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in fig. 593., where a magnetic bar is supported
in the vertical position between pivots which
play in agate cups. A circular mercurial
canal is placed at the centre of the magnet,
and another round the lower pivot. Mercurial
cups communicate with these two canals.
When these cups are put in communication
with the poles of a battery, the current will
pass between the two canals along the lower
pole of the magnet, in the one direction or the
other, according to the mode of connexion ;
and the magnet will turn on its own axis with
a direct or retrograde rotation, according to

the name of the pole on which the current runs, and to the

direction of the current.

Fig. 593.



IF a wire PABCDN, figs. 594., 595., be bent into the form of any
geometrical figure, the extremities being brought near each other


C +**& B

Fig. 594. Fig. 595.

without actually touching, a current entering one extremity and
departing from the other, is called a CIRCULATING CURRENT.


1935. Front and back of circulating current. If such a
current be viewed on opposite sides of the figure formed by the
wire, it will appear to circulate in different directions, on one
side direct, and on the other retrograde (1930). That side on
which it appears direct is called the FRONT, and the other the
BACK of the current.

1936. Axis of current. If the current have a regular figure
having a geometrical centre, a straight line drawn through this
centre perpendicular to its plane is called the AXIS of the

1937. Reciprocal action of circulating current and magnetic
pole. To determine the reciprocal influence of a circulating
current and a magnetic pole placed anywhere upon its axis, let

D^ the axis be xcx', fig. 596.,

"* J^ 1 t ^ ie pl ane f tne current being

o7"-^vNT X at right angles to the paper,
| c s^\L ' A being the point where it as-

* cends, and D the point where

4 it descends through the paper.

F 'S- 596 - 1. Let N be a north mag-

netic pole placed in front of the current.

The part of the current at D will exert a force on N in the
direction NM' at right angles to DN, and the part at A will exert
an equal force in the direction N M at right angles to AN. These
two forces being compounded, will be equivalent to a single
force N o (152) directed from N along the axis towards the

It may be shown that the same will be true for every two
points of the current which are diametrically opposed.

2. Let a south magnetic pole $,fig- 597., be similarly placed
in front of a circulating current. The part D will exert upon
D it a force in the direction sil

*> x perpendicular to SD and to

'^S/ \u K the left of s as viewed from D,
~^ \ , and the part A will exert an

M\ equal force in the direction

s M' to the right of S as viewed
Flg< 597 ' from A. These two equal

forces will have a resultant so directed from the current ; and
the same will be true of every two points of the current which
are diametrically opposed,

p 5


If the magnetic pole be placed at the back of the current, the
contrary effects ensue.

The same inferences may be deduced with respect to any cir-
culating current which has a centre, that is, a point within it
which divides into two equal parts all lines drawn through it
terminating in the current.

It may therefore be inferred generally that when a magnetic
pole is placed upon the axis of a circulating current, attraction
or repulsion is produced between it and the current ; attraction
when a NORTH pole is before, or a SOUTH pole BEHIND, and re-
pulsion when a SOUTH pole is before, or a north pole BKHIND.

1938. Intensity of the force vanishes ivhen the distance of
the pole bears a very great ratio to the diameter of current.
Since the intensity of the attraction between the component
parts of the current and the pole decreases as the square of the
distance is increased, and since the lines NM and NM/, fig. 596.,
and SM and SM', Jig. 597., form with each other a greater angle
as the distance of the pole from the current is increased, it is
evident that when the diameter AD of the current bears an
inconsiderable ratio to the distance of the pole N or s from it,
the attraction or repulsion ceases to produce any sensible

1939. But the directive power of the pole continues. This,
however, is not the case with relation to the directive power of
the pole upon the current. The tendency of the forces im-
pressed by the pole upon the current is always to bring the
plane of the current at right angles to the line drawn from the
pole to its centre. There is, in short, a tendency of the line of
direction of the pole to take a position coinciding with or
parallel to the axis of the current, and this coincidence may be
produced either by the change of position of the pole or of the
plane of the current, or of both, according as either or both are
free to move.

