Bell Labs display of electrical loudspeakers and microphones ca. 1929, from left to right: large round "4181-A Loud-Speaking Telephone, used with baffle for reproducing low notes." - small "596 Receiver, moving coil receiver used for high tones." - small round "555-w Receiver, moving coil type. Introduced for use with exponential horns in 1926. Thousands of these devices are operating in theatres today." - rectangular "196-W Receiver and Folded Exponential Horn, balanced armature driven diaphragm. Was used extensively in Public Address Systems from 1923-1926." - round "540-AW Receiver, cone without baffle. Popular between 1923-1926." - gooseneck "518 Receiver. Widely commercialized about 1920 for radio use."