In this part we will explore the contributions made by Benjamin Franklin,
Coulombe and Poisson made to the fields of electro-statics and magnetism.
Shortly after the discovery of the Leyden jar in 1745 a London apothecary named William Watson
noticed that when the experiment is performed in a certain way the observer feels the shick only
in his arm and chest. From this he inferred that in the act of discharge there is transference of
something which takes the shortest or best conducting path between two points.
He proposed that electrical actions are due to the presence of an electrical aether which in
the act of charging or discharging the leyden jar is transferred, but is not created or destroyed.
He viewed the excitation of electric to be caused by the accumulation of a surplus of electrical
aether by the electric at the expense of some other body, whose stock is accordingly depleted.
All bodies possessed a certain natural store which could be drawn upon for this purpose.
Just 2 years later Benjamin Franklin would stumble upon exactly the same idea as Watson had proposed.
He was also not acquainted with duFay’s work but Franklin’s electric fluid is identical
with the vitreous electricity and resinous electricity in Franklin’s view was merely the
deficiency of a stock of vitreous electricity which all bodies have a natural store of.
Of course he then pinned a positive character to vitreous
which we now know today is actually the electron which is denoted as a negative.
Franklin was the first to suggest that the tingling sensation you feel when your skin
is close to an electrified body was caused by the flow of the electric fluid but instead caused an
action at a distance between the particles in the electric fluid. This later idea did not in itself
cause him to abandon the idea of an electric fluid discharging from objects.
This would come from Franz Ulrich Theodor Aepinus who using Franklin's observations that it is
not possible to blow away the electric field surrounding a charged body. The electric field
remains in tact. He proposed that the electric fluid did not extend beyond the body itself
and is confined to thin layers on the surface. This compelled his to state
that electricity acts ata distance across the intervening air. Action at a distance.
Both Franklin and John Canton had previously shown how it is possible to induce a charge in
another body without direct contact. This they had explained using the idea of the discharge
of the electric fluid. Aepinus that this could equally be explained using action at a distance.
His initial work in this area laid the foundations for the mathematical theory of electrostatics.
Wilcke followed up his work in 1757 describing many experiments relating to electric induction
and discussing the physics of non conductors, and proposing that a dielectric exposed
to an electric field is thereby thrown into a state of electric polarisation.
It was Joseph Priestley, the man who discovered Oxygen who would finally crack the secret of
the force between two charges. In 1766 he was conducting a series of experiments on
a hollow sphere when he discovered that there is no charge on the inner surface
and no electric force in the air inside it. From this he immediately drew the conclusion that the
attraction of electricity falls off with the square of distance. Exactly like gravity does.
Sadly not many took note of his findings and it would take a further 18 years for Dr John Robison
to determine this same law by experimentation.
Priestley’s discovery together with Franklin’s conservation of electric charge idea
raised the electricity to the position of an exact science.
While the theory of electricity was being established a less remarkable
development was taking place in the science of magnetism.
Newton stated this in his manuscript. The power of gravity is of a different nature
from the power of magnetism. For the magnetic attraction is not as the matter attracted.
Some bodies are attracted more by the magnet, others less; most bodies not at all. The power
of magnetism, in one and the same body, may be increased or diminished; and is sometimes
far stronger, for the quantity of matter, than the power of gravity; and in receding from the magnet,
decreases not in the duplicate, but almost in the triplicate proportions of distance.
The first discoverer of the law of force between magnetic poles was John Mitchell.
In 1750 he published a manuscript which stated: ‘Wherever any magnetism is found, whether in the
magnet itself, or any piece of iron excited by the magnet, there are always found two Poles,
which are generally called North and South; and the North pole of one Magnet always attracts
the South Pole, and repels the Norh Pole of another. This was of course adapted from Gilbert.
Each pole attracts or repels exactly equally, at equal distances, in every direction. The
magnetical attraction and Repulsion are exactly equal to each other. This although obvious to us
now was the most important advance in magnetism. The attraction and repulsion of magnets decreases
as the square of distances from the respective poles increases.
