magnets will find it necessary to fix this idea in his mind. He will now
be able to see that if the wire be coiled round the needle, as shown by
the lines and arrows, Fig. 257, so that the same current may circulate
in reverse directions above and below the magnet, its effects in
deviating the needle will everywhere concur—that is, the action of each
part will be to turn the north pole towards the left. It is, therefore,
plain that if the wire conveying the current be passed several times
round the magnetic needle, the deflecting force will be increased; and a
current, which would, by merely passing above or below the magnet,
produce no marked deflection, might be made to produce a considerable
effect if carried many times round it. The arrangement for this purpose
is shown in Fig. 258, where it will be perceived that the needle is
surrounded by a coil of wire, so that the current circulates many times
about it, and the effects of each part of the circuit concur in
deflecting the needle. Such an arrangement of the wire and needle
constitutes what is called the _galvanometer_, an instrument used to
discover the existence and direction of electric currents.
[Illustration:
FIG. 258.—_Galvanometer._
]
The arrangement of metals and acid which we have described is termed a
_voltaic couple_, _element_, or _cell_; and a great controversy has long
been carried on among men of science as to the place at which the
development of electricity has its origin. Three-quarters of a century
ago, the effect was attributed by Volta to the mere contact of the two
dissimilar metals. In the experiment we have described this contact,
supposing the wires to be of copper, would occur at the junction of the
wire and the zinc plate. Now, by joining the copper plate of such a cell
to the zinc plate of another cell, the copper of that to the zinc of a
third, and so on, it is evident that the number of dissimilar contacts
might be indefinitely increased, and the electric power should be
proportionately augmented. It is found that this is really the case, but
Volta’s explanation has been opposed by another which regards the
chemical action in the cells as the real origin of the electric
manifestations. This last explanation, supported by many apparently
conclusive experiments of Faraday and others, has been generally
accepted. Galvanic batteries—as a series of cells joined together in a
certain manner are termed—have been constructed, in which there is no
contact of dissimilar metals; and no electric _current_ can be obtained
from an apparatus in which no chemical action takes place. The contact
theory in a modified form has recently been revived by Sir W. Thomson
and others. In this it is now maintained that some _separation_ of
electricities really does take place by contact of dissimilar
_substances_, but that a _current_ can be produced only when this
separation is continually renewed by chemical actions. Be the true
explanation what it may, the fact is undoubted that by joining cell to
cell, we can really obtain vastly more powerful effects. If we take a
single cell, such as that represented in Fig. 256, and connect the
plates with a long and thin wire, we shall find that the current flowing
through each part of the circuit is much weaker than when we connect the
plates with a short and thick wire. In other words, the action in the
latter case, when the wire is stretched over a magnetic needle, will be
more powerful than in the former. By using a long and thin wire the
current may be so weakened that it becomes necessary to surround the
needle with many coils of the wire to produce a marked deflection.
Again, much depends upon the material; thus a copper wire conveys a much
more powerful current than a German silver one of the same dimensions.
There thus appears to be a certain analogy between the flow of
electricity along conductors to that of water through pipes. The longer
and narrower are the pipes, the less is the quantity of water forced
through them by a given head; and similarly, the resistance to the
passage of a current increases with the length and narrowness of the
conducting wire. When all other circumstances are the same, the
_electrical resistance_ of a conductor varies directly as its length and
inversely as its sectional area. Hence the current flowing in the
apparatus represented in Fig. 256 would be increased by making the wire
thicker, and by making it shorter by bringing Z and C nearer together,
and by making the area they expose to the liquid larger; for in the
liquid also the current flows as indicated by the arrow, a fact which
may be proved by the deflection of a magnetized needle suspended above
the vessel. The magnitude of the current depends, then, upon two
opposing forces, namely, that which continuously separates the
electricities, or drives them apart to re-combine through the circuit,
and that which opposes their passage. The former, which is termed the
_electromotive force_, originates, according to some, from the mere
contact of dissimilar materials, according to others from the chemical
action. Now, we may increase the strength of the current in a given
arrangement, either by increasing the electromotive force, or by
diminishing the resistance. The increase of the strength of the current,
produced by merely pouring more acid into the vessel, Fig. 256, is due,
according to the chemical theory, to the former cause; according to the
contact theory, to the latter. By multiplying the cells we increase the
electromotive forces: the current receives, so to speak, an onward shove
in each cell, but with each cell we introduce an additional resistance.
