The paper covers some of the mechanical requirements of electrical circuit-breakers. Methods to be applied in the design stage to meet these requirements, and to guard against imperfections which may arise during manufacture, are suggested. The effects of various forms of contacts on the frictional characteristics of circuit-breakers are described, and the design steps necessary to prevent a change of friction affecting the reliable performance of circuit-breakers in service are considered.A series of practical routine tests is put forward which should ensure that the calculated design factors are complied with, and which should also lead to an improved standard of workmanship.Emphasis is laid on the importance of accuracy in workmanship and the necessity for careful calculations to comply with rigid standards.The paper concludes with illustrations and information relative to suitable apparatus and shop fixtures considered essential in. carrying out the prescribed routine tests.