Types of Motor Elements

Condenser Motor Element.²⁵ As is well known, mechanical forces exist between the plates of a charged capacitor. Thus, if a special capacitor is constructed with one or more movable plates, if these plates are arranged so that they will radiate sound effectively, and, if speech or program electric voltage waves are impressed on the device, sound waves will be radiated.

Condenser receivers and condenser loudspeakers have not been used extensively. They are more delicate and more expensive, and they require higher voltages than other types. Also, condenser driving elements require a polarizing direct voltage to prevent the radiation of sounds of twice the frequency of the received electric signals.

Piezoelectric Motor Element. Electric signals impressed on the crystal electrodes cause the dimensions of the crystal to change in accordance. The crystal may radiate the sound waves directly or may be coupled mechanically to an acoustic radiator, such as a diaphragm or a paper cone.

Crystal telephone receivers are rugged, light in weight, sensitive, and have an excellent frequency response. The input impedance of one type is about 80,000 ohms at any frequency, and the sensitivity is 1.5 bars per volt at 1000 cycles.* Piezoelectric motor elements have been used to a limited extent in small loudspeakers.²⁶

Magnetic Motor Element. This classification includes most of the motor elements used in modern telephone receivers and loudspeakers. Largely of historical interest are the induction magnetic motor element²⁷ and the magnetostriction motor element. Of practical importance is the magnetic-armature motor element, a device the operation of which involves vibration in some part of the ferromagnetic circuit.¹ Also of practical importance is the moving-coil motor element in which the mechanical forces are developed by the interaction of the field set up by the currents in the conductor and the polarizing field surrounding it.¹ * Advertising literature of the Brush Development Company.