A myoelectric signal, also called a motor action potential, is an electrical impulse that produces contraction of muscle fibers in the body. The term is most often used in reference to skeletal muscles that control voluntary movements. Myoelectric signals have frequencies ranging from a few hertz to about 300 Hz, and voltages ranging from approximately 10 microvolts to 1 millivolt. Myoelectric signals are detected by placing three electrodes on the skin. Two electrodes are positioned so there is a voltage between them when a myoelectric signal occurs. The third electrode is placed in a neutral area, and its output is used to cancel the noise that can otherwise interfere with the signals from the other two electrodes. The output voltage is processed using a device called a differential amplifier. The output of this amplifier has much higher voltage than the myoelectric signals themselves. This higher voltage, which produces significant current, can be used to control electromechanical or electronic devices. Myoelectric signals are of interest to the developers of prosthetic devices, such as artificial limbs. The signals can also be used to facilitate the operation of a computer using small voluntary muscle movements, such as blinking the eyelids. A Japanese company is developing a device that picks up speech using the myoelectric signals from the muscles around the mouth. The eventual goal is to create a mobile telephone that can be worn on, or even implanted in, the body