=441. Three-wire Transmission.=--The currents produced in the three coils just described undergo precisely the same changes as those represented in the _graph_ (Fig. 441) for the three electromotive forces. Careful examination of the graph will show that at any point the sum of the _plus_ e.m.f's. equals the sum of the _minus_ e.m.f's. In other words the algebraic sum of the three e.m.f's. is zero. Therefore if we properly connect a transmission line of three wires to the generator, the sum of the currents leaving the generator will equal the sum of the currents returning to it. Since the algebraic sum of the currents produced by the three coil combination described in Art. 440 is always zero, it is possible to use three wires on three-phase transmission lines. Fig. 442 shows a "tower" carrying three, three-wire transmission lines. Long distance, high tension transmission lines are generally three-wire lines carrying three-phase a.-c. currents. [Illustration: FIG. 442.--A "tower" supporting three, three-phase circuits of a high tension transmission line.] =442. Alternators.=--A dynamo which delivers alternating current is known as an _alternator_. Commercial alternators have many pairs of poles in the field and as a rule the field rotates while the armature is stationary. The field must be supplied with _direct_ current for the polarity of each coil in the field must remain unchanged. Usually a separate "exciter" is used, which is a small direct current generator. The current from this exciter is fed into the rotating field by means of slip rings. Fig. 439 shows a d.-c. (direct current) exciter on the end of the armature shaft of the large alternator. [Illustration: FIG. 443.--Diagram of a "Series Motor."] =443. The A.-C. Series Motors.=--The only type of motor that will run on either alternating or direct current is the _series motor_. The "universal" motor used in household appliances such as electric fans, vacuum cleaners, etc., is a series motor. The reason a series motor will run on either direct or alternating current is because the direction of rotation of the armature of a motor depends on (_a_) the direction of the current in the armature, and (_b_) the polarity of the field. Reversing either of these alone, reverses the direction of rotation of the armature, while reversing both at the same instant leaves the direction of rotation unchanged. Fig. 443 is a diagram of a series motor since the field coils and armature are connected in series. On an a.-c. line, both field and armature current must therefore reverse at the same instant. In a shunt motor (similar to Fig. 286) we have a divided circuit, and the greater self-induction of the field coils causes an a.-c. current through these coils to lag behind that flowing in the armature so that the two currents do not reverse at the same instant.