electricity has been generated in sufficient quantity and under
suitable conditions to prevent its dispersion, a spark of intense
brilliancy and greater or less length, according to the amount of
electricity thus collected, can be obtained when some body, not
similarly electrified, is brought near to what is called the conductor
of the machine. The old-fashioned explanation, still repeated in many
of our books, ran somewhat as follows:--'The positive electricity of
the conductor decomposes the neutral or mixed fluid of the body,
attracting the negative fluid and repelling the positive. When the
tension of the opposite electricities is great enough to overcome the
resistance of the air, they re-combine, the spark resulting from the
heat generated in the process of their combination.' This explanation
is all very well; but it assumes much that is in reality by no means
certain, or even likely. All we _know_ is, that whereas before the
spark is seen the electrical conditions of the conductor and the object
presented to it were different, they are no longer different after the
flashing forth of the spark. It is as though a certain line (straight,
crooked, or branched) in the air had formed a channel of communication
by which electricity had passed, either from the conductor to the
object or from the object to the conductor,--or _possibly_ in both
directions, two different kinds of electricity existing (before
the flash) in the conductor and the object, as the old-fashioned
explanation assumes.[23] Again, we know that the passage of electricity
along the air-track supposing there really is such a passage, but in
any case the observed change in the relative electrical conditions of
the conductor and the object, is accompanied by the generation of an
intense heat along the aërial track where the spark is seen.
In the case of electricity generated by means of a galvanic battery,
we do not note the same phenomena unless the battery is a strong one.
We have in such a battery a steady source of electricity, but unless
the battery is powerful, the electricity is of low intensity, and
not competent to produce the most striking phenomena of frictional
electricity. For instance, voltaic electricity, as used in telegraphic
communication, is far weaker than that obtained from even a small
electrical machine. What is called the positive extremity of the
battery neither gives a spark, nor attracts light bodies. The same
is true of the other, or negative extremity. The difference of the
condition of these extremities can only be ascertained by delicate
tests--the deflections of the needle, in fact, by which telegraphic
communications are made, may in reality be regarded as the indications
of a very delicate electro-cope.
But when the strength of a galvanic battery is sufficiently great,
or, in other words, when the total amount of chemical action brought
into play to generate electricity is sufficient, we obtain voltaic
electricity not only surpassing in intensity what can be obtained from
electrical machines, but capable of producing spark after spark in a
succession so exceedingly rapid that the light is to all intents and
purposes continuous.
Without considering the details of the construction of a galvanic
battery, which would occupy more space than can here be spared, and
even with fullest explanation would scarcely be intelligible (except to
those already familiar with the subject), unless illustrations unsuited
to these pages were employed, let us consider what we have in the case
of every powerful galvanic battery, on whatever system arranged. We
have a series of simple batteries, each consisting of two plates of
different metal placed in dilute acid. Whereas, in the case of a simple
battery, however, the two different metals are connected together by
wires to let the electric current pass (the current ceasing to pass
when the wires are disconnected), in a compound battery, in which (let
us say) the metals are zinc and copper, the zinc of one battery is
connected with the copper of the next, the zinc of this with the copper
of another, and so on, the wire _to_ the copper of the first battery
and the wire _from_ the zinc of the last battery being free, and
forming what are called the poles of the compound battery--the former
the positive pole, the later the negative pole.[24] When these free
wires are connected, the current of electricity passes, when they are
disconnected the current ceases to pass, unless the break between them
is such only that the electricity can, as it were, force its way across
the gap. When the wires are connected, so that the current flows, it
is as though there were a channel for some fluid which flowed readily
and easily along the channel. When the circuit is absolutely broken,
it is as though such a channel were dammed completely across. If,
however, while the poles are not connected by copper wires or by other
freely conducting substances, yet the gap is such as the electricity
can pass over, the case may be compared to the partial interruption
of a channel at some spot where, though the fluid which passes freely
along the channel is not able to move so freely; it can yet force its
way along, with much disturbance and resistance. Just as at such a part
of the course of a liquid stream--say, a river--we find, instead of
the quiet flow observed elsewhere, a great noise and tumult, so, where
the current of electricity is not able to pass readily we perceive
evidence of resistance in the generation of much heat and light (if the
resistance is great enough).
