SWITCHBOARDS AND SWITCHBOARD APPLIANCES. 335 



arrange a switch so that the number of turns of wire in the trans- 

 former coils may be altered at will, thus varying the ratio of trans- 

 formation and changing the secondary voltage even though the 

 primary voltage be constant. Thus, Fig. 285 shows three taps 

 brought out from successive turns at one end of the primaries of 

 each of the step-down transformers. This device for regulating 

 the voltage delivered to a receiving circuit is usually called the 

 dial regulator. The details of the dial regulator are shown in 

 Fig. 286. The contact finger, which is shifted from one to 



tuppljunam service main 



supply main 



Fig. 286. 



another of the contact points I, 2, and 3, consists of two pieces 

 of metal a and b far enough apart to bridge across from one 

 contact point to the next as shown. With this arrangement the 

 circuit is never broken as the contact finger is shifted over the con- 

 tact points, and the short-circuit of one turn of the transformer 

 which takes place when the contact finger touches two contact 

 points includes the two choke coils CC which are so wound on 

 an iron core that they permit current to flow outwards or inwards 

 through both as indicated by the arrows, but do not permit it to 

 flow outwards through one and inwards through the other. 



Another form of voltage regulator is shown in section in Fig. 

 287. Imagine an iron carcase, similar to the carcase of an in- 

 duction motor, to have a single-phase primary winding on its 

 stator and a single-phase secondary winding on its rotor. Such 

 a transformer could have its primary winding connected across 

 the supply mains and its secondary winding connected in series 



