The switches _a a_ and _b b_, shown in Fig. 22, and the bus bars _A B_, are mounted upon a large panel, made of marble or slate, called a switch board. These switches are sufficient for the purpose 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 between 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 upon 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 measuring 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 _Ag_ 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_, however, 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 work. The switches _b b b_, by means of which the current is turned on to the several external circuits, are shown at the extreme end of the switch board.