Permanency.
In electric current conductors the property of possessing conductivity
unaffected by lapse of time. Generally the permanency of conductors is
very high. In some cases a slow annealing takes place which causes a
gradual change with the lapse of time. Annealed German silver wire has
been found to increase in conductivity at about .02 per cent. in a year.
(Matthiessen.) Wire, whether annealed or not, is left in a strained
condition after the drawing operations, and such a change is consonant
with this fact. The figure only applies to the samples tested by
Matthiessen.
Permanent State.
In a telegraph line or other current conductor, the condition when a
uniform current strength obtains over the whole line. When a current is
started it advances through the line with a sort of wave front gradually
increasing in strength. At the further end some time may elapse before
it attains its full intensity. When its does the permanent state
prevails. Until then the variable state, q. v., exists in the line.
Permeameter.
An apparatus for determining the permeability of samples of iron. It
consists of a large slotted block of iron. A coil is placed within the
slot. A hole is drilled through one end, and a rod of the iron to be
tested is passed through this hole and through the coil to the bottom of
the slot. The lower end of the rod must be accurately faced off. The
current is turned on, upon which the rod adheres to the bottom of the
slot. The force required to detach it is determined with a spring
balance. The permeation through its face is proportional to the square
of the force required.
Fig. 258. PERMEAMETER.
Permeance.
The multiplying or the conducting power for magnetic lines of force
possessed by a given mass of material. It varies with the shape and size
of the substance as well as with the inducing force. It is distinguished
from permeability, as the latter is a specific quality proper to the
material, and expressed as such; the permeance is the permeability as
affected by size and shape of the object as well as by its material.