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capable of receiving under these circumstances.

On testing the needle it will be found that its boreal
or south pole is at that end to which the front of
the current is presented; and, consequently, for a right-
handed helix, it will be towards the positive, and for a
left-handed helix towards the negative pole. It ap-
pears, therefore, that the needle acquires a polarity
identical with that which the helix itself is proved to



.1961. Polarity produced by the induction of heli-
acal current. In the case of the right-handed helix,
Fig. 612. represented \nfig. 612., the current passes in the di-
rection indicated by the arrows, and consequently the austral
pole will be at a and the boreal pole at b. In the case
of the left-handed helix, fig. 613., the position of these
poles a and b is reversed in relation to the direction of
the current, but the boreal pole b is in both cases at
that end to which the front of the current looks.

1962. Consequent points produced. If the helix
ID be reversed once or oftener in passing along the tube,
iv; being alternately right-handed and left-handed, as re-
presented in fig. 614., a consequent point will be
produced upon the bar at each change of direction
of the helix.

1963. Inductive action of common electricity pro-
duces polarity. It is not only by the induction of
the voltaic current that magnetic polarity rnay be

. ' imparted. Discharges of common electricity trans-
lg ' 6 3 ' mitted along a wire, especially if it have the form
of an helix, will produce like effects. If the wire be straight,
the influence is feeble. Sparks taken from the prime con-
ductor produce sensible effects on very fine needles ; but if the
wire be placed in actual contact with the conductor at one end
and the cushion at the other, so that a constant current shall
pass along it Irom the conductor to the cushion, no effect is
produced. The effect produced by the spark is augmented as
the spark is more intense and taken at a greater distance from
the conductor.



ELECTRO-MAGNETIC INDUCTION. 333

If the wire be formed into an helix, magnetic polarity will
be produced by a continuous current, that is, by ac-
tually connecting the ends of the wire with the con-
ductor and the cushion ; but these effects are much
more feeble than those produced under like circum-
r stances by the spark.

All these effects are rendered much more intense
when the discharge of a Leyden jar, and still more
that of a Leyden battery, is transmitted along the wire.
When these phenomena were first noticed, it was
assumed that the polai-ity thus imparted by common
electricity must necessarily follow the law which pre-
vails in the case of a voltaic current, and that in the
case of helices the boreal or south pole wpuld be pre-
sented towards the front of the current. Savary,
Fig. 614. however, showed that the effects of common electricity
obey a different principle, and thus established a fundamental
distinction between the voltaic current and the electi-ic discharge.

1964. Conditions on which a needle is magnetized positively
and negatively. When an electric discharge is transmitted
along a straight wire, a needle placed at right angles to the
wire acquires sometimes the polarity of a magnetic needle,
which under the influence of a voltaic current would take a
like position ; that is to say, the austral or north pole will be
to the right of an observer who looks at the needle from the
current, his head being in the direction from which the current
flows. The needle is in this case said to be magnetized posi-
tively. When the opposite polarity is imparted to the needle,
it is said to be magnetized negatively.

1965. Results of Savory's experiments. Savary showed
that needles are magnetized by the discharge of common elec-
tricity, positively or negatively, according to various conditions,
depending on the intensity of the discharge, the length of the
conducting wire, supposing it to be straight, its diameter, the
thickness of the needles, and their coercive force. In a series
of experiments, in which the needles were placed at distances
from the current increasing by equal increments, the magnetiz-
ation was alternately positive and negative ; when the needle
was in contact with the wire, it was positive ; at a small dis-
tance negative, at a greater distance no magnetization was
produced ; a further increase of distance produced positive



334 VOLTAIC ELECTRICITY.

magnetism ; and after several alternations of this kind, the
magnetization ended in being positive, and continued positive
at all greater distances.

The number and frequency of these alternations are de-
pendent on the conditions above-mentioned, but no distinct law
showing their relation to those conditions has been discovered.
In general it may be stated, that the thinner the wire which
conducts the current, the lighter and finer the needles, and the
more feeble their coercive force is, the less numerous will be
those periodical changes of positive and negative magnetization.
It is sometimes found, that when these conditions are observed,
the magnetization is positive at all distances, and that the pe-
riodic changes only affect its intensity.

