Abstract: A telephone transmitter including an electret transducer is connected to an electrical circuit mounted in the transmitter housing. A resistor which can be of varying value is also connected to the circuit, the value of the resistor being adjusted to bring the operating characteristics within predetermined values. This is done by applying a sound signal of known frequency to the transducer and measuring the output characteristics of the transducer. This enables the suitable value of resistor to be connected into the circuit. By this means variations arising from varying atmospheric conditions at assembly, and other causes, can be allowed for. The metallized strip or film of the electret is preferably mounted with a very slight Vee shaped formation to avoid tight manufacturing tolerances which occur with a straight, or flat, strip.

Abstract: A display system is made up of an array of blocks, each block in turn consisting of rows and columns of individual display units each having a thin, pivotally mounted vane movable by electrostatic forces between an upright (WRITE) position and a horizontal (ERASE) position. A message can be displayed by selection of a pattern of vanes moved from one position to the other. Each vane assembly is symmetrically bistable, being movable in both directions by the same triggering electrodes which are subjected to a short pulse of high D.C. voltage that kicks the vane towards a mid position and subsequently allows it to continue its travel under its own momentum and gravity. Selection of units not to be operated is carried out by means of inhibit electrodes located near the vane in its extreme positions, such electrodes, when energized, preventing movement by the triggering electrodes.