Convection, Electric.
The production of blasts or currents of air (convection streams) from
points connected to statically charged conductors. The term is sometimes
applied to electric convection of heat. (See Convection of Heat,
Electric.)
Convection, Electrolytic.
The resistance of acidulated water as a true conductor is known to be
very, almost immeasurably, high. As an electrolytic, its resistance is
very much lower. Hence the current produced between immersed electrodes
is theoretically almost null, unless the difference of potential between
them is high enough to decompose the liquid. Yet a feeble current too
great for a true conduction current is sometimes observed when two
electrodes with potential difference too low to cause decomposition are
immersed in it. Such a current is termed an electrolytic convection
current. It is supposed to be due to various causes. Some attribute it
to the presence of free oxygen from the air, dissolved in the water with
which the hydrogen combines. Others attribute it to the diffusion of the
gases of decomposition in the solution; others assume a partial
polarization of the molecules without decomposition. Other theories are
given, all of which are unsatisfactory. The term is due to Helmholtz.
Convection of Heat, Electric.
The effect of a current upon the distribution of heat in an unevenly
heated conductor. In some, such as copper, the current tends to equalize
the varying temperatures; the convection is then said to be positive, as
comparable to that of water flowing through an unequally heated tube. In
others, such as platinum or iron, it is negative, making the heated
parts hotter, and the cooler parts relatively cooler.
The effect of the electric current in affecting the distribution of heat
in unequally heated metal (Thomson's effect. q. v.), is sometimes so
termed. If a current passes through unequally heated iron it tends to
increase the difference of temperature, and the convection is negative;
in copper it tends to equalize the temperature, and the convection is
positive.
Converter.
An induction coil used with the alternating current for changing
potential difference and inversely therewith the available current. They
generally lower the potential, and increase the current, and are placed
between the primary high potential system that connects the houses with
the central station, and the secondary low potential system within the
houses. A converter consists of a core of thin iron sheets, wound with a
fine primary coil of many convolutions, and a coarse secondary coil of
few convolutions. The ratio of convolutions gives the ratio of maximum
potential differences of their terminals between the primary and
secondary coils. The coil may be jacketed with iron to increase the
permeance. (See Alternating Current System.)
Fig. 110. FERRANTI'S CONVERTER OR TRANSFORMER.
Fig. 111. SWINBURNE'S HEDGEHOG TRANSFORMER.