Similar effects are produced upon needles placed in tubes of
wood or glass, upon which an heliacal current is transmitted.
In these cases, the mere variation in the intensity of the dis-
charge produces considerable effect.

1966. Magnetism imparted to the needle affected by the non-
magnetic substance ivhich surrounds it. Savary also ascer-
tained a fact which, duly studied, may throw much light on the
theory of these phenomena. The quantity of magnetism im-
parted to a needle by an electric discharge, and the character
of its polarity, positive or negative, are affected by the non-
magnetic envelope by which the needle is surrounded. If a
needle be inserted in the axis of a very thick cylinder of
copper, an heliacal current surrounding the cylinder will not
impart magnetism to it. If the thickness of the copper en-
velope be gradually diminished, the magnetization will be ma-
nifested in a sensible degree, and it will become more and more
intense as the thickness of the copper is diminished. This
increase, however, does not continue until the copper envelope
disappears, for when the thickness is reduced to a certain limit,
a more intense magnetization is produced than when the un-
covered needle is placed within the helix.

Envelopes of tin, iron, and silver placed around the needle are
attended with analogous effects, that is to say ; when they con-
sist of very thin leaf metal they increase the quantity of mag-
netism which can be imparted to the needles by the current ;
but when the metallic envelope is much thicker, they prevent
the action of the electric discharge altogether. Cylinders formed
of metallic filings do not produce these effects, while cylinders



ELECTRO-MAGNETIC INDUCTION. 335

formed of alternate layers of metallic and non-metallic sub-
stances do produce them. It is inferred from this that solutions
of continuity at right angles to the axis of the needle, or to that
of the cylinder, have an influence on the phenomena.

1967. Formation of powerful electro-magnets. The in-
ductive effect of a spiral or heliacal current on soft iron is still
more energetic than on steel or other bodies having more or
less coercive force. The property enjoyed by soft iron, of
suddenly acquiring magnetism from any external magnetizing
agent, and as suddenly losing its magnetism upon the suspen-
sion of such agency, has supplied the means of producing the
temporary magnets which are known under the name of

ELECTRO-MAGNETS.

The most simple form of electro-magnet is represented in
fig. 615. It is composed of a bar of soft iron bent into the
form of a horse-shoe, and of a wire wrapped
with silk, which is coiled first on one arm,
proceeding from one extremity to the bend
of the horse-shoe, and then upon the other
from the bend to the other extremity, care
being taken that the convolutions of the
spiral shall follow the same direction in
passing from one leg to the other, since,
otherwise, consequent points would be
Fig. 615. produced. An armature is applied to the

ends of the horse-shoe, which will adhere to them so long as a
voltaic current flows upon the wire, but which will drop off the
moment that such current is discontinued.

1968. Conditions which determine the force of the magnet.
The force of the electro-magnet will depend on the dimensions
of the horse-shoe and the armature, the intensity of the current,
and the number of convolutions with which each leg of the
horse-shoe is wrapped.

1969. Electro-magnet of Faculty of Sciences at Paris. In
1830 an electro-magnet of extraordinary power was constructed
under the superintendence of M. Pouillet at Paris. This appa-
ratus, represented in fig. 616., consists of two horse-shoes, the
legs of which are presented to each other, the bends being
turned in contrary directions. The superior horse-shoe is fixed
in the frame of the apparatus, the inferior being attached to
a cross-piece which slides in vertical grooves formed in the




336



VOLTAIC ELECTRICITY.




sides of the frame. To this cross-
piece a dish or plateau is suspended
in which weights are placed, by the
effect of which the attraction which
unites the two horse-shoes is at length
overcome. Each of the horse-shoes
is wrapped with 10,000 feet of co-
vered wire, and they are so arranged
that the poles of contrary names shall
be in contact. With a current of
moderate intensity the apparatus is
capable of supporting a weight of
several tons.

