Dionysius Lardner.

Hand-book of natural philosophy and astronomy (Volume 2) online

. (page 42 of 45)
Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 42 of 45)
Font size
QR-code for this ebook

1 . A power to develop the electric fluid continuously, and in
the necessary quantity.

2. A power to convey it to any required distance without
being injuriously dissipated.

3. A power to cause it, after arriving at such distant point,
to make written or printed characters, or some sensible signs
serving the purpose of such characters.

The apparatus from which the moving power by which these
effects are produced is derived, is the voltaic pile or galvanic
battery. This is to the electric telegraph what a boiler is to a
steam engine. It is the generator of the fluid by which the
action of the machine is produced and maintained.

We have therefore first to explain how the electric fluid,


generated in the apparatus just explained, can be transmitted
to a distance without being wasted or dissipated in any inju-
rious degree en route.

If tubes or pipes could be constructed with sufficient facility
and cheapness, through which the subtle fluid could flow, and
which would be capable of confining it during its transit, this
object would be attained. As the galvanic battery is analogous
to the boiler, such tubes would be analogous in their form and
functions to the steam-pipe of a steam engine.

2128. Conducting wires. If a wire, coated with a non-con-
ducting substance capable of resisting the vicissitudes of
weather, were extended between any two distant points, one
end of it being attached to one of the extremities of a galvanic
battery, a stream of electricity would pass along the wire
provided the other end of the wire were connected by a con-
ductor with the other extremity of the battery.

To fulfil this last condition, it was usual, when the electric
telegraphs were first erected, to have a second wire extended
from the distant point back to the battery in which the electri-
city was generated. But it was afterwards discovered that the
EARTH ITSELF was the best, and by far the cheapest and most
convenient, conductor which could be used for this returning
stream of electricity. Instead, therefore, of a second wire, the
extremity of the first, at the distant point to which the current
is sent, is attached to a large metallic plate, measuring five or
six square feet, which is buried in the earth. A similar plate,
connected with the other extremity of the battery, at the station
from which the current is transmitted, is likewise buried in the
earth, and it is found that the returning current finds its way
back through the earth from the one buried plate to the other
buried plate.

The manner in which the wires are carried from station to
station is well known. Every one is familiar with the lines of
wire extended along the side of the railways. These wires are
generally galvanised so as to resist oxydation, and are of suffi-
cient thickness to bear the tension to which they are submitted.
They are suspended on posts, erected at intervals of sixty yards,
being at the rate of thirty to a mile.

To each of these poles are attached as many tubes or rollers
of porcelain or glass as there are wires to be supported. Each
wire passes through a tube, or is supported on a roller ; and the


material of the tubes or rollers being among the most perfect of
the class of non-conducting substances, the escape of the elec-
tricity at the points of contact is impeded.

In some cases, as, for example, in the streets of London, it is
found inconvenient to carry the wires on elevated posts. In,
such cases they are wrapped with cotton thread, coated with a
mixture of tar, resin, and grease, which produces a good non-
conductor, or are surrounded with gutta percha, which is better
still, are packed together in a leaden or other metallic pipe, and
are then buried in the ground.

2129. Telegraphic signs. The current being by these means
transmitted instantaneously from any station to another, con-
nected with it by such conducting wires, it is necessary to select
among the many effects which it is capable of producing, such
as may be fitted for telegraphic signs.

There are a great variety of properties of the current which
supply means of accomplishing this. If it can be made to
affect any object in such a manner as to cause such object to
produce any effect sensible to the eye, the ear, or the touch,
such effect may be used as a sign; and if it be capable of being
varied, each distinct variety of which it is susceptible may be
adopted as a distinct sign. Such signs may then be taken as
signifying the letters of the alphabet, the digits composing
numbers, or such single words as are of most frequent occur-

The rapidity and precision of the communication will depend
on the rate at which such signs can be produced in succession,
and on the certainty and accuracy with which their appearance
at the place of destination will follow the action of the pro-
ducing cause at the station from which the despatch is trans-

These preliminaries being understood, it remains to show
what effects of the electric current are available for this

These effects are :

I. The power of the electric current to deflect a magnetic
needle from its position of rest.

n. The power of the current to impart temporary magnetism
to soft iron.

