Platinum.
A metal; one of the elements; symbol, Pt; atomic weight, 197.4;
equivalent, 49.35; valency, 4; specific gravity, 21.5.
It is a conductor of electricity.
The following data refer to the annealed metal at 0° C. (32° F.)
Relative Resistance (Silver annealed = 1), 6.022
Specific Resistance, 9.057 microhms.
Resistance of a wire,
(a) 1 foot long, weighing 1 grain, 2.779 ohms.
(b) 1 foot long, 1/1000 inch thick, 54.49 "
(c) 1 meter long, weighing 1 gram, 1.938 "
(d) 1 meter long, 1 millimeter thick, .1153 "
Resistance of a 1 inch cube, 3.565
Electro-chemical equivalent (Hydrogen = .0105), 0.5181.
The coefficient of expansion by heat is almost the same as that of
glass. It can be passed through holes in glass and the latter can be
melted about it so as to hermetically seal its place of passage through
the glass. It is used in incandescent lamps for leading-in wires and
other similar uses.
Platinum Black.
Finely divided platinum. It is made by boiling a solution of platinic
chloride with excess of sodium carbonate and a quantity of sugar, until
the precipitate is perfectly black and the supernatant liquid is
colorless. It seems to possess a great power of occluding oxygen gas.
When heated to redness it becomes spongy platinum. The negative plates
of a Smee battery are coated with platinum black.
Platinum-silver Alloy.
An alloy of 1 part platinum and 2 parts silver, used for resistance coils.
Relative Resistance (silver annealed = 1 ), 16.21 microhms.
Specific Resistance at 0°C. (32° F.), 24.39
Resistance of a wire,
(a) 1 foot long, weighing 1 grain, 4.197 ohms.
(b) 1 foot long, 1/1000 inch diameter, 146.70 "
(c) 1 meter long weighing 1 gram, 2.924 "
(d) 1 meter long, 1 millimeter diameter, 0.3106 "
Resistance of a 1 inch cube, 9.603 microhms.
Percentage Variation per degree C. (1.8° F.)
at about 20° C. (68° F.), 0.031 per cent.
Synonym--Platinum Alloy.
Platinum Sponge.
Finely divided platinum obtained by igniting platinum black, q.v., and
also by igniting salts of platinum. It has considerable power of
condensing or occluding oxygen. It will, if in good condition, set fire
to a jet of hydrogen impinging upon it.
Plow.
Contact arms projecting downwards from the motors, trucks, or bodies of
electric street cars, which enter the underground conduit through the
slot and carry contact pieces or brushes, to take the current for
driving the motors from the leads within the conduit.