Fading - of radio waves: a variation in the received signal strength arising from variation with time in the conditions of propagation. Fading which affects unequally the components of a modulated wave is known as selective fading.

Ferrohydrodynamics - The study of the mechanics 0.£ strongly magnetizable fluid media in the presence of applied magnetic fields.

Fermi-Dirac Statistics - The form of quantum statistics, having to do with the distribution of particles among various allowed energy values, applicable to an assembly of particles in which only one particle is allowed in each state. Their wave functions have symmetry properties such that the wave function changes sign. if any two particles are interchanged. Such wave functions are said to be antisymmetric. At sufficiently high temperatures, where a large number of energy levels are excited, Fermi-Dirac statistic, like Bose-Einstein statistics, reduce to the classical Maxwell-Boltzmann statistics.
Fermi Energy - Fermi Level. In the elctron theory of metals: the energy of the hightest energy level occupied by electrons in a metal.

Fermi Gas - (1) The ,"free'" electrons contributed by the atoms of a solid metal to provide electrical conductivity. They are considered to form a dense electron gas obeying the Pauli exclusion principal. and Fermi-Dirac .statistics. (2) The aggregate of nucleons in a nucleus of sufficiently large mass number, which may be treated as a statistical "gas". Fermion - A particle to whiCh Fermi-Dirac statistics apply. The Pauli exclusion principle holds for such particles. Electrons, protons, neutrons, and all particles with spin t are fermions. See also:Boson.
Fermi Plot - Kurie Plot. Of a particle specturm: a graph in which a suitable function, of the observed intensity is plotted against the particle energy, the function being chosen so that the graph is a straight line for allowed transitions. It is used in determining the character of the transition and the maximum energy.

Fermi Theory of Beta Decay - A theory giving the probability of the ejection, in unit time, of a -particle whose energy lies between E and E+dE.lt involves the assumption of the creation of leptons in the decay process, the specification of appropriate selection rules, and the conservation of leptons.

Ferric Induction - Defined as B-H. where B is the magnnetic induction in a magnetic field H. It is also called intrinsic induction.

Ferrimagnetism - The existence of a state analogous to the ferromagnetic state but with neighbouring spins anti-parallel instead of parallel. It differs from anti-ferromagnetism in that the individual magnetic moments are not all equal and the vector sum of all spins is not zero, i.e. spontaneous magnetization occurs. 'The saturation magnetization is, however, much lower than the sum of the magnetic moments present.
Faraday Balance - A torsion balance designed for the measurement of the suscptibilities of para and diamagnetic materials. It has also been developed for measuring the susceptibilities of gases.
Faraday.Dark Space - In a glow discharge tube: a relatively dark region immedi tely adjacent to the positive column. .

Faraday Effect - TIle rotation of the plane of polarization produced when plane-polarized light is. passed through a homogenous medium in a magnetic field, the light travelling along the direction of the field. For a given substahce the rotation is proportional to the path length of the light in the substance and to the magnetic field strength. The constant of proportionality is known as the Verdet constant.

Faraday; Faraday Constant - The quantity .of electricity that will liberate 1 mole from solutiqn. It is the product of A vogardro' s number and the electronic charge and is equal to ab.out 96500 c.oulomb.

Faraday Low of Induction - States that the e.m.f. induced jn a closed circuit is equal to the rate of decrease of magnetic flux through the circuit, Dr E = -d$ldt, where E is the e.m.f. and is the magnetic flux linking the circuit.

Farday Laws of Electrolysis - State. that: (a) the amount .of chemical action produced by a current is prop.ortional to the quantity of electricity passed: and (b) the masses Dr' different substances deposited or dissolved by the same quantity of electricity are proportional to their chemical equivalents.

Far Point - Of the eye: the furthest point of clear visi.on when the eye is resolved. It is also known as punctum remotum.

Fast Fission Factor - For nuclear fissi.on in an infinite ntedium: the ratio of the mean number of neutrons produced by fissions due to neutrons of all energies, to the mean number of neutrons producedby thermal fission only.

Fatigue - Of a material subjected to an alternating stress: a phenomenon leading 'to failure after repeated applications (thousands or millions of cycles) of a stress which Would not have caused failure had it been maintained at a steady value.
Ferromagnetic Resonance - The resonant absorption of energy from an external radio-frequency field by ferromagnetic materials when a strong polarizing field. is applied. This resonance occurs at frequencies in the microwave region. Ferrimagnetic resonance is a similar phenomenon, exhibited hy ferrimagnetic materials.

Ferromagnetism - The phenomenon whereby certain materials exhibit a high degree of magnetism in weak fields and possess a very high permeability. Ferromagnetic materials exhibit spontaneous magnetization. residual magnetism, and are subject to hysteresis. Their behaviour is markedly dependent on whether the temperature is above or below that of the Curie point.

Ferromagnetism, Band Theory Of - A theory according to which ferromagnetism maybe attributed to the electrons in the unfilled bands. The equilibrium magnetization depends on the number of electrons, the form of the band, the magnitude of the exchange interaction, and the temperature, and must be calculated on the basis of Fermi-Dirac statistics.

Ferromagnetism, Ewing Theory of - An elaboration of the "molecular" theory of magnetism put forward by Weber according to which each molecule of a magnetic substance contained one or more small permanent which were free to turn.
Ferromagnetism, Heisenberg Theory Of - A theory according to which the explanation of ferromagnetism is given in terms of the exchange forces between electrons, and the electrons themselves are localized on individual atoms rather than shared by the whole crystal as in the collective electron theory.
Ferromagnetbm, Weiss Theory Of - A thory according to which ferromagnetism is attributed to an ensemble of independent molecular magnets which are subject, on the one hand, to the . aligning effect of an applied magnetic field and, on the other, to the disorientation effect of thermal agitation.
Fertile - (1) Of a nuclide capable of being. transformed, directly or indirectly, into a fissile nuclide by neutron capture. (2) Of a material containing one or more fertile nuclides.
Fery Spectrograph - A spectrograph in which. the only optical element is a back-reflecting prism with cylindrically curved faces. The curvatures of these faces are such that rays diverging from a point on a circle are focused on that Circle.
Feynman Diagram - A graphical scheme for representing the interactions between elementary particles and fields, including a pictorial representation of intermediate states. The elements in a Feynman diagram give a graphical picture of matrix elements.
Fibre Diagram - The characteristic X-ray diffraction pattern given by a group of crystallites (such as those in natural fibres) which tend to have one crystallographic direction parallel to a common direction but are otherwise randomly arranged.
Fick Diffusion Laws. First low - states that the rate of diffusion of
particles across a given area, Le. the flux of particles per unit area .per unit time, perpendicular to the gradient of concentration of the particles, is proportional in amount to that gradient and opposit in sign. It may be written as where J is the flux in question, D the diffusion coefficient or
diffusivity, and the gradient of concentration. This law holds only when this concentration is independent of time.