1940. Spiral and heliacal currents. The force exerted by
a circulating current may be indefinitely augmented by causing
the current to circulate several times round its centre or axis.
If the wire which conducts the current be wrapped with silk or
coated with any non-conducting varnish, so as to prevent the
electricity from escaping from coil to coil when in contact, cir-
culating currents may be formed round a common centre or
axis in a ring, a spiral, a helix, or any other similar form, so


that the forces exerted by all their coils on a single magnetic
pole may be combined by the principle of the composition of
force ; and hence an extensive class of electro-magnetic phe-
nomena may be educed, which supply at the same time im-
portant consequences and striking experimental illustrations of
the laws of attraction and repulsion which have been just

1941. Expedients to render circulating currents moveable.
Ampere's and Delarive's apparatus. Two expedients have
been practised to render a circulating current moveable.

1. By the apparatus of AMPERE already described (1915),

the wire conducting the current being bent
at the ends, as represented in Jig. 598., may
be supported in the cups yy 1 as represented
in fig. 564., so that its plane being vertical,
it shall be capable of revolving round the
line yy 1 as an axis. By this arrangement
the plane of the current can take any di-
rection at right angles to an horizontal
plane, but it is not capable of receiving any
Fig. 598. progressive motion.

2. The latter object is attained by the floating apparatus of
M. Delarive.

-. Let a coated wire be formed into a circular

g ( 1 N Tin S composed of several coils. Let one end of
V / it be attached to a copper cell, fig. 599., and

:'if* the other to a slip of zinc which descends

into this celt. The cell being filled with aci-
| dulated water, a current will be established
through the wire in the direction of the
\ arrows. The copper cell may be inclosed in
a glass vessel, or attached to a cork so as to
float upon water, and thus be free to assume
Fig. 599. an y position which the forces acting upon the
current may tend to give it.

1942. Rotatory motion imparted to circular current by a
magnetic pole. If a magnetic north pole be presented in front
of a circular current, fig. 598., suspended on Ampere's frame,
fig. 564., the ring will turn on its points of suspension until its

axis pass through the pole. If the pole be carried round in a
circle, the plane of the ring will revolve with a corresponding


motion, always presenting the front of the current to the pole,
the axis of the current passing through the pole.

If a south magnetic pole be presented to the back of the
current, like effects will be produced.

If a north magnetic pole be presented to the back, or a south
to the front of the current, the ring will, on the least disturb-
ance, make half a revolution round its points of suspension, so
as to turn its point to the north and its back to the south mag-
netic pole.

1943. Progressive motion imparted to it. If c, fig. 600.,

represent a floating circular current,
a north magnetic pole placed anywhere
B on its axis will cause the ring con-
ducting it to move in that direction in
which its front is presented ; for if the
Fig. eoo. pole be before it at A it will attract the

current, and if behind it at B it will repel it (1937). In either
case the ring will move in the direction in which its front looks.
If a south magnetic pole be similarly placed, it will cause the
current to move in the contrary direction ; for if it be placed
before the current at A it will repel it, and if behind it at B it
will attract it. In either case the ring will move in the di-
rection to which the back of the current looks.

1944. Reciprocal action of the current on the pole. If the
magnetic pole be moveable and the current fixed, the motion
impressed on the pole by the action of the current will have a
direction opposite to that of the motion which would be im-
pressed on the current, being moveable, by the pole being fixed.
A north magnetic pole placed on the axis of a fixed circular
current will therefore be moved along the axis in that direction
in which the back of the current looks, and a south magnetic
pole in that direction in which the front looks.

