as it supplies
the means of measuring the force of the galvanic current.

We give it by preference the name reoscope or reometer, as
indicating the presence and measuring the intensity of the

To construct a reometer, let two flat bars of wood or metal be
united at the ends, so as to leave an open space between them of
sufficient width to allow the suspension and play of a magnetic
needle. Let a fine metallic wire of silver or copper, wrapped
with silk, and having a length of eighty or a hundred feet, be
coiled longitudinally round these bars, leaving at its extremities
three or four feet uncoiled, so as to be conveniently placed in
connexion with the poles of the voltaic apparatus from which



the current proceeds. Over the bars on which the conducting
wire is coiled, is placed a dial upon which an index plays,
which is connected with the magnetic needle suspended between
the bars, and which has a common motion with it, the direction
of the index always coinciding with that of the needle. The
circle of the dial is divided into 360, the index being directed
to or 180, where the needle is parallel to the coils of the
conducting wire.

Such an instrument, mounted in the usual manner and co-
vered by a bell-glass to protect it
from the disturbances of the air, is
represented in^. 652.

The needle is usually suspended by
a single filament of raw silk. If the
length of wire necessary for a single
coil be six inches, fifty feet of wire
will suffice for a hundred coils. To
detect the presence of very feeble cur-
rents, however, a much greater number
p ~~ of coils are frequently necessary, and

Fig. 652. in some instruments of this kind there

are several thousand coils of wire.

2033. Nobili's reometer. Without multiplying inconveni-
ently the coils of the conducting wire, Nobili contrived a reo-
scope which possesses a sensibility sufficient for the most
delicate experimental researches. This arrangement consists
of two magnetic needles fixed upon a common centre parallel to
each other, but with their poles reversed as represented in
fig. 653. If the directive forces of
these needles were exactly equal,
such a combination would be as-
tatic ; and although it would indi-
cate the presence of an extremely
feeble current, it would supply no
means of measuring the relative
forces of two such currents. Such
reoscopic, but not reometric. To

Fig. 653.

an apparatus would be
impart to it the latter property, and at the same time to confer
on it a high degree of sensibility, the needles are rendered a
little, and but a little, unequal in their directive force. The
directive force of the combination being the difference of the


directive forces of the two needles, is therefore extremely
small, and the system is proportionately sensitive to the in-
fluence of the current.

2034. Differential reometer. In certain researches a dif-
ferential reometer is found useful. In this apparatus two wires
of exactly the same material and diameter are coiled round the
instrument, and two currents are made to pass in opposite di-
rections upon them so as to exercise opposite deflecting forces
on the needle. The deviation of the needle in this case mea-
sures the difference of the intensities of the two currents.

2035. Great sensitiveness of these instruments illustrated.
The extreme sensitiveness and extensive utility of these reo-
scopic apparatus will be rendered apparent hereafter. Mean-
while it may be observed that if the extremities p and n of the
conducting wires be dipped in acidulated water, a slight che-
mical action will take place, which will produce a current by
which the needle will be visibly affected.

In all cases it is easy to determine the direction of the
current by the direction in which the north pole of the needle
is deflected.



2036. Disturbance of the thermal equilibrium of conductors
produces a disturbance of the electric equilibrium. If a piece
of metal B, fig. 654., or other
conductor, be interposed be-
tween two pieces c, of a dif-
ferent metal, the points of
contact being reduced to dif-
Fig. 654. ferent temperatures, the na-

tural electricity at these points will be decomposed, the positive
fluid passing in one direction, and the negative fluid in the
other. If the extremities of the pieces c be connected by a
wire, a constant current will be established along such wire.
The intensity of this current will be invariable so long as the
temperatures of the points of contact of B with c remain the


same ; and it will in general be greater, the greater the dif-
ference of these temperatures. If the temperatures of the
points of contact be rendered equal, the current will cease.

