In this episode, we will explore the remarkable contributions made by Michael Faraday
Towards the end of the year 1812, Davy received a letter in which the writer,
a bookbinder’s journeyman named Michael Faraday, expressed a desire to escape from trade,
and obtain employment in a scientific laboratory. With the letter was enclosed a neatly written copy
of notes which the 21-year-old Faraday had made of Davy’s own public lectures.
The great chemist replied and arranged an interview at which he leant
that Faraday had educated himself by reading the volumes which came into his hands for binding,
one was the Encyclopaedia Britannica and the other Mrs Jane Marcet’s Conversations on Chemistry.
At Davy’s recommendation, Faraday was appointed to a post in the laboratory
of the Royal Institution. Here he would remain for the whole of his active life,
first as an assistant, then as director of the laboratories
and from 1833 onwards as the chair of chemistry which was founded for his benefit.
In 1821 when the new field of inquiry was opened by Oersted’s discovery Faraday started to write
a memoir. He was hoping to discover an analogy between the behaviour of electricity as observed
in motion in currents, and the behaviour of electricity at rest on conductors. Static
electricity was known to possess the power of induction. It was believed that electric currents
might also possess this property. The idea was that if in any circuit a current were made to
flow, any adjacent circuit would be traversed by an induced current. Faraday found that this was
not the case. A current was induced but it lasted only for an instant. He realised that the induced
current depended not on the mere existence of the inducing current but on its variation.
Faraday now set himself to determine the laws of induction of currents.
In order to do this, he had to devise a new way of representing the state of a magnetic field.
Prior to this magnetic fields would be illustrated by sprinkling iron filings on a sheet of paper and
then observing the curves created. These curves suggested to Faraday the idea of lines of magnetic
force. He conceived all space to be filled with these. Every line of force is a closed curve,
which in some part of its path passes through the magnet to which it belongs. If we take
any closed curve then we can imagine the lines of force that intersect this curve must form a
tubular surface returning to itself. This tube can tell us the direction of the magnetic intensity,
its magnitude. The cross-section multiplied by the magnitude of any tube remains constant along the
entire length of the tube. Faraday used this to conceive the idea of partitioning all space into
compartments by tubes. For simplicity, each of these tubes may be called a unit line of force.
The strength of the field is then indicated by the separation or concentration of the unit
lines of force so that the number of them which intersect a unit area places at right angles
to their direction at any point measures the intensity of the magnetic field at that point.
Faraday was thinking in terms of lines of force. He discovered that a current
is induced in a circuit either when the strength of an adjacent circuit is altered,
or when a magnet is brought near the circuit or when the circuit itself is moved about in
the presence of another circuit or magnet. He saw that the induction in all cases depended
on the relative motion of the circuit and the lines of the magnetic force in the vicinity.
In 1832 he found that the induction consisted in the production of a definitive
electromotive force, independent of the nature of the wire, and dependent only
on the intersections of the wire and the magnetic curves. This electromotive force
is produced in both closed and open circuits. He also discovered that the electromotive force
was proportional to the number of units lines of magnetic force intersected by the wire per second.
This is the fundamental principle of the induction of currents.
Faraday is undoubtedly entitled to the full honour of its discovery.
It would however be many years before the concepts would be understood by his contemporaries.
Faraday himself was not a mathematician and was not able to address the mathematical physicist
of his own generation in their own language. His idea of expressing the concepts with moving lines
of force repelled many of them who had been trained in the school of Laplace and Poisson.
The discoveries of Oersted, Ampere and Faraday had shown the close connection fo magnetic and
electric science. The connection between static and flowing current was less clear.
In 1833 Faraday showed that every known effect of electricity - physiological, magnetic, luminous,
calorific, chemical and mechanical - may be obtained indifferent either with the
electricity obtained from friction or from a voltaic battery. What Faraday proved was that
the process which goes on in a wire connecting the terminals of a voltaic cell is of the same nature
as the process which for a short time goes on in a wire which is used to discharge a Leyden jar.
What Faraday steered clear of was identifying what the substance of electricity was
actually composed of. In one of his laboratory notes he wrote the following:
After much consideration of the manner in which the electric forces are arranged in the various
phenomena generally, I have come to certain conclusions which I will endeavour to note
down without committing myself to any opinion as to the cause of the electricity. If electricity
exists independently of matter, then I think that the hypothesis of one fluid will not stand against
that of two fluids. There are, I think, evidently what I may call two elements of power, of equal
force and acting towards each other. But these powers may be distinguished only by direction
and maybe no more separate than north and south forces in the elements of a magnetic needle.
They may be the polar points of the forces originally placed in the particles of matter.
Faraday now turned his attention to the relations between chemistry and electricity.
At the time it was generally believed that the decomposition of a solution
through the action of an electric current was due primarily to attractive
and repellent forces exercised on its molecules by the metallic terminals
at which the current enters and leaves the solution. Faraday believed that this
was not the case. Instead, he felt that a chain of decompositions and recombinations
takes place all along the course of the current that flows from one terminal to the other.