1970. Form of electro-magnets in general. It is found
more convenient generally to construct electro-magnets of two
straight bars of soft iron, united at one end by a straight bar
transverse to them, and attached to them by screws, so that the
form of the magnet ceases to be that of a horse-shoe, the end at
which the legs are united being not curved but square. The
conductor of the heliacal current is usually a copper wire of
extreme tenuity.

1971. Electro-magnetic power applied as a mechanical
agent. The property of electro-magnets by which they are
capable of suddenly acquiring and losing the magnetic force
has supplied the means of obtaining a mechanical agent which
may be applied as a mover of machinery. An electro- magnet
and its armature, such as that represented in fig. 615., or
two electro-magnets such as those represented in fig. 616., are
placed so that when the electric current is suspended they will
rest at a certain distance asunder, and when the current passes
on the wire they will be drawn into contact by their mutual
attraction. When the current is again suspended they will
separate. In this manner, by alternately suspending and trans-
mitting the current on the wire which is coiled round the
electro-magnet, the magnet and its armature, or the two mag-
nets, receive an alternate motion to and from each other
similar to that of the piston of a steam-engine, or the foot of a
person who works the treddle of a lathe. This alternate
motion is made to produce one of continued rotation by the
same mechanical expedients as are used in the application of
any other moving power.



ELECTRO-MAGNETIC INDUCTION. 337

The force with which the electro-magnet and its armature
attract each other, determines the power of the electro-motive
machine, just as the pressure of steam on the piston determines
the power of a steam-engine. This force, when the magnets
are given, varies with the nature and magnitude of the galvanic
pile which is employed.

1972. Electro-motive power applied in the workshop of M.
Froment. The most remarkable and beautiful application of
electro-motive power as a mechanical agent which has been
hitherto witnessed is presented in the workshops of M. Gustave
Froment, of Paris, so celebrated for the construction of in-
struments of precision. It is here applied in various forms to
give motion to the machines contrived by M. Froment for
dividing the limbs of astronomical and surveying instruments
and microscopic scales. The pile used for the lighter descrip-
tion of work is that of Daniel, consisting of about 24 pairs.
Simple arrangements are made by means of commutators, reo-
meters, and reotropes, for modifying the current indefinitely in
quantity, intensity, and direction. By merely turning an index
or lever in one direction or another, any desired number of
pairs may be brought into operation, so that a battery of
greater or less intensity may be instantly made to act, subject
to the major limit of the number of pairs provided. By
another adjustment, the copper elements of two or more pairs,
and at the same time their zinc elements, may be thrown into
connexion, and thus the whole pile, or any portion of it, may
be made to act as a single pair, of enlarged surface. By
another adjustment, the direction of the current can be re-
versed at pleasure. Other adjustments, equally simple and
effective, are provided, by which the current can be turned on
any particular machine, or directed into any room that may be
required.

The pile used for heavier work, is a modification of Bunsen's
charcoal battery, in which dilute sulphuric acid is used in the
porous porcelain cell containing the charcoal, as well as in the
cell containing the zinc. By this expedient the noxious fumes
of the nitric acid are removed, and although the strength of the
battery is diminished, sufficient power remains for the purposes
to which it is applied.

The forms of the electro-motive machines constructed by

ii. Q



338 VOLTAIC ELECTRICITY.

M. Froment are very various. In some the magnet is fixed and
the armature moveable ; in some both are moveable.

In some there is a single magnet and a single armature.
The power is in this case intermittent, like that of a single
acting steam-engine, or of the foot in working the treddle of a
lathe, and the continuance of the action is maintained in the
same manner by the inertia of a fly-wheel.

In other cases two electro-magnets and two armatures are
combined, and the current is so regulated, that it is established
on each during the intervals of its suspension on the other.
This machine is analogous in its operation to the double-acting
steam-engine, the operation of the power being continuous, the
one magnet attracting its armature during the intervals of sus-
pension of the other. The force of these machines may be
augmented indefinitely, by combining the action of two or more
pairs of magnets.

