The carbon brush depends for its frictional properties on atmospheric water. At high altitudes, where the atmosphere is very dry, friction is high and wear rate is catastrophic. Laboratory apparatus for the study of the phenomenon must simulate the conditions in the upper atmosphere. Conventional lubricants are unsuitable on account of their effect on the electrical contact and their failure to withstand the conditions at the rubbing face. Effective inorganic lubricants give immunity by building up a stable film on the commutator surface, but they adversely affect machine performance.