III. The power of the current to decompose certain chemical


2130. Signs made with the needle system. Let us now see
how these three properties have been made instrumental to the
transmission of intelligence to a distance.

We have explained how a magnetic needle over which an
electric current passes will be deflected to the right or to the
left, according to the direction given to the current. Now, it
is always easy to give the current the one direction or the other,
or to suspend it altogether, by merely changing the end of the
galvanic trough with which the wires are connected, or by
breaking the contact.

A person, therefore, in London, having command over the
end of a wire which extends to Edinburgh, and is there con-
nected with a magnetic needle, in the manner already de-
scribed, can deflect that needle to the right or to the left at

Thus a single wire and a magnetic needle are capable of
making at least two signals.

By repeating the same signals a greater or less number of
times, and by variously combining them, signs may be multi-
plied ; but it is found more convenient to provide two or more
wires affecting different needles, so as to vary the signs by
combination, without the delay attending repetition.

Such is, in general, the nature of the signals adopted in the
electric telegraphs in ordinary use in England, and in some
other parts of Europe.

It may aid the conception of the mode of operation and com-
munication if we assimilate the apparatus to the dial of a clock
with its two hands. Let us suppose that a dial, instead of
carrying hands, carried two needles, and that their north poles,
when quiescent, both pointed to twelve o'clock. When the
galvanic current is conducted under either of them, the north
pole will turn either to three o'clock or to nine o'clock, according
to the direction given to the current.

Now, it is easy to imagine a person in London governing the
hands of such a clock erected in Edinburgh, where their indi-
cations might be interpreted according to a way previously
agreed upon. Thus, we may suppose that when the needle
No. 1. turns to nine, the letter A is expressed ; if it turn to
three, the letter B is expressed. If the needle No. 2. turn to
nine o'clock, the letter c is expressed ; if it turn to three, the
letter D. If both- needles are turned to nine, the letter E is


expressed ; if both to three, the letter F. If No. 1. be turned
to nine, and No. 2. to three, the letter G is expressed ; if No. 2.
be turned to nine, and No. 1. to three, the letter H, and so

2131. Telegraphs operating by an electro-magnet. Tele-
graphs depending on the second and third principles adverted
to above, have been brought into extensive use in America, the
needle system being in no case adopted there.

The power of imparting temporary magnetism to soft iron by
the electric current has been applied in the construction of tele-
graphs in a great variety of forms ; and indeed it may be stated
generally that there is no form of telegraph whatever in which
the application of this property can be altogether dispensed

To explain the manner in which it is applied, let us suppose
the conducting wire at the station of transmission, London for
example, to be so arranged that its connexion with the voltaic
battery may, with facility and promptitude, be established and
broken at the will of the agent who transmits the despatch.
This may be effected by means of a small lever acting like the
key of a pianoforte, which being depressed by the finger,
transmits the current. The current may thus be transmitted
and suspended in as rapid alternation as the succession of notes
produced by the action of the same key of a pianoforte.

At the station to which the despatch is transmitted, Edin-
burgh for example, the conducting wire is coiled spirally round
a piece of soft iron, which has no magnetic attraction so long
as the current does not pass along the wire, but which acquires
a powerful magnetic virtue so long as the current passes. So
instantaneously does the current act upon the iron, that it may
be made alternately to acquire and lose the magnetic property
several times in a second.

