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A switch-board in "skeleton form " is one built as a frame-work, in distinction from a solid board or partition. It gives good ventilation, has a minimum amount of combustible material, and affords little opportunity for leakage. (Fig. 16.)
The switch-board has on it a number of devices controlling the full power of the dynamos, and there
is here the full electrical pressure. These conditions often expose the switch-board to intense heat, and sometimes shutting down the machinery is the only way to stop the cause. It is consequently best to have everything about the switch-board of non-combustible material, even to the supports for the slate or marble slabs. With the high potentials that are brought to this point, it is necessary, too, that the board be perfectly dry and clean, and that the material of which the slabs are made does not have conducting veins in it. All the leakage effects are greatly increased with high potential, and even slate slabs have been known to split into many pieces from heat generated by leakage through metallic veins.
5. Resistance-boxes and Equalizers:
a. Must be equipped with metal or other non-combustible frames.
b. Must be placed on the switch-board, or if not thereon, at a distance of a foot from combustible material, or separated therefrom by a non-inflammable, non-absorptive, insulating material.
[Section a. The word "frame" in this section relates to the entire case and surrounding of the rheostat, and not alone to the upholding supports.]
Resistance-boxes, even in normal working, develop heat, for their office is to introduce resistance into the circuit, and all the energy necessary to overcome this resistance is converted into heat. It is not only a possibility, therefore, that is here to be guarded against, but an actual normal condition. Moreover, as many of the resistance-boxes are made, there is liability of the coils of wire coming together, forming shorter paths for the current and thereby causing much greater temperature than was anticipated. There is clearly but one way to treat this piece of apparatus, and that is to have it wholly non-combustible, and arranged so that it can do no harm even if it heats
Fio. 17.—Resistance-Box or Rheostat. A, Handle for adjusting resistance. B, Slate top and back. C, Screen allowing ventilation. D, Coils.
excessively. As the coils may accidentally become connected to the metal frame, it is well to have the frame insulated as the rules indicate.
a. Must be attached to each side of every overhead circuit connected with the station.
b. Must be mounted on non-combustible bases in plain sight on the switch-board, or in any equally accessible place, away from combustible material.
c. Must be connected with at least two "earths" by separate wires, not smaller than No. 6 B. & S., which must not be connected to any pipe within the building, and must be run as nearly as possible in a straight line from the arresters to the earth connection.
d. Must be so constructed as not to maintain an arc after the discharge has passed.
A "lightning-arrester" would perhaps more properly be called a lightning diverter, since it does not arrest the lightning, but diverts it from the wiring by offering a comparatively easy path to the earth. If there
were not this easy path, the lightning would pierce the insulation in some other place, because of the great force urging a discharge. The lightning-arrester is made so that the connection with the earth is broken as soon as the lightning or "static discharge" has passed, for otherwise there would be an opportunity for the leakage of the dynamo current.
Figure 18 shows the principle of a lightning-arrester. At A is a very short air-space that is an effectual insulator as far as the normal electrical pressure on the line is concerned. When a lightning charge accumu
lates on the line there is a very strong tendency for it to hreak through the insulation to the earth, and the short air-space is easily sparked across. The air is heated somewhat by the discharge, and the normal electrical pressure on the line is sufficient to maintain a constant leakage to the earth if there is not some arrangement that breaks the connection. This connection can be broken in several ways. Often the arrester is made so that after a discharge the air-space is increased until the electrical pressure can no longer force a current through it, and sometimes a strong magnet is directed across the space; it being a curious fact that a magnet will repel an arc and thus "blow it out."
"Each side" of the circuit means each wire of the circuit; the one going out and the one coming in.
An "earth" is a connection with the earth, or the special arrangements, such as buried metal, pipes, etc., that insure the electrical connection. A good earth is made by imbedding a copper or iron plate about two feet square in broken coke and burying the whole in moist earth. All connections should be soldered.
The wire gauge of Brown & Sharpe is the one by which copper wire is usually measured in this country, and it is commonly called the "B. & S. gauge."
The electric "arc " has been explained. (Pages 33, 47.) It is familiar in the arc lamp. A current of elec