1945. Action of a magnet on a circular floating current.

If a bar magnet s N,
fig. 601., be placed in
a fixed position with
its magnetic axis in
Fig- 601. the direction of a

floating circular current A, its north pole x being directed to
the front of the current, the current will be attracted by x and
repelled by s ; but the force exerted by x will predominate in


consequence of its greater proximity to A, and the current will
accordingly move from A towards N. After it passes N, the
bar passing through the centre of the ring, it will be repelled
by N and also by s (1937) ; but so long as it is between N and
the centre c of the bar, as at B, the repulsion of N will pre-
dominate over that of s in consequence of the greater proximity
of N, and the current will move towards c. Passing beyond c to
B', the repulsion of s predominates over that of N, and it will be
driven back to C, and after some oscillations on the one side
and the other it will come to rest in stable equilibrium, with its
centre at the centre of the magnet, its plane at right angles to
it, the front looking towards s and the back towards N.

1946. Reciprocal action of the current on the magnet. If
the current be fixed and the magnetic bar moveable, the latter
will move in a direction opposite to that with which the cur-
rent would move, the bar being fixed. Thus, if the current
were fixed at A, the bar would move to it in the direction of N A,
and the pole N passing through the ring, the bar would come
to rest, after some oscillations, with its centre at the centre of
the ring.

1947. Case of instable equilibrium of the current. If the
ring were placed with its centre at c and its front directed to
N, it would be in instable equilibrium, for if moved through any
distance, however small, towards N or s, the attraction of the
pole towards which it is moved would prevail over that of the
other pole which is more distant, and the ring would conse-
quently be moved to the end of the bar and beyond that point,
when, being still attracted by the nearest pole, it would soon be
brought to rest. It would then make a half revolution on its
axis and return to the centre of the bar, where it would take
the position of stable equilibrium.

All these are consequences which easily follow from the ge-
neral principles of attraction and repulsion established in (1937).
J948. Case of a spiral current. If the wire which con-
ducts the current be bent into the form of a spiral,
fig. 602., each convolution will exert the force of
\\ a circular current, and the effect of the whole will
be the sum of the forces of all the convolutions.
Such a spiral will therefore be subject to the con-
Fig. 602. ditions of attraction and repulsion which affect a
circular current (1937).


1949. Circular or spiral currents exercise the same action as
a magnet. In general it may be inferred that circulating
currents exercise on a magnetic pole exactly the same effects as
would be produced by another magnet, the FRONT of the current
playing the part of a SOUTH pole, and the BACK that of a NORTH

1950. Case of heliacal current. It has been shown that a
helix or screw is formed by a point which is at the same time
affected by a circular and progressive motion, the circular motion
being at right angles to the axis of the helix, and the progressive
motion being in the direction of that axis (496). In each con-
volution the thread of the helix makes one revolution, and at
the same time progresses in the direction of the axis through a
space equal to the distance between two successive convolutions.

1951. Method of neutralizing the effect of the progressive
motion of such a current. If a current therefore be transmitted
on a heliacal wire, it will combine the characters of a circular
and rectilinear current. The latter character, however, may be
neutralized or effaced by transmitting a current in a contrary
direction to the progression of the screw, on a straight wire
extended along the axis of the helix. This rectilinear current
being equal, parallel, and contrary in direction to the pro-
gressive component of the heliacal current, will have equal and
contrary magnetic properties, and the forces which they exert
together on any magnetic pole within their influence will
counteract each other.

1952. Right-handed and left-handed helices. Helices are
TWWVWWy???'??^ of two forms : those in which the wire

HHimilD turns like the thread of a cork9creW)

lg< ' that is, in the direction of the hands of a

HffiSHHEffiH watch, fig. 603. ; and those in which it
Fig. 6O4. turns in a contrary direction,^. 604.

1953. Front of current on each kind. If a current traverse
a right-handed helix, its front will be directed to the end at
which it enters, and in the left-handed helix to the end at
which it departs.

1954. Magnetic properties of heliacal currents their poles
determined. Hence it follows, that in a right-handed heliacal
current, the end at which the current enters, and which is the
positive pole, has the magnetic properties of a south pole ; and
in a left-handed helix this end has the properties of a north pole.


1955. Experimental illustration of these properties. The
magnetic properties of spiral and heliacal currents may be illus-
trated experimentally by means of Ampere's arrangement,^.
564., or by a floating apparatus constructed on the same prin-
ciple as that represented in^. 599.

