Abstract
Hygroscopicity of microphone carbon powder was measured by weight increase, after 4 hours' exposure at 30°C to moist air having 80 % relative humidity. Experiments were made on the effect of pretreatment of anthracite by acid, temperature of heat treatment, and atmospheric gas. Carbon powder baked in hydrogen atmosphere, was found more hygroscopic than the one baked in nitrogen and in hydrocarbon gas, and this may be due to the reaction of hydrogen with carbon to form methane.
