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e.g. milled on white onlus and plain on brass or bronze. Furthermore, to ensure distinguishability by coin sorting machines it is desirable that a minimum difference of 14 millimetres in outside diameter should exist between any two coins irrespective of colour or shape. When the choice of size is severely limited non-circular shapes may be adopted, though these generally add to cost and make more difficult the design of reliable mechanisms for vending machines. Shapes available include square (with rounded corners), polygonel and scalloped. Polygenrl coins can be made with curved sides in such a way that the coins possess constant breadth and will roll freely when
placed on edge on an inclined plane. The ability of a coin to roll is important in some types of vending machine.
10. As alternatives to milled and plcin edges, coins can be supplied with rcised cr
incused lettered edges, or with a grooved "security" edge. These special edge
finishes make counterfeiting more difficult but depending on local circumstances it
is arguable whether the benefits are commensurate with the considerable additional
cost. Most countries which at one time used security edge coins have now abandoned
then, and lettered edged pieces are generally confined to the highest denominations and have a decorative pather than a security purpose.
Choice of Metals
11.
The metals in widest use at present are pure nickel, cupro-nickel, brass, bronze,
and aluminium, all of which wear well and maintain a reasonable appearance. They
may be employed either in solid homogeneous form, or as the outer layers of a
sandwich of which the inner layer is usually mild steel. Numerous other metals are
also used, but much less widely, end the differences in their cost and properties
are either only marginal or actually unfavourable. Nickel-silver is about the same
price es cupro-nickel which it resembles, but it has a yellowish tinge and is less
resistant to corrosion. Aluminium-bronze resembles brass and in some conditions
retains a better appearance, but it is substantially more expensive. Some forms of
stainless steel resemble pure nickel and cre appreciably cheaper, but they are
difficult to strike and therefore impose severe limitations on the types of obverse
and reverse designs that may be employed. Zinc is an inexpensive metal that resembles cupro-nickel when new, but it werrs much less well and rapidly becomes
dull grey in use.
12.
Sandwich materials with a mild steel core offer the advantage that they
cannot readily be melted down to produce saleable scrap, and particularly for larger coins they may be substantially cheaper than coins made wholly of the outer material (in smaller coins higher nenufacturing costs ney tend to offset the savings on metrl, especially when small quantities are involved). The disadvantages cre that the exposed edges are subject to corrosion, particularly in humid climates, and that the scrap value of the coins when they are withdrawn is generally very low.
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