These facts may be verified by connecting the extremities of
c with the wires of any reoscopic apparatus. The moment a
difference of temperature is produced at the points of contact,
the needle of the reoscope will be deflected ; the deflection will
increase or diminish with every increase or diminution of the
difference of the temperatures ; and if the temperatures be equal-
ized, the needle of the reoscope will return to its position of
rest, no deflection being produced.

2037. Thermo-electric current. A current thus produced
is called a thermo-electric current. Those which are pro-
duced by the ordinary voltaic arrangements are called for dis-
tinction hydro-electric currents, a liquid conductor always
entering the combination.

2038. Experimental illustration. A convenient and simple
apparatus for the experimental illustration of a thermo-electric
current is represented in fig. 655., consisting of a narrow strip

of copper bent so as to form
three sides of a rectangle, the
fourth part of which is a cy-
linder of bismuth, about half
an inch in diameter, which
is soldered at both ends to
the copper so as to ensure
perfect contact. A magnetic
needle is placed within the
Fig. 655. rectangle, which is directed

in the plane of the magnetic meridian, so that the needle, when
undisturbed by the current, shall rest in the direction of the
rectangle, its north pole pointing to the zinc cylinder.

If a lamp be placed under the end of the bismuth cylinder, so
as to raise its temperature above that of the upper end, the
needle will be immediately deflected, and the deflection will in-
crease as the difference of the temperatures of the lower and
upper end of the zinc cylinder is increased.

2039. Conditions which determine the direction of the cur-
rent. When the temperature of the lower end of the bismuth
cylinder is more elevated than that of the upper end, the north
pole of the needle is deflected towards the east, from which it


appears that the current in this case flows from the upper to the
lower end of the cylinder, and passes round the rectangle in
the direction represented by the arrows.

If the heat be applied to the upper end of the bismuth, or,
what is the same, if cold be applied to the lower end, the north
pole of the needle will be deflected to the west, showing that
the direction of the current will be reversed, the positive fluid
always flowing towards the warmer end of the bismuth.

2040. A constant difference of temperature produces a con-
stant current. If means be taken to maintain the extremities
of the bismuth at a constant difference of temperature, the needle
will maintain a constant deflection. Thus, if one end of the
bismuth be immersed in boiling water and the other in melting
ice, so that their temperatures shall be constantly maintained at
212 and 32, the deflection of the needle will be invariable.
If the temperature of the one be gradually lowered, and the
other gradually raised, the deflection of the needle will be
gradually diminished ; and when the temperatures are equalized,
the needle will resume its position in the magnetic meridian.

2041. Different metals have different thermo-electric energies.
This property, in virtue of which a derangement of the electric
equilibrium attends a derangement of the thermal equilibrium,
is common to all the metals, and, indeed, to conductors generally;
but, like other physical properties, they are endowed with it in
very different degrees. Among the metals, bismuth and anti-
mony have the greatest thermo-electric energy, whether they are
placed in contact with each other, or with any other metal. If
a bar of either of these metals be placed with its extremities in
contact with the wires of a reometer, a deflection of the needle
will be produced by the mere warmth of the finger applied to
one end of the bar. If the finger be applied to both ends, the
deflection will be redressed, and the needle will return to the
magnetic meridian.

It has been ascertained that if different parts of the same
mass of bismuth or antimony be raised to different temperatures,
the electric equilibrium will be disturbed, and currents will be
established in different directions through it, depending on the
relative temperatures. These currents are, however, much less
intense than in the case where the derangement of temperature
is produced at the points of contact or junction of different con-

2042. Pouillefs thermo-electric apparatus. M. Pouillet


has with great felicity availed himself of these properties of
thermo-electricity to determine some important and interesting
properties of currents. The apparatus constructed and applied
by him in these researches is represented in jig. 606.

