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for each size of wire the current that in passing through it will heat it to this maximum allowable temperature. The results of these experiments are embodied in a table of safe carrying capacities for all sizes of wire commonly used. (Page 135.)

In dynamo rooms the conductors are usually numerous and very compactly placed, so that there is need of special precaution in construction. Moreover, the

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wires are the main conductors of the wiring system, and any devices protecting them work only when the full capacity of the system is exceeded. Short circuits or leaks would consequently be more serious in their effects. In a part of the dynamo-room wiring, it is not practicable to have safety devices, and where so much has thus to be trusted to insulation and noncombustibility, the only satisfactory construction is

to have the wires in contact with nothing but glass, porcelain, slate, or similar material.

Bus-bars are in a place where they are not exposed to contact with foreign substances, but they are to be supported on non-combustible material, as there is a possibility of their overheating.

4. Switch-boards:

a. Must be so placed as to reduce to a minimum the danger of communicating fire to adjacent combustible material.

b. Must be accessible from all sides when the connections are on the back; or may be placed against a brick or stone wall when the wiring is entirely on the face.

c. Must be kept free from moisture.

d. Must be made of non-combustible material, or of hard wood in skeleton form, filled to prevent absorption of moist

ure.

e. Bus-bars must be equipped in accordance with Rule 3 for placing conductors.

[Section a. Special attention is called to the fact that switch-boards should not be built down to the floor, nor up to the ceiling, but a space of at least eighteen inches, or two feet, should be left between the floor and the board, and between the ceiling and the board, in order to prevent fire from communicating from the switch-board to the floor or ceiling, and also to prevent the forming of a partially concealed space very liable to be used for storage or rubbish and oily waste.]

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

FIG. 16.-Switch-board in Skeleton Form.

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

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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

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FIG. 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.

6. Lightning-arresters:

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

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