The field coils vary in number and position. The purpose of their construction is always to send the largest possible number of lines of force through the armature. Some dynamos are _bipolar_, or have _two_ poles, others are multipolar or have more than two. In Fig. 288 No. 4 has four poles. The _armature_ of a dynamo differs from a magneto armature in that it consists of a series of coils of insulated copper wire wound in numerous slots cut in the surface of a cylindrical piece of iron. Fig. 289 shows a _side_ view of the iron core of such an armature. Iron is used to form the body of the armature since the magnetic lines of force flow easily through the iron. The iron by its permeability also concentrates and increases the magnetic flux. The best armatures are made of many thin sheets of soft iron. These are called _laminated_ armatures. An armature made of a solid piece of iron becomes hot when revolving in a magnetic field. This is due to electric currents induced in the iron itself. This heating is largely reduced by _laminating_ the armature. Why? [Illustration: FIG. 290.--Armature connected to slip rings producing an alternating current.] =303. Methods of Collecting Current from the Armature.=--The electric currents produced in the armature are conducted away by _special sliding contacts_. The stationary part of the sliding contact is called a _brush_. The moving part is a _slip ring_ or a _commutator_. Fig. 290 shows an armature coil connected to slip rings. As the armature revolves, the coils and slip rings revolve with it. The two ends of the armature coils are connected to the two rings respectively. Now as the armature revolves it cuts the lines of force first in one direction and then in the other. This produces in the coils an E.M.F. first one way and then the other. This E.M.F. sets up a current which is conducted to the outside circuits through the slip rings and brushes. Such a current which repeatedly reverses its direction is called an _alternating current_. Fig. 291 (1) indicates graphically how the current moves alternately one way and then the other. Alternating currents are extensively used for electric _light, heat, and power_. _Direct currents_ or those going continuously in one direction are however in much demand especially for _street car service_, _for electrolysis_, and for _charging storage batteries_.