=445. A synchronous motor= is one that keeps step with the alterations of an alternating current. The line current is fed into the armature by means of two slip rings and brushes. The principle of the synchronous motor is illustrated in Fig. 447. This shows a motor having a two-pole field. The armature current must be reversed twice in each revolution. The reversal must take place when the armature winding is perpendicular to the lines of force of the field. In a direct current motor this reversal is brought about by the commutator. In a synchronous motor the armature reaches the 90 degree position at the exact instant at which the current reverses in the line. Thus in the case of a two-pole motor the armature must make exactly one revolution for each cycle; it is, therefore, a constant speed motor. Such motors are frequently employed in converter stations where alternating current is converted into direct current by what are called _rotary converters_. In practice the synchronous motor has a number of pairs of field poles. It is essentially an alternating current generator running as a motor. One of the principal uses of the synchronous motor is that of a converter, receiving alternating current and delivering direct current. Synchronous motors are also used in transmission lines to aid in maintaining constant voltage. Important Topics The wireless telephone, essential parts, action, arrangement. Alternating currents, alternating fields. Transformers, voltage relation of coils, power and core losses. Self-induction, inductance, and coke coils, uses, applications. Impedance, reactance, and resistance; relation and effects. Condensers, uses and applications with a-c. circuits. Alternating current power transmission; uses, advantages. Power factor, lag, lead, volt-amperes, true watts. Single- and three-phase currents; uses and nature of each. Three-wire transmission systems, alternators, construction, and action. A-c. motors, series, induction, synchronous. INDEX Aberration, spherical, 408 Absolute scale of temperature, 164 Absorptions of gases by solids and liquids, 29 Accelerated motion, 86 Acceleration, 87 Adhesion, 21 Aeroplane, 97 Air, aspirator, 67 brake, 74 cushion, 46 height (of atmosphere), 64 pressure, 56 pump, 66 weight, 56 Alternators, 481 Alternating current, 337, 466 Amalgamation, 273 Ammeter, 291 Ampere, 291 Archimedes' principle, 48 Arc light, 321 Armature, 335 Artesian wells, 44 Audion, 463 Aurora borealis, 453 Balloon, 72 Barometer, 59 Beats, 376 Boiling, laws, 208 point, 207 Boyle's Law, 63 Breezes, land and sea, 181 British thermal unit, 162 Brownian movements, 16 Calorie, defined, 162 Camera, 426 Candlepower, 394 Capillary action, 25 Cartesian diver, 71 Cathode rays, 453 Centrifugal force, 91 Charles' Law, 165 Chladni's figures, 381 Choke coils, 470 Coefficient of expansion, definitions, 170 gases, 167 liquids, 168 solids, 169 Coherer, 449