Arc light Switchboards. 5 5 2 2 matter more fully, suppose it is desired to connect machine No. I with circuit No. 1, then the plugs B2 C2, Ba Ce, B10 C10 must be inserted; but if it is desired to run machine No. 1 on circuit No. 2, then plugs B2 C2 are not inserted. In their stead plugs B, D, and D2 are inserted to connect machine terminal B with circuit terminal D, and plugs C, E, E2 are inserted to connect machine terminal C with circuit terminal E. In making this connection we have used the horizontal bars 4 and 5; the remaining four bars, 3, 7, 8, 9 can be used in like manner to make any desired combination of the other two machines and circuits. The row of A sockets is for the purpose of connecting the horizontal bars of one panel with those of the adjoining panel. The sockets C, E, G on line II are constructed so that a twin cable with split plug may be inserted for the purpose of cutting the ammeter into the circuit. The construction of the plugs can be understood from Fig. 55 and that of the sockets from Fig. 56. The tube which forms the body of the socket, and the support for the vertical bars, is made of hard fiber, and is provided with a metallic tube lining at the entering end, and a split metallic sleeve at the other end. The metallic lining is connected with the horizontal bar and the split sleeve with the vertical bar. When the plug is inserted its point is forced into the split sleeve, and a spring projecting from the side like the spring of an umbrella forces the back end against the side of the metallic lining and also acts to hold the plug in place. The plug being a metal rod, its insertion in the socket closes the circuit between the tube lining at one end and the split sleeve at the other. Switchboards for alternating currents. The construction of alternating current switchboards is substantially the same as that of continuous current boards; whatever difference there may be between them is in the way of increased strength of insulation. Where currents of very high voltage are handled the switches. are generally placed at a considerable elevation above the floor and are separated from each other by means of marble slabs, so as to prevent arcing from one to the other. The construction of the switches is also considerably modified, and for strong currents they are frequently made so as to be operated by electromagnets or compressed air. Switches of this kind are also made so as to be immersed in oil for the purpose of reducing the size of the sparks when the switch contacts are separated. All the apparatus and instruments used on continuous current switchboards are used on alternating current boards, and, in addition, several other devices that are not required on the former. The object of these devices, and their location in the circuit, can be made clear by the aid of simplified diagrams. Fig. 57 shows the switchboard instruments and apparatus, and the manner in which they are connected in the circuit for controlling one single phase alternating current generator. The generator is shown at A. The field magnetizing coils are energized by a continuous current derived from a small continuous current generator shown at E. The current from the alternator reaches the switchboard through the main switch D, and the right-side Single phase Switchboards. wire runs to and through an ammeter Am, and through a fuse C to the terminal L'. The other side of the circuit runs directly to fuse C and thence to terminal L. At H Switchboard for single phase alternating current. is shown a voltmeter transformer, the primary of which is connected across the circuit between switch D and the Single phase Switchboards. alternator. If the voltage of the alternator is low this transformer is not required, and the voltmeter is connected directly across the line, just as in a continuous current system. For high voltages a transformer is used so as to obtain a current of low e. m. f. to actuate the voltmeter. It is not advisable to make voltmeters for very high e. m. fs. on account of the very small size wire required for the coils. If the current delivered by the alternator is large, a transformer may be provided to furnish a secondary current to actuate the ammeter. If a transformer is provided for this purpose, its primary coil is connected in series with the main circuit, and not across it, as is the case with the voltmeter transformer H. The latter is connected across the line so that the current traversing its primary coil may vary with the e. m. f. of the alternator; but as the ammeter is to indicate changes in the strength of the alternator current, its primary must be connected in series with the main line. The voltage of the current furnished by the exciter E is regulated by means of a rheostat R' which, as will be seen, is connected in the field circuit of this machine. Another rheostat R is provided in the main circuit of E, by means of which the field exciting current of A can be adjusted. By means of the switch F the circuit through the field coils of A can be opened or closed. As can be seen, the strength of the field magnetizing current of A can be regulated by both the rheostats, as R varies the current by changing the resistance in the circuit, and R' varies it by changing the voltage. When economy is practiced one of the rheostats can be dispensed with, generally R'. The fuses C C can be replaced by circuit breakers, or both devices can be used. The main switch D can be a |