It will be observed that I have spoken in the preceding paragraph
of the passage of a current along the wire connecting the two poles
of a powerful electric battery, or along any substance connecting
those poles which possesses the property of being what is called a
good conductor of electricity. But the reader is not to assume that
there is such a current, or that it is known to flow either from the
positive to the negative pole, or from negative to positive pole; or,
again, that, as some have suggested, there are two currents which flow
simultaneously in opposite directions. We speak conventionally of the
current, and for convenience we speak as though some fluid really
made its way (when the circuit is complete) from the positive to the
negative pole of the compound battery. But the existence of such a
current, or of any current at all, is purely hypothetical. I should
be disposed, for my own part, to believe that the motion is of the
nature of wave-motion, with no actual transference of matter, at least
when the circuit is complete. According to this view, where resistance
takes place we might conceive that the waves are converted into
rollers or breakers, according to the nature of the resistance--actual
transference of matter taking place through the action of these changed
waves, just as waves which have traversed the free surface of ocean
without carrying onward whatever matter may be floating on the surface,
cast such matter ashore when, by the resistance of the shoaling bottom
or of rocks, they become converted either into rollers or into breakers.
I may also notice, with regard to good conductors and bad conductors
of electricity, that they may be compared to substances respectively
transparent and opaque for light-waves, or again, to substances which
allow heat to pass freely or the reverse. Just as light-waves fail to
illuminate a transparent body, and heat-waves fail to warm a body which
allows them free passage, so electricity-waves (if electricity really
is undulatory, as I imagine) fail to affect any substance along which
they travel freely. But as light-waves illuminate an opaque substance,
and heat-waves raise the temperature of a substance which impedes
their progress, so waves of electricity, when their course is impeded,
produce effects which are indicated to us by the resulting heat and
light.
A powerful galvanic battery is capable of producing light of intense
brilliancy. For this purpose, instead of taking sparks between the
two metallic poles, each of these is connected with a piece of carbon
(which is nearly as good a conductor as the metal), and the sparks are
taken between these two pieces of carbon, usually set so that the one
connected with the negative pole is virtually above the one connected
with the positive pole, and at a distance of a tenth of an inch from
each other or more, according to the strength of the battery. Across
this gap between the carbons an arc of light is seen, which in reality
results from a series of electric sparks following each other in rapid
succession. This arc, called the voltaic arc, is brilliant, but it is
not from this arc that the chief part of the light comes. The ends
of the carbon become intensely bright, being raised to a white heat.
Both the positive and negative carbons are fiercely heated, but the
positive is heated most. As (ordinarily) both carbons are thus heated
in the open air, combustion necessarily takes place, though it is to be
noticed that the lustre of the carbons is not due to combustion, and
would remain undiminished if combustion were prevented. The carbons
are thus gradually consumed, the positive nearly twice as fast as
the negative. If they are left untouched, this process of combustion
soon increases the distance between them till it exceeds that which
the electricity can pass over. Then the light disappears, the current
ceasing to flow. But by bringing the carbon points near to each other
(they must, indeed, be made to touch for an instant), the current is
made to flow again, and the light is restored.
The following remarks by M.H. Fontaine (translated by Dr. Higgs) may
help to explain the nature of the voltaic arc:--'In truth, the voltaic
arc is a portion of the electric circuit possessing the properties of
all other parts of the same circuit. The molecules swept away from
point to point (that is, from one carbon end to the other) 'constitute
between these points a mobile chain, more or less conductive, and
more or less heated, according to the intensity of the current and
the nature and separation of the electrodes' (that is, the quality
and distance apart of the carbon or other substances between which
the arc is formed). 'These things happen exactly as if the electrodes
were united by a metallic wire or carbon rod of small section' (so as
to make the resistance to the current great), 'which is but saying
that the light produced by the voltaic arc and that obtained by
incandescence arise from the same cause--that is, the heating of a
resisting substance interposed in the circuit.'
The intensity of the light from the voltaic arc and the carbon points
varies with circumstances, but depends chiefly on the amount of
electricity generated by the battery. A fair idea of its brilliancy,
as compared with all other lights, will be gained from the following
statements:--If we represent the brightness of the sun at noon on a
clear day as 1,000, the brightness of lime glowing under the intense
heat of the oxy-hydrogen flame is about 7; that of the electric
light obtained with a battery of 46 elements (Bunsen's), 235. With a
battery of 80 elements the brightness is only 238. (These results were
obtained in experiments by Fizeau and Foucault.) The intensity does not
therefore increase much with the number of the component elements after
a certain number is passed. But it increases greatly with the surface,
for the experimenters found that with a battery of 46 elements, each
composed of 3, with their zinc and copper respectively united to form
one element of triple surface, the brightness became 385, or more than
one-third of the midday brightness of the sun (that is, the apparent
intrinsic lustre of his disc's surface), and 55 times the brightness of
the oxy-hydrogen lime-light.
