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Figure 81. Wiring diagram of same. Figures 79-80. Outline diagrams of alternating current arc light boards.

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Resistance

Wattmeter

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How they are made.

furnishes a current to actuate the ammeter, and the other furnishes one of the currents for the wattmeter. A resistance is also provided with the last-mentioned transformer, to adjust the current it delivers. The ammeter transformer is seen at the top of the board in Fig. 66, and the wattmeter transformer is located directly above the resistance used in connection with it, this resistance being located at the bottom of the board as shown in the figure. The construction of the board, and the arrangement of the circuits and various apparatus can be more fully understood from the explanatory outline drawings, Figs. 79 and 80, and the wiring diagram, Fig. 81. The location of the various devices in Fig. 61 is not the same as in Fig. 79, but as each part is clearly marked, there will be no difficulty in understanding the diagram. As will be seen, the primary current that energizes the transformer, that furnishes the current for the arc lights, runs to the two lower plugs of the switchboard, and the light circuit is connected with the two upper plugs.

The wattmeter shown in these illustrations is an in-strument very rarely used in connection with continuous current arc switchboards; this, however, is not because the wattmeter is a necessary adjunct of the alternating current system, but simply because the conditions under which the current is used are such that it is desirable to measure it. In continuous current arc lighting, the switchboard and generators are located in the station, and it would be of no special benefit to provide wattmeters. With alternating series arc lighting, the transformer that furnishes the current, as well as the switchboard, may be located on the premises of the consumer, and in that case it is necessary to have a meter, so as to know what amount of current to charge for.

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High-tension switchboards.

In power transmission plants, where the energy of waterfalls is transmitted to a long distance, by means of polyphase currents, very high e. m. fs. are used, ranging all the way from 10,000 to 40,000 volts, and the switchboards made for such installations have to be arranged so as to safely withstand such pressure. The switches and circuit breakers used are of special construction so as to produce a long break when they are opened. In addition, these devices are separated from each other by barriers made of suitable insulating material, so that the arc produced on opening the circuit may not jump across from one to the other. The type of switchboards used for such purposes is well illustrated in Figs. 82 and 83, which show the boards at a receiving station at the end of a transmission line. The line current which is impelled by an e. m. f. of 11,000 volts is received in the board, Fig. 82, and from here passes to the primary coils of transformers, which are designed so as to develop secondary currents of 2,200 volts. These secondary currents are conveyed to board, Fig. 83, and from there pass to the distributing circuits, or to other transformers, where they are again reduced and rectified.

The switchboard, Fig. 82, handles the high-tension transmission currents that come from the power station, It consists of three panels, each one made of two marble slabs placed one behind the other. The switching is accomplished by means of plugs and sockets in a manner similar to that employed in the arc light boards already explained. The terminals connected by the plugs are mounted on the marble slabs, the latter being separated

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