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A B, are mounted upon a large panel, made of marble or slate, 

 called a switch board. These switches are sufficient for the pur- 

 pose of turning the current on or off any track or for connecting 

 and disconnecting the generators, but for the successful operation 

 of the plant it is necessary to have other devices by means of 

 which the strength of the current may be ascertained, and also the 

 electro-motive force. It is necessary to provide each generator 

 with means for varying the electro-motive force of the current it 

 generates, otherwise the load could not be properly equalized be- 

 tween the several machines. All these different devices are located 

 upon the switch board, so as to have them in an accessible position. 

 A railway switch board, arranged for four generators and a large 

 number of distributing circuits, is shown in Fig. 24. The four 

 generator switches are shown at a a a a, and the circles marked R, 

 directly under them, are the devices by means of which the electro- 

 motive force of the current is regulated. These devices are called 

 field regulators, from the fact that their office is to regulate the 

 strength of the field magnets of the generators, making them 

 stronger to increase the electro-motive force and weaker to reduce 

 it. The part seen iipon the front of the switch board is not the 

 regulator proper, but only the handle and the contact points over 

 which this swings. The instruments marked A A A A are for meas- 

 uring the strength of the current of each individual generator, and 

 are called ammeters. The instruments marked V V V V are for 

 the purpose of indicating the electro-motive force of the currents 

 of the several generators, and are called voltmeters. Ag is an 

 ammeter used to measure the strength of the total current, and 

 Vg is a voltmeter that indicates the electro-motive force of the 

 current passing out to the cars on the various lines. The ammeter 

 ^^ is not an actual necessity, for the strength of the total current 

 can be ascertained by adding the readings of the four instruments 

 connected with the generators, but it is a convenience, as it saves 

 the trouble of performing the addition. The voltmeter Vg, how- 

 ever, can not be regarded in this light; in fact, its presence is 

 decidedly serviceable, for it indicates the average electro-motive 

 force of all the generators; therefore if any one of the instruments 

 V V V V is higher or lower it shows at once that the generator 

 to which it is attached is out of adjustment and not doing its proper 

 share of the w^ork. The switches hhh,hj means of which the cur- 

 rent is turned on to the several external circuits, are shown at the 

 extreme end of the switch board. 



The instrument marked W, located between the a switches, is 

 called a wattmeter, and its office is to indicate the amount of power 

 furnished by the generators. This instrument is not always used. 