Another way of obtaining an intense heat and light from the electric
current generated by a strong battery is to introduce into the
electric circuit a substance of small conducting power, and capable
of sustaining an intense heat without disintegration, combustion, or
melting. Platinum has been used for this purpose. If the conductive
power of copper be represented by 100, that of platinum will be
represented by 18 only. Thus the resistance experienced by a current in
passing through platinum is relatively so great, that if the current
is strong the platinum becomes intensely heated, and shines with a
brilliant light. A difficulty arises in using this light practically,
from the circumstance that when the strength of the current reaches a
certain point, the platinum melts, and the circuit being thus broken,
the light immediately goes out.
The use of galvanic batteries to generate an electric current strong
enough for the production of a brilliant light, is open to several
objections, especially on the score of expense. It may, indeed, be
safely said that if no other way of obtaining currents of sufficient
intensity had ever been devised, the electric light would scarcely
have been thought of for purposes of general illumination, however
useful in special cases. (In the electric lighting of the New Opera
House at Paris, batteries are used.) The discovery by Orsted that an
electric current can make iron magnetic, and the series of discoveries
by Faraday, in which the relation between magnetism and electricity
was explained, made electric lighting practically possible. One of
these shows that if a properly insulated wire coil is rapidly rotated
in front of a fixed permanent magnet (or of a set of such magnets),
currents will be induced in the coil, which may be made to produce
either alternating currents or currents in one direction only, in wire
conductors. An instrument for generating electric currents in this way,
by rapidly rotating a coil in front of a series of powerful permanent
magnets fixed symmetrically around it, is called a magneto-electric
machine. Another method, now generally preferred, depends on the
rotation of a coil in front of an electro-magnet; that is, of a bar
of soft iron (bent in horseshoe form), which can be rendered magnetic
by the passage of an electric current through a coil surrounding it.
The rapid rotation of the coil in front of the soft iron generates
a weak current, because iron always has some traces of magnetism
in it, especially if it has once been magnetised. This weak current
being caused to traverse the coil surrounding the soft iron increases
its magnetism, so that somewhat stronger currents are produced in
the revolving coil. These carried round the soft iron still further
increase its magnetism, and so still further strengthen the current.
In this way coil and magnet act and react on each other, until from
the small effects due to the initial slight magnetism of the iron,
both coil and the magnet become, so to speak, saturated. Machines
constructed on this principle are called dynamo-electric machines,
because the generation of electricity depends on the dynamical force
employed in rapidly rotating the coils.
We need not consider here the various forms which magneto-electric
and dynamo-electric machines have received. It is sufficient that
the reader should recognise how we obtain an electric current of
great intensity in one case from mechanical action and permanent
magnetism,[25] and in the other from mechanical action and the mere
residue of magnetism always present in iron.
In the cases here considered it is in reality the sudden presentation
of the coil (twice at each rotation) before the positive and negative
poles of the magnet, which induces a momentary but intense current
of electricity. The rotation being exceedingly rapid, these currents
succeed each other with sufficient rapidity to be appreciably
continuous. A similar principle is involved in the use of what is
called the inductive coil, except that in this case the sudden
beginning and ceasing of a current in one coil (and not magnetic
action) induces a momentary but strong current: matters are so arranged
that the current induced by the starting of the inducing current,
immediately causes this to cease; while the current induced by the
cessation of the inducing current immediately causes this current
to begin again: so that by a self-acting process we have a constant
series of intense induced currents, succeeding each other with great
rapidity, so as to be practically continuous, as with those produced by
magneto-electric and dynamo-electric machines.
All that I have said about the voltaic arc, the incandescence
resulting from resistance to the current's flow, and so forth, in
relation to electricity generated by galvanic batteries, applies to
electricity generated by induction coils, or by magneto-electric and
by dynamo-electric machines. Only it is to be noticed that in some of
these machines the currents alternate in direction with each revolution
of the swiftly turning coil, in others the currents are always in the
same direction, and in yet others the currents may be made to alternate
or not, as may be most convenient.
We have now to consider how light suitable for purposes of illumination
may be obtained from the electric current. Hitherto we have considered
only light such as might be used for special purposes, where a bright
and very intense light was required, where expense and complexity of
construction might not be open to special objections, and where in
general the absolute steadiness of the light was not an essential
point. But those who have seen the electric light used even by the
most experienced manipulators for the illustration of lectures will
know that the light as so obtained, though of intense brilliancy, is
altogether unsuited for purposes of ordinary illumination.
If we consider a few of the methods which have been devised for
overcoming the difficulties inherent in the problem of electric
lighting, the reader will recognise at once the nature of these
difficulties, and the probability of their being effectually overcome
in the future, for though much has been done, much yet remains to be
done in mastering them.