Hence, it follows, that when the resistance of the circuit outside of
the cells is extremely small, the current produced by a single cell is
as powerful as that produced by a thousand. But when the external
resistance is great, as when long thin wires are used, the united
electromotive forces of a number of cells are needed to drive the
current through the circuit. The strength of a current, C, is therefore
expressible by the following simple formula, in which _r_ stands for the
internal resistance, and _e_ for the electromotive force in each cell;
_n_ represents the number of cells in the battery, these being supposed
exactly similar in every respect; R is the sum of the resistances in the
circuit outside of the battery.
_ne_
C = ————————
_nr_ + R
It is easily seen that the smaller R is made, the more nearly does the
strength of the current become independent of the number of cells.
[Illustration:
FIG. 259.—_Daniell’s Cell and Battery._
]
[Illustration:
FIG. 260.—_Grove’s Cell and Battery._
]
But many modifications have been made in the materials and form of the
cells, by which greater power and duration of action have been attained.
Our space permits a description of only two forms, and these must be
described without a discussion of the principles upon which their
increased efficiency depends. Daniell’s constant cell is represented in
Fig. 259, where D is a battery of ten such cells, A is a cylindrical
vessel of copper, C is a tube of porous earthenware, closed at the
bottom, and within it is suspended the solid rod of amalgamated zinc, B.
The copper vessel and the zinc rod are provided with screws by which
wires may be attached. In the copper vessel is placed a saturated
solution of sulphate of copper, and some crystals of the same substance
are placed on the perforated shelf within the vessel. The porous tube is
filled with diluted sulphuric acid. When the battery is in action the
zinc is dissolved by the sulphuric acid, and metallic copper is
continually deposited upon the internal surface of the copper vessel.
Daniell’s battery, in some form or other, is much used for telegraphs
and for electrotyping. Grove’s cell is shown in section in Fig. 260. The
external vessel is made of a rectangular form in glazed earthenware or
glass. It contains a thick plate of amalgamated zinc, A, A, bent
upwards, and between the two portions a flat porous cell, C, C, is
placed, filled with strong nitric acid, in which is immersed a thin
sheet of platinum. The outside vessel is charged with water, mixed with
about ⅛th of sulphuric acid. D represents a battery of four such cells,
in which the mode of connecting the platinum of one to the zinc of the
next may be noticed. The terminal platinum and zinc form the _poles_ of
the battery, and to them the wires are attached which convey the
current. The substitution of plates of coke for the platinum gives the
form of battery known as Bunsen’s, which is also sometimes made with
circular cells. Gover’s and Bunsen’s are much more powerful arrangements
than Daniell’s, but the latter has the advantage as regards the duration
and uniformity of its action.
[Illustration:
FIG. 261.—_Wire ignited by electricity._
]
When the current produced by a battery of a dozen or more such cells is
conveyed by a wire, it is observed that this wire becomes sensibly hot,
and, if the wire be thin enough, the heat may be sufficiently great to
heat the wire to redness. By stretching a piece of platinum wire between
two separate rods which convey the current, as represented in Fig. 261,
the length of wire through which the current passes may be adjusted so
as to give any required amount of light, and the wire may even be heated
to the fusing-point of platinum. This property of electricity has some
interesting applications, as, for example, in firing mines and other
explosive charges, and in some surgical operations. A still more
interesting exhibition of heating and luminous effects is observed when
the terminals of a battery of many cells are connected with two rods of
coke, or gas-retort carbon. When the pointed ends of the rods are
brought into contact, the current passes, and the points begin to glow
with an intensely bright light, and if they are then separated from each
other by an interval of ⅒th of an inch or more, according to the power
of the battery, a luminous arc extends between them, emitting so intense
a light that the unprotected eye can hardly support it. This luminous
arc is called the _voltaic arc_, and it excels all other artificial
lights in brilliancy, a fact due to the extremely high temperature to
which the carbon particles are heated, the temperature being, perhaps,
the highest we can attain. It must not be supposed that in this
brilliant light we see electricity: the light is due to the same cause
as the light of a candle or gas flame, namely, incandescent particles of
solid carbon. These particles are carried from one carbon point to the
other, and it is found that the positive pole rapidly loses its
substance, which is partly deposited on the negative pole. But in order
to obtain a steady light, it is requisite to keep the pieces of carbon
at one invariable distance; and therefore the transference of the
material from one pole to the other, and the loss by combustion, must be
compensated by a slow movement of the carbons towards each other.