The manner of forming spiral currents adapted to Ampere's
apparatus is represented \njigs. 605. and 606. In Jig. 605. the

Fig. 605.

spirals are both in the same plane, passing through the axis of
suspension yy'. In Jig- 606. they are in planes parallel to this
axis, and at right angles to the line joining their centres, which
is therefore their common axis.

1956. The front of a circulating current has the properties
of a south, and the back those of a north, magnetic pole. Ac-
cording to what has been explained, the front of such a spiral
current will have the properties of a south magnetic pole, and
will therefore attract and be attracted by the north, and repel
and be repelled by the south pole of a magnet. If the spirals
in fig. 605., therefore, be so connected with the poles of a voltaic
system, as to present their fronts on the same side, they will be
both attracted by the north pole, and both repelled by the
south pole of a magnet presented to them, that which is nearer
to the magnet being more attracted or repelled than the other.
If the magnetic pole be equally distant from them, they will
be in equilibrium, and the equilibrium will be stable if they
are both repelled, and instable if they are both attracted by
the magnet.

To demonstrate this, let s, fig. 607., be the south pole of a
magnet placed in front of the two spirals, whose centres are at
A and B, equally distant from s. It is evident that a perpen-
dicular so drawn from s to AB will in this case pass through
the middle of AB. The pole s will therefore, according to what



has been already explained, repel the two spirals with equal

forces. If the spirals be re-
moved from this position to
the positions A'B', A', being
nearer to S than B', will be
repelled by a greater force,
and therefore A' will be driven
back towards A, and B' to-
wards B. In like manner, if
they were removed to the
positions A"B", the force re-
pelling B" would be greater
than that which repels A",

F 'S- 607 - and therefore B" will be driven

back to B, and A" to A.

It follows, therefore, that the position of equilibrium of AB is in
this case such that the system will return to it after the slightest
disturbance on the one side or the other, and is therefore stable.
If the pole s were the north pole, it would attract both
currents, and in that case A' would be more strongly attracted
than B', and B" than A", and consequently the spirals would de-
part further from the position A after the least disturbance. The
equilibrium would therefore be instable.

It will be found, therefore, that when a NORTH POLE is pre-
sented BEFORE, or a south pole BEHIND, such a pair of spiral
currents, the system,^. 605., will, on the least disturbance from
the position of instable equilibrium, turn on its axis yy' through
half a revolution, presenting the fronts of the currents to the
south pole, and will there come to rest after some oscillations.

In the position of stable
equilibrium, the front of the
currents must therefore be
presented to the south pole
of the magnet, or the back
to the north pole.

1957. Adaptation of an
heliacal current to Ampere's
and Delarive's apparatus.
The manner of adapting
an heliacal current to Am-
pere's arrangement,^. 564.,

Fig. 608.

is represented in fig. 608.


and the manner of adapting it to the floating method is repre-
sented in Jiff. 609.

The positive wire is carried down from y,fig. 608., and then
coiled into an helix from the centre to
, . mu -i j

the extremity. Thence it is carried in a

straight direction through the centre of
the helix to the other extremity, from
whence it is again conducted in heliacal
coils back to the centre, where it is bent
upwards and terminates at the negative
pole#'. In one half of the helix the cur-
rent therefore enters at the centre and
<ig. 609. issues from the extremity, and in the

other half it enters at the extremity and issues from the centre.
If the helices be both right-handed, therefore, the end from
which the current issues will have the properties of a north,
and that at which it enters those of a south, magnetic pole. If
they be both left-handed, this position of the poles will be re-
versed (1954).

The wire which is carried straight along the axis neutralizes
that component of the heliacal current, which is parallel to the
axis, leaving only the circular elements effective (1951).

These properties may be experimentally verified by present-
ing either pole of a magnetic bar to one or the other end of the
heliacal current. The same attractions and repulsions will be
manifested as if the helix were a magnet.

