Approximately, with the exception of iron and mercury, the resistance of
a metallic conductor varies with the absolute temperature. This is very
roughly approximate.
Except for resistance energy would not be expended in maintaining a
current through a circuit. The resistance of a conductor may be supposed
to have its seat and cause in the jumps from molecule to molecule, which
the current has to take in going through it. If so a current confined to
a molecule would, if once started, persist because there would be no
resistance in a molecule. Hence on this theory the Ampérian currents
(see Magnetism, Ampere's Theory of) would require no energy for their
maintenance and Ampére's theory would become a possible truth.
When metals melt their resistance suddenly increases.
Light rays falling on some substances, notably selenium, q. v., vary the
resistance.
Longitudinal stretching of a conductor decreases it, it increases with
longitudinal compression, and increases in iron and diminishes in tin
and zinc when a transverse stress tends to widen the conductor.
(b) The term resistance is used to express any object or conductor used
in circuit to develop resistance.
[Transcriber's note: At room temperatures, the thermal motion of ions in
the conductor's crystal lattice scatters the electrons of the current.
Imperfections of the lattice contribute slightly. At low temperatures
superconductivity (zero resistance) can occur because an energy gap
between the electrons and the crystal lattice prevents any interaction.
At the time of this book, none of this was known. "Jumps from molecule
to molecule" is a good guess.]
Resistance, Apparent.
Impedance; the virtual resistance of a circuit including the spurious
resistance due to counter-electromotive force. It may be made up of true
resistance and partly of an inductive reaction, as it represents the net
factor, the entire obstruction to the passage of a current, and not
merely a superadded resistance or counter-electro-motive force.
Synonym--Impedance.
[Transcriber's note: Impedance can also have a component due to
capacitance.]
Resistance, Asymmetrical.
Resistance which varies in amount in different directions through a
conductor. It implies a compound or composite conductor such as the
human system. The presence of counter-electro-motive force in different
parts of a conductor may bring about asymmetrical resistance.
Resistance, B. A. Unit of.
The British Association Ohm. (See Ohm, B. A.)