Several kinds of apparatus are used for this purpose, but they all
depend upon the principle of regulating the motions by the action of an
electro-magnet, formed by the current itself, which becomes weaker as
the carbons are farther apart. The movement is communicated to the
apparatus by clockwork. Duboscq’s electric lantern is shown in Fig. 262,
with enlarged images of the carbon points projected on a screen. The
mechanism of the regulator is contained within the cylindrical box
immediately below the lantern. The supports of both carbons are moved;
that which bears the positive carbon pole being advanced twice as fast
as the other, and thus the light is maintained at the same level, for
the positive carbon wears away twice as fast as the other. The light is
more brilliant when charcoal is used instead of coke, but then it is
necessary to operate in a vacuum, to avoid the combustion of the
charcoal. The voltaic arc has recently been applied to illuminate
lighthouses, and for other purposes, and will probably soon be more
widely employed, for a cheap and convenient mode of producing a uniform
current of electricity has recently been discovered and will be
presently described.
[Illustration:
FIG. 262.—_Duboscq’s Electric Lantern and Regulator._
]
[Illustration:
FIG. 263.—_Decomposition of Water._
]
The current which is maintained by the chemical action taking place in
the cells of the battery can also be made to do chemical work outside of
the battery. When the poles of the battery are terminated by wires or
plates of platinum, and these are plunged into water acidulated with
sulphuric acid, bubbles of gas are seen to rise rapidly from each wire,
or _electrode_, as it is termed. Fig. 263 shows an arrangement by which
these gases may be collected separately, and examined, by simply placing
over each electrode an inverted glass tube, filled also with the
acidulated water. The gases collect at the tops of the tubes, displacing
the water, and it is found that from the wire connected with the zinc
end of the battery, or negative electrode, hydrogen gas is given off,
while at the positive electrode oxygen gas is liberated, in volume
precisely equal to half that of the hydrogen. This being the proportion
in which these two substances combine to produce water, it appears that
in the passage of the current a certain quantity of water is decomposed;
and the quantity thus decomposed is in reality a measure of the current,
all the other effects of which are found to be proportional to this.
When the electricity in a current is said to be measured, it is simply
the power of the current to deflect a magnet, or the quantity of gas it
can liberate, or some other such effect, which is in fact measured. The
discharge of a Leyden jar through such an apparatus as that represented
in Fig. 263 would present no perceptible decomposition of the water; yet
such a discharge passed through the arms and body produces, as everybody
knows, a painful shock, and is accompanied by a bright spark and a
noise, while the simultaneous contact of the fingers with the positive
and negative poles of the galvanic battery occasions neither shock nor
spark. Thousands of discharges from large jars must be passed through
acidulated water to liberate the amount of gas which a battery current
of a second’s duration will produce. The electricity of the jar is often
spoken about as having a higher _tension_ than that of the battery, but
the latter sets an immensely greater quantity of electricity in motion.
The idea may be illustrated thus: Suppose we have a small cistern of
water placed at a great height, and that this water could fall to the
ground in one mass. The fall of the small quantity from a great height
would be capable of producing very marked instantaneous effects, such as
smashing, as with a blow, any structure upon which it might fall. This
would correspond with the small quantity of electricity which passes in
the discharge of a Leyden jar. Contrast this with the case in which we
allow a very large quantity of water to descend from a very small
height—as when the water of a reservoir is flowing down a gently
inclined channel. It is plain that a different kind of effect might be
produced in this case; the current might be made, for instance, to turn
a water-wheel, which the more forcible impact of the small quantity of
water in the case first supposed would have broken into pieces.
[Illustration:
FIG. 264.—_Electro-plating._
]
It is probable that the apparent decomposition of water by the electric
current is in reality a secondary effect, and that it is the sulphuric
acid which is decomposed. When, instead of acidulated water, we place in
the apparatus a solution of sulphate of copper, it is found that
metallic copper is deposited on the negative electrode, and sulphuric
acid collects at the positive electrode. The metal is deposited in a
firm and coherent state, and the useful applications of this deposition
of metals are of great interest and importance. For, in a similar
manner, gold, silver, lead, zinc, and other metals may be made to form
thin uniform layers over any properly prepared surface. The immense
advantages which the arts have derived from electro-plating illustrate
in a convincing manner the benefits which physical science can confer on
society at large.
The process of electro-plating may be practised by the aid of apparatus
of very simple character. Fig. 264 shows all that is necessary for
obtaining perfect casts in copper of seals, small medals, &c. A A is a
section of a common tumbler; B B is a tube, made by rolling some brown
paper round a ruler, uniting the edge with sealing-wax, and closing the
bottom by a plug of cork, round which the paper may be tied by a string,
or in any other convenient manner. The tumbler contains a solution of
sulphate of copper, and the tube is filled with water, to which about
one-twentieth of its bulk of sulphuric acid has been added. A strip of
_amalgamated_ zinc, or a piece of thick amalgamated zinc wire, is placed
in the tube, and a piece of copper bell-wire is twisted round the top of
it, and has attached to its other extremity, and immersed in the copper
solution, the article which is to be covered with copper. We may suppose
that this is to be a cast in white wax or in plaster of one side of a
medal. The cast is carefully covered with black lead by means of a soft
brush, and the copper wire is inserted in such a manner as to be in
contact with the black lead at some part. When the apparatus has been
left for some hours in the position represented, a deposit of copper
will be found over the blackleaded surface, and it will be a perfect
impression of the wax cast.
