He is considered the founder of electrology.
After he effected some important experiences about the voltages originated from contact
between two metals, and discussed with Galvani, a physiologist at Bologna University,
about the existence of the animal electricity, Volta in 1799 constructed the column
battery , the first electrochemical device generating a direct current.
The realization of his battery started a century of fundamental researches about electric
and magnetic phenomena.
Before devoting himself to the researches that led him to the realization of his battery,
he studied the laws governing the thermal expansion of the gases and realized a particular
gun that exploited the violent expansion of a gaseous mixture of hydrogen and oxygen, by
means of an electric spark.
Besides he invented the disk electrophorus, a special electrostatic induction machine that
permits to accumulate rapidly a great quantity of static electricity.
He is considered the most great experimenter in the field of the electric and magnetic
phenomena.
In 1831 he had the idea to effect an inverse experiment in comparison with the one made by
Oersted, who in 1820 discovered the magnetic field produced by an electric current,
observing the deviation that suffers a magnetic needle, initially in equilibrium in the
terrestrial magnetic field, whenever an electric current is made to flow in a wire placed
near the needle.
Faraday discovered that if a magnet is being approached to an electric circuit, in which
initially there isn't flowing an electric current, while the magnet is moving, in the
circuit is being produced a temporary induced current (that is the principle of
electromagnetic induction ), and that, even while the magnet is being removed, is produced
another temporary induced current, with an opposite direction in comparison with the
precedent one.
Faraday's law, that is worth for any temporal variation of the magnetic flux
concatenated with an electric circuit, is represented mathematically by the Neumann-Lenz
formula,on which is based the operation of both the electric generators and transformers.
Other important contributions of Faraday concern the electrostatic shield ( the so-called
Faraday cage ), the laws of electrolysis (that is the decomposition by means of electric current of a chemical compound that has been solved in water or melted), the concept of field and the relative graphic representation (Faraday's criterion), founded on the increasing and the decreasing density
of force lines passing across a surface, and the rotating effect of a magnetic field
acting on the polarization plane of a light ray propagating in any optically inactive
medium (that is a medium unable to rotate, without the magnetic field, the polarization plane of a light ray) .
His researches on the forces acting between electric circuits and magnets led him to
enunciate the equivalence theorem of a magnetic needle to a coil in which is flowing
electric current.
His exceptional scientific intuition pushed him to extend to the microscopic world the
validity of his theorem, supposing that the magnetic field produced by a permanent magnet
is owing to a great number of elementary electric currents, equivalent to as much
microscopic magnetic needles.
His hypothesis was confirmed brightly, after about a century, by the discovery of
spin and of the associated magnetic moment of electron,
proton, neutron and of many other elementary particles, in the context of quantun
mechanics.
Ampere was besides the founder of classical electrodynamics, since enunciated the laws
describing the electrodynamic attractive or repulsive forces, acting between two electric
circuits.
He,as a very skilled experimenter,is considered one of the founders of
electrotechnics, since discovered the law that establishes the direct proportionality
between the intensity of electric current in a conductor and the voltage applied to its
terminals .
He enunciated contemporarily the law that allows to calculate for a conductor the constant
of proportionality (that is the resistivity or specific resistance), between the
voltage and the current intensity.
The resistance depends on the specific resistance of a metal and is directly proportional
to the lenght and inversely proportional to the section of a conductor.
He is considered one of the founders of electrotechnics, since discovered the laws of
electric circuits.
Other his fundamental contributions concern optics, since he invented, together with
Bunsen, the spectroscope, that make it possible to analyse the chromatic components of a
light radiation, and thermodynamics, since he enunciated the fundamental principle
according to which the specific energy (that is the energy per unity of
surface ) emitted by means of thermal radiation, by a body at an assigned temperature,
is the greater, the greater is the specific thermal radiation absorbed by the body, at the same temperature.