Another variety of the application of this moving principle,
presents an analogy to the rotatory steam-engine. Electro-
magnets are fixed at equal distances round a wheel, to the
circumference of which the armatures are attached at corre-
sponding intervals. In this case the intervals of action and
intermission of the currents are so regulated, that the magnets
attract the armatures obliquely, as the latter approach them,
the current, and consequently the attraction, being suspended
the moment contact takes place. The effect of this is, that all
the magnets exercise forces which tend to turn the wheel on
which the armatures are fixed constantly in the same direction,
and the force with which it is turned is equal to the sum of the
forces of all the electro-magnets which act simultaneously.

This rotatory electro-motive machine is infinitely varied, not
only in its magnitude and proportions, but in its form. Thus
in some the axle is horizontal, and the wheel revolves in a
vertical plane ; in others the axle is vertical, and the wheel
revolves in an horizontal plane. In some the electro-magnets
are fixed, and the armatures moveable with the wheel ; in
others both are moveable. In some the axle of the wheel
which carries the armatures is itself moveable, being fixed
upon a crank or excentric. In this case the wheel revolves
within another, whose diameter exceeds its own by twice the
length of the crank, and within this circle it has an hypo-
cycloidal motion.



ELECTRO-MAGNETIC INDUCTION.



339



Each of these varieties of the application of this power, as
yet novel in the practical operations of the engineer and manu-
facturer, possesses peculiar advantages or convenience, which
render it more eligible for special purposes.

1973. Electro-motive machines constructed by him. To
render this general description of M. Froment's electro-motive
machines more clearly understood, we shall add a detailed ex-
planation of two of the most efficient and useful of them.

In the machine represented in fig. 617., a and b are the two
legs of the electro-magnet ; c d is the transverse piece uniting




Fig. 617.

them, which replaces the bend of the horse-shoe ; ef is the
armature confined by two pins on the summit of the leg a

Q 2



340 VOLTAIC ELECTRICITY.

(which prevent any lateral deviation), the end / being jointed
to the lever g h, which is connected with a short arm projecting
from an axis k by the rod i. When the current passes round
the electro-magnet, the lever /is drawn down by the attraction
of the leg b, and draws with it the lever gh, by which i and the
short lever projecting from the axis k are also driven down.
Attached to the same axis k is a longer arm m, which acts by a
connecting rod n upon a crank o and a fly-wheel v. When the
machine is in motion, the lever gh and the armature /attached
to it recover their position by the momentum of the fly-wheel,
after having been attracted downwards. When the current is
again established, the armature /and the lever gh are again
attracted downwards, and the same effects ensue. Thus, during
each half-revolution of the crank o, it is driven by the force of
the electro-magnet acting on/ and during the other half-revo-
lution it is carried round by the momentum of the fly-wheel.
The current is suspended at the moment the crank o arrives at
the lowest point of its play, and is re-established when it returns
to the highest point. The crank is therefore impelled by the
force of the magnet in the descending half of its revolution, and
by the momentum of the fly-wheel in the ascending half.

The contrivance called a distributor, by which the current is
alternately established and suspended at the proper moments,
is represented in Jig. 618., where y represents the transverse
section of the axis of the fly-wheel ; r, a spring
which is kept in constant contact with it ; x, an ex-
centric fixed on the same axis y, and revolving with
it and r another spring similar to r, which is acted
upon by the excentric, and is thus allowed to press
_ = against the axis y during half the revolution, and
Fi 618 remove d from contact with it during the other half-
revolution. When the spring r' presses on the axis
y the current is established ; and when it is removed from it
the current is suspended.

It is evident that the action of this machine upon the lever
attached to the axis k is exactly similar to that of the foot on
the treddle of a lathe or a spinning-wheel ; and as in these
cases, the impelling force being intermittent, the action is un-
equal, the velocity being greater during the descending motion
of the crank o than during its ascending motion. Although the
inertia of the fly-wheel diminishes this inequality by absorbing



ELECTRO-MAGNETIC INDUCTION.



341



a part of the moving power in the descending motion, and re-
storing it to the crank in the ascending motion, it cannot alto-
gether efface it.

Another electro-motive machine of M. Froment is represented
in elevation in fig. 619., and in plan in^. 620. This machine




Fig. 619.

has the advantage of producing a perfectly regular motion of
rotation, which it retains for several hours without sensible
change.