Two rods A and B of bismuth, each about sixteen inches in
length and an inch in thickness, are bent at the ends at right
angles, and being supported on vertical stands are so arranged
that the ends CD and EF may be let down into cups. The
cups c and E are filled with melting ice, and D and F with
boiling water, so that the ends c and E are kept at the constant
temperature of 32, and the ends D and F at the constant tem-
perature of 212.

A differential reometer (2033) is placed at M. Two conducting
circuits are formed either of one or several wires, one com-

Fig. 656.

mencing from F, and after passing through the wire of the reo-
meter M, returning to E ; the other commencing from D, and
after passing through the wire of the reometer in a contrary
direction to the former, returning to c. The wires conducting
the current are soldered to the extremities c, D, E, F of the
bismuth rods, which are immersed in the cups.

If the two currents thus transmitted, the one between F and
E, and the other between D and G, have equal intensities, the
needle of the reometer M will be undisturbed ; but if there be.
any difference of intensity, its quantity and the wire on which
the excess prevails will be indicated by the quantity and di-
rection of the deflection of the needle,
s 2


The successive wires along which the current passes are
brought into metallic contact by means of mercurial cups, a, b,
c, d, &c., into which their ends are immersed.

The circuits through which the current passes may be simple
or compound. If simple, they consist of wire of one uniform
material and thickness. If compound, they consist of two or
more wires differing in material, thickness, or length.

The wire composing a simple circuit is divided into two
lengths, one extending from D or F to the cup e or d, where the
current enters the convolutions of the reometer, and the other
extending from the cup b or/, where the current issues from
the reometer to c or E, where it returns to the thermo-electric
source. The wires composing a compound current may consist
of a succession of lengths, the current passing from one to
another by means of the metallic cups. Thus, as represented
in the figure, the wires FC, cd, and/E, forming, with one wire
of the reometer, one circuit, and the wires ve, ba, and a c,
forming with the other wire of the reometer the other circuit,
may differ from each other in material, in thickness, and in

The currents pass as indicated by the arrows, from the extre-
mity of the bismuth which has the higher temperature through
the wires to the extremity which has the lower temperature.

2043. Relation between the intensity of the current and the
length and section of the conducting wire. If the two circuits
be simple and be composed of similar wires of equal lengths,
the intensity of the two currents will be found to be equal, the
needle of the reometer being undisturbed. But if the length of
the circuit be greater in the one than in the other, the inten-
sities will be unequal, that current which passes over the
longest wire having a less intensity in the exact proportion in
which it has a greater length.

If the section of the wire composing one circuit be greater
than that of the wire composing the other circuit, their lengths
being equal, the current carried by the wire of greater section
will be more intense than the other in exactly the proportion
in which the section is greater.

If the wire composing one of two simple circuits have a length
less than that composing the other, and a section also less in
the same proportion than the section of the other, the currents
passing over them will have the same intensity, for the excess


of intensity due to the lesser length of the one is compensated
by the excess due to the greater section of the other.

In general, therefore, if i and ^ express the intensities of the
two currents transmitted from D and r, Jig. 656., over two
simple circuits of wire of the same metal, whose sections are
respectively s and s', and whose lengths are L and i/, we shall
have :

that is to say, the intensities are directly as the sections and
inversely as the lengths of the wire.

If two simple circuits be compared, consisting of wires of
different metals, this proportion will no longer be maintained,
because in that case wires of equal length and equal section
will no longer give the currents equal intensities, because they
will not have equal conducting powers. That circuit which,
being alike in other respects, is composed of the metal of
greatest conducting power, will give a current of proportionally
greater intensity. The relative intensities, therefore, of the
currents carried by wires of different metals of equal length
and thickness are the exponents of the relative conducting
powers of these metals.

In general, if c and c' express the conducting powers of the
metals composing two simple circuits, we shall have :

s s'

i : i' : : c x - : c' x ->

Li it

2044. Conducting powers of metals. M. Pouillet ascertained
on these principles the conducting powers of the following

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 37 of 45)