His ingeniousness brought it to deal with several sectors of physics. Effecting a
rigorous physical-mathematical analysis of the electric and magnetic phenomena discovered
by Oersted, Faraday, Ampere and Ohm, he developed from 1864 to 1873 his genial theory of
electromagnetism, exposed in the treatise "Electricity and magnetism",
creating by his well-known equations the unified theory of electric and magnetic phenomena
and arriving at foreseeing, exclusively on theoretical previsions, that light consists of
electromagnetic waves, for which are valid the reflection, refraction, interference,
diffraction and polarization laws, experimented for light waves, as it was shown
experimentally, toward the end of the XIX century, by Heinrich Hertz.
He founded besides statistical mechanics, that served him to develop, on the basis of his
distribution formula of the molecular velocities, the kinetic theory of gases, that is
fundamental for the statistic study of the thermodynamic systems.
He is considered one of the founders of thermodynamics, in which introduced the
absolute scale of temperatures (measured in Kelvin degrees).
In the field of electromagnetism he studied the oscillatory currents that originate in a
resistive-inductive-capacitive circuit, and furnished the formula of the relative
resonance frequency.
He developed besides the theory of propagation of electric signals across transmission
lines, formulating the so called "equations of telegraphists ",
fundamental for projecting telegraphic and telephone lines.
In 1887 he, as a very skilled experimenter, after several famous experiences on
electromagnetic waves,showed brightly that they propagate with the velocity of
light,following laws of light waves.
After the Hertz experiments was definitely shown the electromagnetic nature of the light
phenomena, as it had been foreseed by Maxwell in 1873, founding exclusively on
physical-mathematical considerations.
In 1887 discovered the photoelectric effect, that consists in the emission of electric
negative charges (electrons) by an illuminated metallic surface.
In 1895, while making experiments on cathode rays, he arrived at the discovery of the
X-rays,the so-called Roentgen rays, that are electromagnetic waves with a very high
frequency and a wavelength between some billionths of centimetre and some hundred
millionths of centimeter, emitted during the collision of electrons accelerated by high
voltages ( from about some tens to any hundreds of kV ) against the atoms of matter, that
in such a mode are excited toward higher energy levels.
The acquired energy is successively emitted by X-rays ( X-photons ) with a wavelength
depending on the atom used as a target ( X-rays with line spectra ).
X-rays with a continuous spectrum are instead emitted whenever an electron with high
energy suffers an acceleration or a deceleration, according to the forecasts of the
classical electromagnetic theory, that implicates emission of electromagnetic waves by a
charged body that is moving with an accelerated (or decelerated) motion.
He was the first physicist to which was conferred in 1901 the Nobel prize, just founded.
In 1896, during any researches on the phosphorescent materials, for discovering if they
were able to emit radiations like X-rays, by experimenting with various substances, he
discovered that salts of uranium emitted some radiations that were able to impress a
photographic plate and to discharge an electrically charged conductor, even after having
passed across an aluminium plate.
Becquerel had discovered by a sheer chance natural radioactivity, that consists in the
emission of ionizing radiations by atomic nuclei : a particles
,that is helium nuclei with a positive electric charge equal to 2 electron charges, b particles (high energy electrons) and g-rays,
with a wave-like nature as the one of X-rays, but much more penetrating than these.
He was conferred the Nobel prize in 1903 together with Mr. e Mrs. Curie.
Subsequently to Becquerel's discovery, studying the radioactivity of uranium
minerals, they discovered that any pitchblende variety, although containing a too low
percentage of uranium,emitted a radioactivity much more intense than the one of uranium,
which did they suppose that was contained an element more intensely radioactive of
uranium.
After long and laborious researches , that required the employment of about 6 pitchblende
tons, they succeeded in 1898 to extract a few less of a gram of the new element, to which
gave the name of radium.
To the discovery of radium followed the one of polonium, much more radioactive than
radium.
In 1903 they were conferred the Nobel prize together with Becquerel.
He is the founder of quantum physics, since formulated his famous postulate of the
quantization of energy, by which he succeeded to give a solution to the old problem of
explaining the spectrum of the radiant energy emitted by a "black body", that is
an ideal body which is able to absorb the whole radiant energy it receives.
Several clever physicists, as Rayleigh, Jeans and Wien had tried in vain to furnish a
theory that could explain the experimental curve of the emitted radiant energy, at various
temperatures, by a body keeped at a constant temperature and behaving approximately as a
black body.