He introduced the terms which have ever since been generally used to describe the phenomena
of electrochemical decomposition. To the terminals through which the electric current passes,
he gave the name electrodes. The electrode of high potential, at which oxygen, chlorine,
acids, etc are evolved he gave the name anode, and the electrode of low potential, at which metals,
alkalis and bases are evolved, the cathode. Those bodies which are decomposed directly
by the current he names electrolytes; the parts into which they are decomposed ions, those that
travelled to the anode he called anions and the metallic ions that pass to the cathode, cations.
He discovered that the rate at which an electrolyte is decomposed depends
solely on the intensity of the electric current passing through it, and not at
all on the size of the electrodes or the strength of the solution.
He showed that the electricity which passes through the electrolyte is the exact equivalent
of that which is possessed by the atoms separated by the electrodes; which implies that there is a
certain absolute quantity of the electric power associated with each atom of matter.
In 1837 Faraday reflected on the behaviour of substances like sulphur or oil turpentine when
places between two electrodes. These substances do not conduct electricity and are not decomposed.
If however, they are placed between two metallic plates maintained at a specific
potential difference, then the nature of the insulating substance directly impacts the
charge on either plate. Faraday termed this the specific inductive capacity of the insulator.
The discovery of this property of the insulating substance or dielectrics raised questions as to
whether it could be harmonised with the old ideas of electrostatic action. Faraday put forward the
idea that the behaviour of a dielectric is exactly the same as that of an electrolyte, up to the
point at which the electrolyte breaks down under the electrical stress; a dielectric being in fact,
a body which is capable of sustaining the stress without suffering decomposition.
Thus at the root of Faraday’s conception of electrostatic induction lay the idea that the
whole of the insulting medium through which the action takes place is in a state of polarisation
similar to that which precedes decomposition in an electrolyte.
Insulators can be said to be bodies whose particles can retain the polarised state,
whilst conductors are those whose particles cannot be permanently polarised.
He supposed that in the ordinary or unpolarised condition of a body,
the molecules consist of atoms which are bound to each other by the forces of chemical affinity,
these forces are exerted, though to a lesser degree, between atoms which belong to different
molecules, thus producing the phenomena of cohesion. When an electric field is set up,
a change takes place in the distribution of these forces; some are strengthened and some
are weakened, the effect being symmetrical about the direction of the applied electric force.
He then applied the same idea of magnetic force to this idea. The changes which take place in
an electric field when the dielectric is varied may be very simply described
in terms of lines of force. So when a mass of sulphur is introduced into the field,
the effect is as if the lines of force tend to diminish. By studying the figures of the
lines of force he noticed that they always dispose themselves as if they were subject
to mutual repulsion, or as if the tubes of force had an inherent tendency to dilate.
In 1838 he expanded his theory further by using the analogy originally put forward by Poisson
when he had described magnetisation.
Faraday suggested that the particles of an insulating dielectric whilst under induction
may be compared to a series of small magnetic needles, or more correctly still, a series of
small insulated conductors. Once discharged they would return to their normal state.
This would allow Faraday to finally come up with the fundamental equations of electrostatics
as modified to take into account the effect of the specific inductive capacity.
The conception of action propagated step by step through a medium
by the influence of particles in contact with each other
had a firm hold on Faraday’s mind and was applied by him in almost every part of physics.
In 1838 he wrote ‘It appears to me, possible and even probable, that magnetic action may
be communicated to a distance by the action of the intervening particles,
in a manner having a relation to the way in which the inductive forces of static electricity
are transferred to a distance the intervening particles assuming for the time more or less
of a peculiar condition, which I have several times expressed by the term electrotonic state.
The same set of ideas were also used to explain electric currents.
Faraday felt conduction might be an action of contiguous particles, depending on the force
developed in electrical excitement these forces bring the particles into a state of
tension or polarity and being in this state the contiguous particles have a power or capability
of communicating these forces, one to the other, by which they are lowered and discharge occurs.
Sir John Herschel had suggested there might be a connection between magnetism and light. In 1845
Faraday succeeded in demonstrating that a magnetic field was capable of rotating polarised light.
The discovery of the connection between light and magnetism led Faraday to publish a short
paper of a speculative character titles Thoughts of Ray Vibrations.
Considering first the nature of ponderable matter, he suggested
that an atom may be nothing else than a field of force - electric,
magnetic and gravitational - surrounding a point centre. An atom would have no definitive size
but should be considered to be conceived as completely penetrable, and extending
throughout all space; and the molecule of a chemical compound would consist not of atoms
side by side, but of spheres of power mutually penetrated, and the centres even coinciding.
All space being permeated by lines of force he suggested that light and radiant heat might be
transverse vibrations propagated along these lines of force. In this way he proposed to
dismiss the aether, or rather replace it by lines of force between centres,
the centres together with their lines of force constituting the particles of material substances.
If the existence of a luminiferous aether were to be admitted; Faraday
suggested that it might be the vehicle of the magnetic force. He believed that
if there was an aether it should have other uses than simply conveyance of radiations.
This sentence may be regarded as the origins of the electromagnetic theory of light.
Faraday would go on to study diamagnetic bodies. He applied the idea of the conduction of lines
of magnetic force by different substances to explain the phenomena of diamagnetism.
Among experimental philosophers, Faraday holds by universal consent the foremost place.
His memoirs form a lasting legacy.