Let us consider first the Jablochkoff candle, the invention of which
brought about, in July 1877, the first great fall in the value of gas
property.
The Jablochkoff candle consists of two carbons placed side by side
(instead of one above the other in a vertical line). Thus placed,
with a slight interval between them, the carbon rods would allow the
passage of the electric current at the place of nearest approach, and
therefore of least resistance to its passage. A variable and imperfect
illumination would result. M. Jablochkoff, however, interposes between
the separate carbon rods a slip of plaster of Paris, which is a
non-conducting material. The upper points of the carbon rods are thus
the only parts at which the current can cross. They are connected by
a little bridge of carbon, which is necessary for the starting of the
light--just as in the case of the ordinary electric light, the two
carbons must, in order to start the light, be brought into contact.
When the current flows, the small bridge of carbon connecting the two
points is presently consumed, but the arc between the points is still
maintained: for the plaster becomes vitrified by the intense heat of
the two carbon points on each side, and melts down as the carbons are
consumed. If the light is in any way put out, however, a small piece of
carbon must be set again, to form a bridge between the carbon points.
Throughout the burning of the Jablochkoff candle the fused portion
of the insulating layer forms a conducting bridge between the carbon
points; and hence there is a considerable loss of electric force
(probably about thirty per cent.), which in the ordinary arrangement
would increase the intensity of the light. The great advantage of the
candle consists in the circumstance that throughout its consumption
the carbon ends are at a constant distance from each other without any
mechanical or other arrangement being necessary to maintain them in due
position.
One point should be noticed here. In the ordinary arrangement of carbon
points, the positive carbon, as we have already said, is much more
intensely heated, and consumes twice as fast as the negative carbon.
Now, if one carbon of the Jablochkoff candle were connected with the
positive, and the other with the negative pole of the battery or of a
machine, the former side would consume twice as fast as the latter, and
the two points would no longer remain at the same horizontal level,
which is essential to the proper burning of the Jablochkoff candle.
By using a machine which produces alternating currents, M. Jablochkoff
obviates this difficulty, the carbons being alternately positive and
negative (in extremely rapid succession), and therefore consuming at
the same rate.
The Jablochkoff candle lasts only about an hour and a half. But four,
six, or more candles may be used in the same globe or lantern, and
automatic arrangements adopted to cause a fresh candle to be ignited at
the moment when its predecessor is burnt out.
In Paris and elsewhere (as in Holborn, for instance), each Jablochkoff
lamp is enclosed in an opal glass globe. Mr. Hepworth remarks on this,
that in his opinion the use of the opal globe is a mistake, as it
shuts off quite 50 per cent. of the light without any corresponding
advantage, except the correction of the glare. 'This wasteful
disadvantage will no doubt be remedied in the future,' he says, by
the use of some less dense medium. 'Mr. Shoolbred states that from a
series of careful photometric experiments carried out by the municipal
authorities with the Jablochkoff lights, each naked light is found to
possess a maximum intensity of 300 candles. With the opal globe this
was reduced to 180 candles, showing a loss of 40 per cent., while
during the darker periods through which the light passed the light
was as low as 90 candles. It may be mentioned here that Mr. Van der
Weyde, who has long used the electric light for photographic purposes,
has given much attention to the important problem of rendering the
electric light available as an illuminator without wasting it, and yet
without throwing the rays directly upon the object to be illuminated.
The rays are intercepted by an opal disc about four inches in diameter,
and the whole body of the rays is gathered up by a concave reflector
(lined with a white material), and thrown out in a flood of pure white
light, in which the most delicate shades of tint are discernible. He
can use any form of electric candle in this way. Only it should be
noticed, before the employment of his method is advocated for street
illumination, that there is a difference between the problems which
the photographer and the street-lighter have to solve. The Jablochkoff
candle, for instance, must be screened on all sides, and even above,
when used to illuminate the streets. If its direct light is allowed
to escape in any direction, there will be a mischievous and unsightly
beam, and from every point along the path of the beam, the intensely
bright light of the candle will be directly visible. Again: it is
essential that whatever substance is used to screen the light should
be dense enough to cause the whole globe to seem uniformly bright or
nearly so. The only modification which seems available (when these
essential points have been secured) is that the tint of the globe
should be such as to correct any colour which the light may be found to
have in injurious excess. We may, however, remark that the objection
which has been often raised against the colour of the electric light
can hardly be just--the injury to the eyes in certain cases arising
probably from the strong contrast between the light and the background
on which it is projected. For, as to colour, the electric light
derived either from the glowing carbon or from incandescent metal is
appreciably the same as sunlight.