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.
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.