The great discrepancies among theory and experiments were brightly eliminated by Planck in
1901.
Planck postulated that the electromagnetic energy is absorbed exclusively by multiples of
an elementary "packet", a quantum with an energy directly proportional to the
frequency f of the radiation, E = hf, where h is an universal constant , the Planck's
constant.
Subsequently he extended his postulation to all the energy forms, determining a fruitful
breakup with the classical physics, in which it wasn't possible to explain the
experimental results.
So originated quantum theory, that gave impulse, in the first decades of the XX century,
to the creation of both quantum mechanics and quantum electrodynamics, on
which are founded the quantum theory of the fields and the actual theories , the
electro-weak theory and the quantum chromo-dynamics, in the context of the "standard
model" of microcosm.
He was conferred the Nobel prize in 1918.
His genial theories, that revolutionized physics in the first years of the XX century,
furnish the bases needful to understand the frontiers of today physics.
After he moved to Switzerland in 1902 to get a job as an experienced official at Bern
patent office, devoted himself with an exceptional critical spirit to the analysis of the
main scientific problems that had led to crisis of classical physics, after the Maxwell
admirable unification ( 1873 ) of electric and magnetic phenomena.
In 1905 Einstein, submitting to deep analysis space and time concepts and taking account
of the negative results of several experiences made in 1887 by the American physicists
Michelson and Morley, that showed the non-existence of the ether,an hypothetical medium
introduced by the physicists to explain the propagation of the electromagnetic waves, he
was convinced it was necessary to postulate the independence of the velocity of light on
the motion of the source and the extension of Galilean relativity principle, limited
initially to the only mechanical phenomena, to all the physical phenomena.
To do that, he used the formulae of transformation that Lorentz had introduced to show the
invariance of Maxwell's equations with respect to two observers in rectilinear and
uniform motion each other, and formulated the special relativity theory, so denominated
because it allows to treat in a new mode all the physical phenomena that happen in
non-accelerated frames of reference and in absence of gravitational fields, considering
the time no more in an absolute sense, that is no more independently from the observer,
but depending on space coordinates by means of Lorentz' s transformations.
In the same period (1905), studying the quantum theories enunciated by Planck in
relationship to the problem of the radiant energy emitted by a black body, thought to use
those to explain the characteristics of the photoelectric effect,discovered by Hertz and
Hallwachs in 1887 and consisting in the emission of electrons by an illuminateded metallic
surface.
His theory, founded on the hypothesis of photons, the "quanta" of
electromagnetic energy of entity proportional to the frequency of the light radiation,
allowed him to explain entirely all the characteristics of the photoelectric emission,
otherwise quite inexplicable by applying the laws of classical physics.
For his theory, that extended Planck's quantum theory, creating quantum physics, he
was conferred the Nobel prize in 1921.
The hypothesis of the existence of photons was brightly confirmed in 1923 by Compton, that
succeeded to identify the X-photons diffused, with a wavelength change, by
atomic electrons.
In the same period Einstein contributed besides definitively to explain the
continuous, chaotic, motions of particles of a colloidal solution (Brownian
motions), caused by thermal agitation.
From 1905 to 1916 Einstein extended the theory of relativity, elaborating his genial
"theory of the general relativity", that is a new theory of the gravitation that
explains the motions of heavenly bodies on a geometric base, by means of the dependence of
the space-time bend on the mass.
The theory of the general relativity, published in 1916, allows to explain effects of
fundamental importance for the astrophysics and the cosmology, as the red shift of the
spectral lines of the light emitted by a star, the bend of the light rays that propagate
in proximity of a star and the existence of the black holes, invisible heavenly bodies
deriving from the contraction of stars in the final phase of their evolution, and
characterized by a matter concentration so great to prevent even the emission of the
photons.
Einstein devoted the rest of his life to the research of a relativistic theory
permitting to unify the gravitational phenomena and the
electromagnetic ones, setting up the bases for the research of a theory which permits to
treat in an unified mode all the fundamental forces of nature (gravitational,
electromagnetic, weak and strong forces).
The unitary theory of all the fundamental forces is
at present the objective of an extraordinary
and stimulating challenge for the physicists of the XXI century.
