In regular machines, the field-magnets are electromagnets, the whole or a part of the current from the dynamo passing around them on its way out, to excite them and make a powerful field between the poles. To lessen the resistance to the lines of force on their way from the N to the S pole of the field-magnets, the armature coils are wound on an iron core; this greatly increases the strength of the field, as the lines of force have to jump across but two small air-gaps. There are many loops of wire on regular armatures, and many segments to the commutator, carefully insulated from each other, each getting its current from the coil attached to it. =130. Types of Dynamos.= While there is an almost endless number of different makes and shapes of dynamos, they may be divided into two great types; the _continuous_ or _direct current_, and the _alternating current_ dynamo. Direct current machines give out a current which constantly flows in one direction, and this is because a commutator is used. Alternating currents come from collectors or rings, as shown in Fig. 141; and as an alternating current cannot be used to excite the fields, an outside current from a small direct current machine must be used. These are called exciters. [Illustration: Fig. 149.] In direct current machines enough residual magnetism is left in the field to induce a slight current in the armature when the machine is started. This immediately adds strength to the field-magnets, which, in turn, induce a stronger current in the armature. =131. Winding of Dynamos.= There are several ways of winding dynamos, depending upon the special uses to be made of the current. The _series wound_ dynamo, Fig. 143, is so arranged that the entire current passes around the field-magnet cores on its way from the machine. In the _shunt wound_ dynamo, Fig. 144, a part, only, of the current from the machine is carried around the field-magnet cores through many turns of fine wire. The _compound wound_ dynamo is really a combination of the two methods just given. In _separately-excited_ dynamos, the current from a separate machine is used to excite the field-magnets. =132. Various Machines.= Fig. 145 shows a hand power dynamo which produces a current for experimental work. Fig. 146 shows a magneto-electrical generator which produces a current for medical use. Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps are connected in series to dynamos. [Illustration] CHAPTER XVIII. HOW THE ELECTRIC CURRENT IS TRANSFORMED. =133. Electric Current and Work.= The amount of work a current can do depends upon two factors; the strength (amperes), and the pressure, or E. M. F. (volts). A current of 10 amperes with a pressure of 1,000 volts = 10 × 1,000 = 10,000 watts. This furnishes the same amount of energy as a current of 50 amperes at 200 volts; 50 × 200 = 10,000 watts.