Now let us suppose this soft iron to be placed under an iron
lever, like the key of a pianoforte, so that when the former has
acquired the magnetic property, it shall draw this key down as
if it were depressed by the finger, and when deprived of the
magnetic property, it will cease to attract it, and allow it to
recover its position of rest. It is evident in this case that
movements would be impressed by the soft iron, rendered mag-
netic, on the key at Edinburgh simultaneous and exactly
identical with the movements impressed by the finger of the
u 2


agent upon the key in London. In fact, if the key in Edin-
burgh were the real key of a pianoforte, the agent in London
could strike the note and repeat it as often and with such in-
tervals as he might desire.

This lever at Edinburgh, which is worked by the agent in
London, may, by a variety of expedients, be made to act upon
other moveable mechanism, so as to make visible signals, or to
produce sounds, to ring a bell or strike a hammer, or to trace
characters on paper by means of a pen or pencil, so as actually
to write the message, or to act upon common moveable type so
as to print it. In fine, having once the power to produce a
certain mechanical effect at a distant station, the expedients are
infinitely various, by which such mechanical effect may be
made subservient to telegraphic purposes.

2132. Morse's system. The telegraph of Morse, extensively
used in the United States, affords an example of this. To compre-
hend its mode of operation, let us suppose the lever on which
the temporary magnet acts to govern the motion of a pencil or
style under which a ribbon of paper is moved with a regulated
motion by means of clockwork. When the current passes, the
style is pressed upon the paper, and when the current is sus-
pended, it is raised from it. If the current be maintained for
an interval more or less continued, the style will trace a line on
the ribbon, the length of which will be greater or less according
to the duration of the current. If the current be maintained
only for an instant, the style will merely make a dot upon the
ribbon. Lines, therefore, of varying lengths, and dots sepa-
rated by blank spaces, will be traced upon the ribbon of paper
as it passes under the style, and the relative lengths of these
lines, their combinations with each other and with the dots,
and the lengths of the blank intervening spaces, are altogether
under the control of the agent who transmits the despatch.

It is easy to imagine how a conventional alphabet may be
formed by such combinations of lines and dots.

Provisions are made, so that the motion of the paper does not
begin until the message is about to be commenced, and ceases
when the message is written. This is easily accomplished.
The cylinders which conduct the band of paper are moved by
wheel-work and a weight properly regulated. The motion is
imparted by a detent detached by the action of the magnet,
which stops the motion when the magnet loses its virtue.


2133. Electro-chemical telegraphs. The following descrip-
tion of the telegraph of Mr. Bain will convey some idea of the
general principle on which all forms of electro-chemical tele-
graphs are based :

Let a sheet of writing paper be wetted with a solution of prussiate of
potash, to which a little nitric and hydrochloric acid have been added. Let
a metallic desk be provided corresponding in magnitude with the sheet of
paper, and let this desk be put in communication with a galvanic battery so
as to form its negative pole. Let a piece of steel or copper wire forming a
pen be put in connection with the same battery so as to form its positive pole.
Let the sheet of moistened paper be now laid upon the metallic desk, and let
the steel or copper point which forms the positive pole of the battery be
brought into contact with it. The galvanic circuit being thus completed,
the current will be established, the solution with which the paper is wetted
will be decomposed at the point of contact, and a blue or brown spot will
appear. If the pen be now moved upon the paper, the continuous succession
of spots will form a blue or brown line, and the pen being moved in any
manner upon the paper, characters may be thus written upon it as it were in
blue or brown ink.

In this manner, any kind of writing may be inscribed upon the paper, and
there is no other limit to the celerity with which the characters may be
written, save the dexterity of the agent who moves the pen, and the suffi-
ciency of the current to produce the decomposition of the solution in the
time which the pen takes to move over a given space of the paper.

The electro-chemical pen, the prepared paper, and the metallic desk
being understood, we shall now proceed to explain the manner in which a
communication is written at the station where it arrives.