This great discovery is the basis of the mathematical theory of magnetism.
The idea of electricity being caused by an electric fluid
led to attempts to construct a similar model for magnetism.
Aepinus thought that the poles were places at which a magnetic fluid was present
in amounts exceeding or falling short of the normal quantity. The permanence of magnets was accounted
for by supposing the fluid to be entangled in the pores, making it very difficult to be displaced.
The particles of this fluid were thought to repel each other,
and attract the particles of iron and steel. However in order to explain all magnetic phenomena
Aepinus had to assume that there was also a mutual repulsion of the material particles of the magnet.
Subsequently two magnetic fluids were proposed called boreal and austral.
These fluids were supposed to have properties of mutual attraction
and repulsion similar to those possessed by vitreous and resinous electricity.
The next person of note to move both camps forward was a french physicist Charles Augustin Coulomb.
Coulomb did not accept the one fluid theory but preferred a rival hypothesis which had been put
forward by Robert Symmer in 1759. His idea was the operation of electricity did not depend upon on
a single positive power, but instead required two upon two distinct, positive, and active powers,
which by contrasting, and counteracting each other produce the various phenomena of electricity.
Coulomb developed this idea and stated. Whatever be the cause of electricity we can
explain all the phenomena by supposing that there are two electric fluids,
the parts of the same fluid repelling each other according to the inverse square of the distance
and attracting the part of th either fluid according to the same inverse square law.
When Coulomb published his work it lead to controversy between the one
fluid and two fluid groups. The later quickly adopted in
France but rejected in Holland by Can Marum and in Italy by Volta.
The main difference between the theories is that in the two fluid theory,
both the electric fluids are moveable within the substance
of a solid conductor while in the one fluid theory the actual electric fluid is mobile,
but the particles of the conductor are fixed. The dispute could therefore be settled only the actual
motion of the electricity in discharges and this was beyond the reach of experiments at the time.
Coulombe also contributed to the further development of magnetism.
He by simple mechanical reasoning was able to completely overthrow the hypothesis of vortices.
He was also the first to state that the two fluids of magnetism could not be obtained separately,
for when a magnet is broken into two pieces, one containing its north and the other its south pole,
it is found that each piece is an independent magnet possessing two poles of its own,
so that it is impossible to obtain a north or south pole in a state of isolation.
Coulombe reasoned that this was caused because the magnetic fluids
are permanently imprisoned within the molecules of magnetic bodies,
so as to be incapable of crossing from one molecule to the next.
In 1812 Simeon Poisson would advance electro statics significantly. He accepted the idea
of a two fluid electricity and assumed that the natural state of a body contained equal
quantities of both types. Any disturbance of this distribution would lead to an electrified state.
He also felt that not all material bodies behave in the same way with respect to this fluid. Metals
do not appear to exert any influence on it, but permit it to move about freely in their substance;
for this reason they are called conductors. Other
materials oppose the passage of the electric fluid in their interior.
Poisson went on to study the distribution of charge across the surface of various shaped
bodies. He borrowed the idea of from Lagrange’s memoirs on motion of gravitational bodies
from Lagrange, that the sum of the components of the attractive force
at any point can be simply expressed as the derivatives of the function which is
obtained by adding together the masses of all the particles of an attractive system, each
divided by its distance from the point. Poisson use this to come up with the following idea
Here phi denotes the density and x y and z are points within the substance. He
called attention to the utility of the function , remarking that
its value over the surface of any conductor must be constant.
Poisson then turned his attention to magnetism and defined an equation for the magnetic moment
and the magnetic intensity at various external points.
This function V was later named the magnetic potential by Green.
George Green would go on to generalise many of Poisson’s ideas.
Electrostatics had by this time developed to such an extent
that further development would either come from more complex maths or from experimentation
showing some new character that had not been accounted for.
Before we return to the idea of the aether we must consider one more branch of electrical philosophy
which we will cover in the next installment.