Variometer, 108 Vibrating string, study of, 154 Vocal cords, 153 Voice frequencies, 163 Volt, definition of, 57; measurement of, 61 Voltmeter, calibration of, 62; construction of, 205 Watt, 235 Wave form, 182 Wave length, relation to frequency, 98, 122; defined, 122 Wire, inductance of, 104 Wire, movement of electrons in, 14; emission of electrons from, 37 Wire telephony, 253 Wired wireless, 255; advantages of, 257 X-rays, 147 Zero coupling, 177 Zinc, electrode for battery, 23