Uprights of I-beams. I-beams are used for the uprights, the arrangement being as shown in Fig. 38, which represents a plan in section through a beam located opposite a joint in the marble. At the side of the board the I-beam is placed flush with the edges. The use of I-beams cannot be said to be as desirable as the angle iron, because it is more expensive and not so substantial, so far as the slabs are concerned, owing to the fact that the bolt holes must be drilled near A Figure 38. Use of I-beam in switchboard construction. to the edges, and, therefore, are more liable to break out. The bottom of a switchboard should be of such height that the attendant can see the lower instruments or switches without inconvenience, and the top should be of such a height that the highest instruments can be reached from the floor. If space is limited the bottom of the board may not be over a foot and a half from the floor, but two feet should be allowed whenever possible. Height of instruments. If the board cannot be made of such width as to permit of placing all the switches that have to be handled within easy reach of a man of average stature, then the board should be made in two stories, with a platform from which the device upon the upper part may be manipulated. The space between the back of the board and the wall should be four feet or more, and if possible the location should be such that there is an abundance of daylight back of the board, so that connections may be made when necessary without the aid of artificial light. Artificial lights should also be provided so as to facilitate work after dark. If there are windows back of the board they should be so arranged that rain and snow cannot enter through them and reach the board, and in damp weather they should be closed tightly. A perfectly watertight ceiling should be constructed over the entire switchboard so as to obviate all danger of injury by water falling from overhead. Switchboard fronts are made of marble and marbleized slate, both of which are suitable for the purpose, when free from seams and metallic veins. Marbleized slate is much superior to the plain stone, as the enamel makes it waterproof. Polished marble is also better than the rough stone, on account of the polish rendering the surface more impervious to moisture. When there is much dampness in the air there is a liability of more or less. leakage in the switchboard if the voltage is high. This can be reduced by varnishing the back of the slabs after they have been well dried out by heat. Fig. 39 shows the back of a switchboard arranged for two generators connected in the three-wire system. The The back of the switchboard. leads from the generators come up through the floor and are made of large flexible cables. As will be noticed, all the bars are supported entirely by the connections with the switches and other devices, just as in Fig. 27. A side elevation in section is shown in Fig. 40. Fig. 41 is a photographic view of the back of the switchboard, the front of which is shown in Fig. 42. The five large disks, seen in the back view, are the field rheostats, and by looking at Fig. 42 it will be seen that. they correspond in position with the five small hand wheels located in the center panel of the board. This |