The metallic desk is a circular di&k, about twenty inches in diameter. It
is fixed on a central axis, with \\ hich it is capable of revolving in its own
plane. An uniform movement of rotation is imparted to it by means of a
small roller, gently pressed against its under surface, and having sufficient
adhesion with it to cause the movement of the disk by the revolution of the
roller. This roller is itself kept in uniform revolution by means of a train
of wheel -work, deriving its motion either from a weight or main spring, and
regulated by a governor or fly. The rate at which the disk revolves may
be varied at the discretion of the superintendent, by shifting the position of
the roller towards the centre ; the nearer to the centre the roller is placed,
the more rapid will be the motion of rotation. The moistened paper being
placed on this disk, we have a circular sheet kept in uniform revolution.

The electro-chemical pen, already described, is placed on this paper at a
certain distance from its centre. This pen is supported by a pen-holder,
which is attached to a fine screw extending from the centre to the circum-
ference of the desk in the direction of one of its radii.

On this screw is fixed a small roller, which presses on the surface of the
desk, and has sufficient adhesion with it to receive from it a motion of
revolution. This roller causes the screw to move with a slow motion in a
direction from the centre to the circumference, carrying with it the electro-
chemical pen. We have thus two motions, the circular motion carrying the
moistened paper which passes under the pen, and the slow rectilinear motion
of the pen itself directed from the centre to the circumference. By the
combination of these two motions, it is evident that the pen will trace upon
the paper a spiral curve, commencing at a certain distance from the centre,
u 3


and gradually extending towards the circumference. The intervals between
the successive coils of this spiral line will be determined by the relative
velocities of the circular disk, and of the electro-chemical pen. The relation
between these velocities may likewise be so regulated, that the coils of the
spiral may be as close together as is consistent with the distinctness of the
traces left upon the paper.

Now, let us suppose that the galvanic circuit is completed in the manner
customary with the electric telegraph, that is to say, the wire which termi-
nates at the point of the electro-chemical pen is carried from the station of
arrival to the station of departure, where it is connected with the galvanic
battery, and the returning current is formed in the usual way by the earth
itself. When the communication between the wire and the galvanic battery
at the station of departure is established, the current will pass through the
wire, will be transmitted from the point of the electro-chemical pen to the
moistened paper, and will, as already described, make a blue or brown line
on this paper. If the current were continuous and uninterrupted, this line
would be an unbroken spiral, such as has been already described; but if the
current be interrupted at intervals, during each such interval, the pen will
cease to decompose the solution, and no mark will be made on the paper.
If such interruption be frequent, the spiral, instead of being a continuous
line, will be a broken one, consisting of lines interrupted by blank spaces.
If the current be allowed to act only for an instant of time, there will be a
blue or brown dot upon the paper ; but if it be allowed to continue during
a long interval, there will be a line.

Now, if the intervals of the transmission and suspension of the current
be regulated by any agency in operation at the station of departure, lines
and dots corresponding precisely to these intervals, will be produced by the
electro-chemical pen on the paper, and will be continued regularly along the
spiral line already described. It will be evident, without further explanation,
that characters may thus be produced on the prepared paper corresponding
to those of the telegraphic alphabet already described, and thus the language
of the communication will be written in these conventional symbols.

There is no other limit to the celerity with which a message may be thus
written, save the sufficiency of the current to effect the decomposition while
the pen passes over the paper, and the power of the agency used at the
station of departure to produce, in rapid succession, the proper intervals in
the transmission and suspension of the current.

But the prominent feature of this system is the extraordinary celerity of
which it is susceptible. In an experiment performed by M. Le Verrier and
Dr. Lardner before Committees of the Institute and the Legislative Assembly
at Paris, despatches were sent a thousand miles, at the rate of nearly 20,000
words an hour.*

Lardner on the Great Exhibition, p. 89. et seq.