Such a copper cast, or any article in copper having a perfectly clean
surface, can be readily covered by a film of silver by means of a
similar arrangement, where a solution of cyanide of potassium, in which
some chloride of silver has been dissolved, is made to take the place of
the sulphate of copper. Electro-plating with the precious metals has
become a commercial industry of great importance; and this process has
completely superseded the old plan of covering the metallic article to
be plated with an amalgam of silver or of gold, and then exposing it to
heat, which volatized the mercury, leaving a thin film of gold or of
silver adhering to the baser metal. On the large scale a battery of
several cells is used for electro-plating, and the articles are immersed
in the metallic solutions as the negative poles of the battery; any
required thickness of deposit being given according to the length of the
time they remain. At the works of Messrs. Elkington, of Birmingham,
these operations are conducted on a grand scale. The liquid there
employed for silvering is a solution of cyanide of silver in cyanide of
potassium, and the positive pole is formed of a plate of silver, which
dissolves in proportion as the metal is deposited on the negative pole.
As the charging of batteries is a troublesome operation, and their
action is liable to variations which affect the strength of the
currents, the more uniform, more convenient, and more economical mode of
producing currents by magneto-electricity, which will presently be
described, has been to a great extent substituted for the voltaic
battery.
[Illustration:
FIG. 265.—_A Current producing a Magnet._
]
The wire conveying a current not only affects a magnetic needle in the
manner already described, but itself possesses magnetic properties, of
which, indeed, its action on the needle is the result and the
indication. If such a wire be plunged into iron filings, it will be
found that the filings are attracted by it: they cling in a layer of
uniform thickness round its whole circumference and along its whole
length, and the moment the connection with the battery is broken they
drop off. This experiment shows that every part of the wire conveying a
current is magnetic, and it may be proved that the action is not
intercepted by the interposition of any non-magnetic material. Thus the
action of the wire upon the magnetic needle takes place equally well
through glass, copper, lead, or wood. Consequently, if we cover the wire
with a layer of gutta-percha, or over-spin it with silk or cotton, we
shall obtain like results on our filings, and if we coil the covered
wire round a bar of iron, while the non-conducting covering of the wire
will compel the current to circulate through all the turns of the coil,
it will not interfere with the magnetic action on each particle of the
bar. Whenever this is done it is found that the iron is converted into a
powerful magnet so long as the current passes. Fig. 265 represents in a
striking manner the result when the current is made to circulate through
numerous convolutions of the wire; and as each turn adds its effect to
that of the rest, magnets of enormous strength may be formed by
sufficiently increasing the number of the turns. The end of the iron bar
is shown projecting from the axis of the coil, and below it is placed a
shallow wooden bowl, containing a number of small iron nails. The
instant the battery connection is completed these nails leap up to the
magnetic pole, and group themselves round it in the manner shown in the
cut; and again, when the current is interrupted, the iron reverts to its
ordinary condition, the magnetism vanishes, and the nails drop down in
an instant. These effects may be produced again and again, as often as
the current flows and is broken. A magnet so produced is called an
_electro-magnet_, to distinguish it from the ordinary permanent steel
magnets. By coiling the conducting wire round a bar of iron which has
been bent into the form of a horse-shoe, very powerful magnets may be
produced, and enormous weights may be supported by the force of the
magnetic attraction so evoked. Fig. 266 represents the apparatus for
experiments of this kind, in which weights exceeding a ton can be
sustained.
[Illustration:
FIG. 266.—_An Electro-magnet._
]
Here, then, we have a striking instance of the subtile agent
electricity, evoked by the contact of a few pieces of zinc with dilute
acid, showing itself capable of exerting an enormous mechanical force.
Engines have been constructed in which this force is turned to account
to produce rotatory motion as a source of power. Such engines have
certain advantages for special purposes; but the money cost for
expenditure of material for power so obtained is, at least, sixty times
greater than in the case of the steam engine. It is, however, in
producing mechanical effects at a distance that the electric current
finds the most interesting practical application of its magnetic
properties. These are the actions which are so extensively utilized in
the construction of telegraphic instruments, of clocks regulated by
electric communication with a standard time-keeper, and of many
ingenious self-registering instruments. The telegraph will be described
in the next article, and we shall also have occasion in subsequent
articles to describe some of the other applications of electro-magnetic
and electro-chemical force.
_INDUCED CURRENTS._