0.441 lb. or 129 dwt. per cu. in. It is practically unworkable.
Ruthenium is an excellent hardener for palladium or platinum, being approximately twice as effective as iridium. The hardness and strength of an alloy of 4.5% ruthenium 95.5% palladium are about the same as the alloy made of 10% iridium and 90% platinum.

Airplane magnetos utilize a 10% ruthenium. and 90% platinum alloy for the contact points that continuously make and break the primary electrical circuit. Unusually high resistance to wear and spark erosion are required of contact points in this service.

IRIDIUM

Pure iridium, a very heavy element of white color, weighs 0.813 lb. or 237 dwt. per cu. in. and melts at approximately 4449°F.

Being harder than rhodium, iridium is very difficult to roll into wire or strip. It is used principally as an alloying material, usually as a hardener for platinum. An iridium-platinum alloy containing 10% iridium is used in fine jewelry. A proportion as high as 30% iridium is used to harden platinum for hypodermic needles and other special purposes.

OSMIUM

Pure osmium has the greatest density of all the elements, weighing 0.82 lb. or 238 dwt. per cu. in. It is a white metal, very hard and unworkable .

Alloys containing osmium and ruthenium are used to tip the points of fountain pens and phonograph needles of the permanent type. . ' The hardest alloys used for such purposes contain more than 600/0 osmium.

COMPOSITE METALS

The cost of articles may be reduced by making them of a composite metal composed of a base metal to which a covering layer of precious metal or precious metal alloy is bonded. "Gold Filled", "Rolled Gold Plate" and "Sheffield Plate" are some terms applied to composite metals.

The serviceability of composite metals is related to the thickness of the precious metal coating and to the suitability of the base metal used. The better composite metals are made using a substantial coating of precious metal bonded to a strong and corrosion resisting base metal.

The material for "Gold Filled" and "Rolled Gold Plate" is made by bonding a sheet or tube