2134. Conditions on which calorific power of current de-
pends, When a voltaic current passes over a conductor, an
elevation of temperature is produced, the amount of which will
depend on the quantity and intensity of the electricity trans-
mitted, upon the conductability of the material composing the
conductor, and upon the magnitude of the space which it offers
for the passage of the electric fluid. Although the conditions
which determine this development of heat are not ascertained
with much certainty or precision, it may be stated generally
that the quantity of heat produced is augmented with the
quantity of the fluid transmitted, and the obstacles opposed to
its passage. A given current, therefore, will develop less heat
on good than on bad conductors, and on those having a large
than on those having a small transverse section.

The development of heat, so far as it depends on the current
itself, appears to increase in a much larger ratio with the
quantity, than with the intensity, of the electric fluid. Thus,
while it is greatly augmented by increasing the extent of sur-
face of the elements of the pile, it is very little affected by
augmenting their number. For a like reason it is greatly
augmented by selecting the elements from the extremes of the
electromotive series (1847), since in that case the quantities
of electricity developed are increased in proportion to the electro-
motive energy of the exciting surfaces.

When a voltaic current of a certain intensity passes along a
metallic wire, the wire becomes heated. If the intensity of the
current be increased, the wire will become incandescent, and
will, by a further increase of the force of the current, be fused,
or burned.

The same current which will produce only a slight elevation
of temperature upon a wire of a certain diameter, will render a
finer wire incandescent, and will fuse or burn one which is still

2135. Calorific effects : Hare's and Children's deflagrators.

U 4


The calorific power of a battery depending chiefly on the extent
of the heating surface, and the electromotive energy of its ele-
ments, those forms which, within a given volume, present the
most extensive surfaces, such as Hare's spiral arrangement
(1861), and others, on a like principle, contrived by Children
and Strating, and denominated deftagrators, and the systems
of Grove (1865) and Bunsen (1866), in which platinum or
carbon is combined with zinc, and excited by two fluids, are
the most efficient. With piles of the latter kind, consisting of
ten to twenty pairs, the development of heat is so considerable
that substances which resist the most powerful blast-furnaces
are easily fused and burned. Extraordinary effects are pro-
duced by this calorific agency. Metallic wire, submerged in
water, is rendered incandescent, and may be fused either in
vacuo or in an atmosphere of any gas, such as azote or carbonic
acid, which is not a supporter of combustion.

2136. Wollastoris thimble battery. A combination thus

designated, which has acquired a sort of
z ^ historical scientific interest, is represented
in its actual size in fig. 664.

A strip of thin silver or platinum leaf
p, is bent double, so as to include between
its folds a plate z of amalgamated zinc,
|P the distance between the surfaces being
about the twentieth of an inch. The sur-
faces are kept separated by small bits of
cork thrust between them. Two short
copper wires, one z, attached to the zinc,
and the other, p, to the platinum, form the

poles of the combination, and when these are connected by a wire
a voltaic current will pass from p to z. Each of these polar
wires has a fine slit made in it, into which an extremely fine
platinum wire is inserted, extending between z and p, the inter-
vening length between these points being not more than the
tenth of an inch. The wire handle h is provided, to enable
the operator to immerse the apparatus in a wine-glass, contain-
ing a solution composed of three parts of water and one of sul-
phuric acid. The current which will then be established upon
the wire between p and z will render it incandescent.

2137. Experimental illustration of the conditions which
effect calorific power of a current. If the poles of a powerful


battery be connected by iron or platinum wire from two to
three feet in length, the metal will become incandescent. If its
length or thickness be diminished it will fuse or burn. If its
length or thickness be increased it will acquire first a darker
degree of incandescence, and then will be only heated without
being rendered luminous. The same current which will render
iron or platinum wire incandescent or fuse it, will only raise
the temperature of silver or copper wire of the same length and
thickness without rendering it incandescent. If, on the other
hand, the iron or platinum be replaced by tin or lead of much
greater length or thickness, these metals will be readily fused
by the same current.

These phenomena are explained by the different conducta-

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 42 of 45)