Plumbago.
Soft lustrous graphite, a native form of carbon; sometimes chemically
purified. It is used in electro-plating to give a conducting surface to
non-conducting objects, such as wax moulds. The surface, after coating
with plumbago, is sometimes dusted over with iron dust, which
precipitates the metal of the bath and starts the plating. It is
sometimes plated with copper, silver or gold, and is then termed
coppered, silvered, or gilt plumbago. It is gilded by moistening with
etherial solution of gold chloride and exposing to the air, and drying
and igniting.
Plunger.
A movable core which is used in connection with a so-called solenoid
coil, to be drawn in when the coil is excited. (See Coil and Plunger.)
Fig. 267 COIL AND PLUNGER WITH SCALES TO SHOW ATTRACTION.
P. O.
Abbreviation for Post Office, q.v.
Poggendorf's Solution.
An acid depolarizing and exciting fluid for zinc-carbon batteries. The
following is its formula: Water, 100 parts; potassium bichromate, 12
parts; concentrated sulphuric acid, 25 parts. All parts by weight. Use
cold.
Point, Neutral.
(a) On a commutator of a dynamo the points at the ends of the diameter
of commutation, or where the brushes rest upon the surface of the
commutator, are termed neutral points. At these points there is no
generation of potential, they marking the union of currents of opposite
direction flowing from the two sides of the armature into the brushes.
(b) In electro-therapeutics, a place in the intra-polar region of a
nerve so situated with reference to the kathode and electrode as applied
in treatment, that its condition is unaffected.
Synonym--Indifferent Point.
(c) In a magnet the point of no attraction, situated between the two
poles, at about an equal distance from each, so as to mark the centre of
a magnet of even distribution of polarity.
(d) In thermo-electricity the point of temperature where the
thermo-electric powers of two metals are zero; in a diagram the point
where the lines representing their thermo-electric relations cross each
other; if the metals are arranged in a thermo-electric couple, one end
at a temperature a given amount above, the other at a temperature the
same amount below the neutral point, no current or potential difference
will be produced.