Meter connections. there are four instruments at the bottom of the board of rectangular outline with cylindrical fronts. These are integrating wattmeters, and are for the electric system what gas meters are for gas systems, that is, they measure the amount of electrical energy that passes through them. One of these meters is connected in the circuit of each generator, and thus the output of each machine can be read and compared, and by adding the readings of the four instruments the total output of the plant can be ascertained. When the capacity of a system is large and the e. m. f. rather low, or even if the capacity is moderate, if the e. m. f. is very low, the strength of the current becomes so great, that it taxes the ingenuity of the designer to Allow access to back of board. provide room enough at the back of the board for the conductor bars without making the dimensions of the board much larger than they need be for the accommodation of the switches and other devices. As an illustration of such cases is shown Fig. 45, which is the back of a board used in a manufacturing establishment where the e. m. f. is very low, and the current strength runs up into several thousand amperes. As will be seen, all the connections are of a massive kind, and there is very little waste room between the conducting bars. Another example of a somewhat different design is shown in Fig. 46. It has already been stated that the switchboard should be set far enough from the wall to allow free access to Switchboards for arc-lighting circuits. the various parts of the back, and it may be added that the connections should be kept as close to the board as possible. A good example of a snugly arranged board is shown in Fig. 47, which is an end view and shows well the clear space back of the board, and also the way in which the wiring is arranged so as to not encroach upon the space. Switchboards are often made in a number of panels so that they may be increased in capacity at any time without destroying their symmetry or crowding any of the parts together. A three-panel board of this type is shown in Figs. 48 and 49, the first showing the front and the second the back. The panels a and b are generator panels and are provided with a main switch, a field regulator, a circuit breaker, and an ammeter. The third panel c, is a distributing panel, and is provided with two circuit breakers, two single pole switches, one ammeter and one voltmeter. On the back of the board are also provided lightning arresters for all the conductors. This board is arranged for operating an electric railway, and it only requires one distributing panel, because the road is small. It will be noticed that this board is made up of panels that are duplicates of each other, so that if at any time it becomes necessary to enlarge the plant, the switchboard can also be enlarged by the addition of more panels. Switchboards for arc-lighting circuits. The construction of arc light switchboards is very simple in comparison with constant potential boards. This difference arises from the fact that each machine Arc lighting. is arranged so as to feed into a single circuit, and also because circuit breakers and voltmeters are not required. A simple arc light board for four generators is shown in Fig. 50. It consists of a marble slab which carries eight plug receptacles at the upper edge, and eight more at the bottom, and an ammeter in the central space. Eight wires running from the terminals of four machines are connected with the eight lower receptacles, and eight line wires are connected with the eight upper receptacles. By means of the flexible cables shown, any one of the lower receptacles can be connected with any one of the upper ones, as is clearly indicated in the illustration. A Arc lighting. voltmeter is not required, owing to the fact that the e. m. f. of the generator is continually rising and falling in accordance with the demands of the circuit. If the num ber of lamps in service is increased, the e. m. f. must correspondingly increase, and if the number of lamps is re |