1958. Action of an heliacal current on a magnetic needleplaced
in its axis. If HH' represent an heliacal current, the front of
H ' ir which looks towards A, a north

c" ^li?- ^ A magnetic pole placed any where

B in its axis, either within the

Fi g- 61 - limits of the helix or beyond its

extremities, will be urged by a force directed from A towards c.
Between A and H it will be attracted by the combined forces
of the fronts of all the convolutions of the helix. Between
H and H' it will be attracted by the fronts of those convo-
lutions which are to the left of it, and repelled by the
backs of all those to its right. Beyond H' towards C, it will be
repelled by the backs of all the convolutions. In all positions,
therefore, it will, if free, be moved from right to left, or in a


direction contrary to that towards which the front of the cur-
rent is directed.

If the pole were fixed and the current moveable, the helix
would move from right to left, or in that direction towards
which the front of the current looks.

If a magnetic needle SN,y?^r. 611., be placed in the centre of
the axis of an heliacal current, with its poles equidistant from

the extremities, the south
pole s being presented to-
wards that end F to which
the front of the currents looks,
it will be in equilibrium, the
pole N being repelled to-
. en. wards B, and the pole s to-

wards F by equal forces ; for in this case the pole N will be at-
tracted towards B by all the convolutions of the helix between
N and B, and will be repelled in the same direction by all the
convolutions between N and F ; while the pole s will in like
manner be attracted towards F by all the convolutions between
s and F, and repelled in the same direction by all the convolu-
tions between s and B.

The needle SN, being thus impelled by two equal forces di-
rected/row its centre, will be in stable equilibrium.

If the directions of the poles were reversed, they would be
impelled by two equal forces directed from its extremities to-
wards its centre, and the equilibrium would be instable.

When the magnetic needle is sufficiently light, and the heli-
acal current sufficiently powerful, a curious effect may be
observed, if the needle be placed within the helix so as to rest
upon the lower parts of the wire. Before the current is trans-
mitted, the needle will rest on the wires under the position SN
represented in^. 611. ; but the moment the connexion with the
battery is made, and the current established, it will start up and
place itself in the middle of the axis of the helix, as in the
figure, where it will remain suspended in the air without any
visible support.




1959. Inductive effect of a voltaic current upon a magnet. >
The forces which a voltaic current impresses upon the poles of
a permanent magnet, being similar in all respects to those with
which the same poles would be affected by another magnet, it
may be expected that the natural magnetism of an unmagnetized
body would be decomposed, and polarity imparted to it by the
approach of a voltaic current, in the same manner as by the
approach of a magnet. Experiment accordingly confirms this
consequence of the analogy suggested by the phenomena. It is,
in fact, found that a voltaic current is capable of decomposing
the natural magnetism of magnetic bodies, and of magnetizing
them as effectually as the most powerful magnets.

Soft iron rendered magnetic by voltaic currents. If the wire
upon which a voltaic current flows be immersed in filings of
soft iron, they will collect around it, and attach themselves
to it in the same manner as if it were a magnet, and will
continue to adhere to it so long as the current is maintained
upon it ; but the moment the connexions with the battery are
broken, and the current suspended, they will drop off.

Sewing needles attracted by current. Light steel sewing
needles being presented to the wire conducting a current will
instantly become magnetic, as will be apparent by their as-
suming a position at right angles to the wire, as a magnetic
needle would do under like circumstances. When the current
is suspended or removed, the needles will in this case retain the
magnetism imparted to them.

1960. Magnetic induction of an heliacal current. To exhibit
these phenomena with greater effect and certainty, the needles
should be exposed to the influence not of one, but of several
currents, or of several parts of the same current flowing at
right angles to them. This is easily effected by placing them
within an heliacal current.

Let a metallic wire coated with silk or other non-conductor


be rolled heliacally on a glass tube, fig. 612., and the current
being made to pass along the wire, let a needle or bar
of steel or hard iron be placed within the tube. It
will instantaneously acquire all the magnetism it is

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 32 of 45)