Alloy are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is called a "substitutional alloy". Examples of substitutional alloys include bronze and brass, in which some of the copper atoms are substituted with either tin or zinc atoms respectively. In the case of the interstitial mechanism, one atom is usually much smaller than the other and can not successfully substitute for the other type of atom in the crystals of the base metal. Instead, the smaller atoms become trapped in the
Copper conductor alloys are offset by their lower electrical conductivities. Design engineers weigh the advantages and disadvantages of the various types of copper and copper alloy conductors when determining which type to specify for a specific electrical application. An example of a copper alloy conductor is cadmium copper wire, which is used for railroad electrification in North America. In Britain the BPO (later Post Office Telecommunications) used cadmium copper aerial lines with 1% cadmium for extra strength; for local lines 40 lb/mile (1.3 mm dia) and for toll lines 70 lb/mile (1.7 mm dia). Some of the major application markets for copper