The problem of finding a light alloy of aluminium which will satisfactorily eliminate the steel core for overhead electrical conductors is of importance at the present time. The ½ per cent magnesium alloy, under the name of Aldrey, is being used on the Continent without steel core, and the present paper is concerned with its exposure to London atmosphere. The conductors are in the form of wires and have been exposed on the roof of King's College since October,1928. The variations in their electrical and mechanical properties have been studied with special reference to type of corrosion and mechanical stress has proved beneficial so far as variations in tensile properties are concerned, it has had but a small effect upon electrical conductivity. An account is given of experiments which have been made to explain the reason for the beneficial results obtained. The paper refers to published work on the atmospheric exposure of light nickel alloys, duralumin, and high-purity aluminium, and comparisons with the present experiments have been made and discussed.