=137. Dynamotors.= Fig. 153. In Dynamotors the motor and dynamo armatures are combined in one, thus requiring a single field only. The primary armature winding, which operates as a motor to drive the machine, and the secondary or dynamo winding, which operates as a generator to produce a new current, are upon the same armature core, so that the armature reaction of one winding neutralizes that of the other. They therefore have no tendency to spark, and do not require shifting of the brushes with varying load. Having but one field and two bearings, they are also more efficient than motor-dynamos. _Applications._ They have largely displaced batteries for telegraph work. The size shown, occupying a space of about 8-inch cube, and having an output of 40 watts, will displace about 800 gravity cells, occupying a space of about 10 feet cube. The cost of maintenance of such a battery per year, exclusive of rent, is about $800, whereas the 1-6 dynamotor can be operated at an annual expense of $150. Dynamotors are largely used by telephone companies for charging storage batteries, and for transforming from direct to alternating current, for ringing telephone bells. Electro-cautery, electroplating, and electric heating also give use to dynamotors. CHAPTER XIX. HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE. [Illustration: Fig. 154.] [Illustration: Fig. 155.] [Illustration: Fig. 156.] =138. Conductors and Insulators.= To carry the powerful current from the generating station to distant places where it is to give heat, power, or light, or even to carry the small current of a single cell from one room to another, _conductors_ must be used. To keep the current from passing into the earth before it reaches its destination _insulators_ must be used. The form of conductors and insulators used will depend upon the current and many other conditions. It should be remembered that the current has to be carried to the lamp or motor, through which it passes, and then back again to the dynamo, to form a complete circuit. A break anywhere in the circuit stops the current. Insulators are as important as conductors. [Illustration: Fig. 157.] [Illustration: Fig. 158.] =139. Mains, Service Wires, etc.= From the switchboard the current flows out through the streets in large conductors, or _mains_, the supply being kept up by the dynamos, just as water-pressure is kept up by the constant working of pumps. Branches, called _service wires_, are led off from the mains to supply houses or factories, one wire leading the current into the house from one main, and a similar one leading it out of the house again to the other main. [Illustration: Fig. 159.] [Illustration: Fig. 160.]