A drum, which revolves on a vertical axis xy, carries on its
circumference eight bars of soft iron a placed at equal distances
asunder. These bars are attracted laterally, and always in the
same direction, by the intermitting action of six electro-magnets
b, mounted in a strong hexagonal frame of cast iron, within
which the drum revolves. The intervals of action and sus-
pension of the current upon these magnets are so regulated that
it is established upon each of them at the moment one of the
bars of soft iron a is approaching it, and it is suspended at the
moment the bar begins to depart from it. Thus the attraction
accelerates the motion of the drum upon the approach of the
piece a towards the magnet b, and ceases to act when the piece
a arrives in face of b. The action of each of the six impelling
forces upon each of the eight bars of soft iron attached to the
Q 3



342 VOLTAIC ELECTRICITY.

drum is thus intermitting. During each revolution of the
drum, each of the eight bars a receives six impulses, and there-




Fig. 620.

fore the drum itself receives forty-eight impulses. If we suppose
the drum to make one revolution in four seconds, it will there-
fore receive a succession of impulses at intervals of the twelfth
part of a second, which is practically equivalent to a con-
tinuous force.

. The intervals of intermission of the current are regulated by
a simple and ingenious apparatus. A metallic disc c is fixed
upon the axis of rotation. Its surface consists of sixteen equal
divisions, the alternate divisions being coated with non-con-
ducting matter. A metallic roller h, which carries the current,
presses constantly on the surface of this disc, to which it im-
parts the current. Three other metallic rollers efg press against
the edge of the disc, and, as the disc revolves, come alternately
into contact with the conducting and non-conducting divisions of
it. When they touch the conducting divisions, the current is
transmitted ; when they touch the non-conducting divisions,
the current is interrupted.

Each of these three rollers efg is connected by a conducting
wire with the conducting wires of two electro-magnets diame-



ELECTRO-MAGNETIC INDUCTION. 343

trically opposed, as is indicated in fig. 620., so that the current
is thus alternately established and suspended on the several
electro-magnets, as the conducting and non-conducting divisions
of the disc pass the rollers e, f, and g.

M. Froment has adapted a regulator to this machine, which
plays the part of the governor of the steam-engine, moderating
the force when the action of the pile becomes too strong, and
augmenting it when it becomes too feeble.

A divided circle mn,fiy. 619., has been annexed to the ma-
chine at the suggestion of M. Pouillet, by which various im-
portant physical experiments may be performed.

1974. Applied as a sonometer. This machine has been
applied with much success as a sonometer, to ascertain and
register directly the number of vibrations made by sonorous
bodies in a given time.

1975. Momentary current by induction. If a wire A, on
which a voltaic current is transmitted, be brought into proximity
with and parallel to another wire B, the ends of which are in
metallic contact either with each other, or with some continuous
system of conductors, so as to form a closed circuit, the electric
equilibrium of the wire B will be disturbed by the action of the
current A, and a current will be produced upon B in a direction
opposite to that which prevails on A. This current will, how-
ever, be only momentary. After an instant the wire B will
return to its natural state.

If the wire A, still carrying the current, be then suddenly
removed from the wire B, the electric equilibrium of B will be
again disturbed, and as before, only for a moment ; but in this
case the current momentarily produced on B will have the same
direction as the current on A.

If the contact of the extremities of the wire B, or either of
them with each other, or with the intermediate system of con-
ductors which complete the circuit, be broken, the approach or
removal of the current A will not produce these effects on the
wire B.

If, instead of moving the wire A to and from B, the wires,
both in their natural state, be placed parallel and near to each
other, and a current be then suddenly transmitted on A, the
same effect will be produced on B as if A, already bearing the
current, had been suddenly brought into proximity with B.
And in the same way it will be found that if the current es-



344 VOLTAIC ELECTRICITY.

tablished on A be suddenly suspended, the same effect will
be produced as if A, still bearing the current, were suddenly
removed.

These phenomena may be easily exhibited experimentally, by
connecting the extremities of the wire A with a voltaic pile, and
the extremities of B with the wires of a reoscope. So long as



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 33 of 45)