Effects Associated with the Thermal Response of the T1 Telephone Transmitter

T h e design of an effective telephone t r a n s m i t t e r requires t h a t an acoustic signal (voice) be efficiently converted to an electrical output. One method of performing this function is to use a moving electrode (attached to a diaphragm) as one wall of a chamber containing granular carbon. Thus, if a dc bias current is impressed on the t r a n s mitter the resulting variations can be used to operate remote telephone apparatus. An important factor in the design of such a t r a n s m i t t e r is the control of thermal effects, not only caused by variations in ambient conditions but also arising from the heat generated each time t h e telephone set is connected to the line. These thermal effects result in dimensional changes in the transmitter body, resistance change resulting from temperature rise of the granular carbon itself, and the resistance change caused by thermal expansion of the carbon granules when they are heated by the biasing current. T h e total effect, which produces a loss of carbon t r a n s m i t t e r efficiency, is referred to as "thermal packing." To assay the relative importance of the various thermal effects it is first useful to ascertain whether the carbon granules should be 1615