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wood. If moisture were to penetrate, it would make a semi-conductor of the wood through which currents of electricity might leak. The frames on which the smaller machines are set are usually made in two parts, the top part being adjustable so that the belt may be tightened by moving the machine.

Almost all damp substances are fairly good conductors, and even damp and dirty surfaces will allow considerable leakage. Precautions have to be taken against the beginning of these small leaks. They rapidly become worse, for they are accompanied by heating and sparking which carbonizes surrounding substances, making it possible for still larger currents to flow. If the wires of a dynamo become damp, there is apt to be leakage from one wire to another across the damp insulation, and once started, there will soon be a path of very low resistance, or a "short circuit," that will result in a current heavy enough to melt metal and destroy the usefulness of the machine. Trouble comes in another way, if the dynamo be damp and not well insulated from the ground: though the circuit is supposed to be insulated from the ironwork of the machine, there is liability of a connection, and if the wooden frame under the dynamo be damp and partly conducting, the circuit will be more or less well connected with the earth through the ironwork, wood, masonry, etc.

Suppose, in Figure 10, there is a connection to the iron of the dynamo near B, and a path thence through wet wood to a bolt, and then to the earth at E. If this be all, no current will flow, for there is no path up from the earth to the other wire of the circuit. Suppose, however, that somewhere in a building there is a gas-pipe in electrical connection with the other wire of the circuit, as at A. The gas-pipe runs, of course, to the earth, and the current then has a

B

E

FIG. 10. Showing Effect of Ground Connections.

path through pipe, earth, bolt, wet wood, and the iron of the dynamo, between the two sides of the circuit, A and B There is thus the full electrical pressure of the dynamo to force the current through this path, and a hole may be burned in the gas-pipe, with easily imagined result.

A dynamo often emits small sparks, and when working badly throws out bits of molten metal, so that it

is evidently unsafe to have the machine placed where there is inflammable matter.

The waterproof cover that is required, serves to keep the dynamo clean and dry under ordinary conditions, and will be ready to use in case of a leak above the machine or for protection from water damage in case of a fire.

2. Care and Attendance:

A competent man must be kept on duty in the room where generators are operating.

Oily waste must be kept in approved metal cans, and removed daily.

[Approved waste cans shall be made of metal, with legs raising can three inches from the floor, and with self-closing covers.]

3. Conductors:

From generators, switch-boards, rheostats or other instruments, and thence to outside lines, conductors

a. Must be in plain sight, and readily accessible.

b. Must be wholly on non-combustible insulators, such as glass or porcelain.

c. Must be separated from contact with floors, partitions, or walls, through which they may pass, by non-combustible insulating tubes, such as glass or porcelain.

d. Must be kept rigidly so far apart that they cannot come in contact.

e. Must be covered with non-inflammable insulating mate

rial sufficient to prevent accidental contact, except that "busbars" may be made of bare metal.

f. Must have ample carrying capacity, to prevent heating. (See Capacity of Wires Table, page 135.)

The "switch-board" is the frame, board, partition, or slab on which are grouped the different instruments, and the devices for controlling the dynamos and for properly distributing the currents to the different circuits. (Page 64.)

A "resistance-box" contains usually a number of coils of wire, and its object is to introduce into the electric circuit a resistance that will reduce the strength of the current. It is thus a sort of electrical throttling device. A "rheostat " is an adjustable resistance-box. An "equalizer" is a resistance-box or rheostat used for the special purpose of equalizing conditions in different parts of a circuit. "Resistance-box" is the general term and is applied to all this species of apparatus. (Page 66.)

There are innumerable types of "insulators" for supporting wire, and they vary in shape according to the special purpose for which they are used. Sketches of a few of them are given in Figures 11-14.

Where wires pass through floors, walls, and such places, there is particular liability to dampness, and to contact with material that offers a path for leakage. In such places, there is also liability that the insulation

on the wire will become abraded in drawing the wire through, or that it will gradually be abraded by the movement of the wire afterward. For protection in these places "insulating-tubes" are used. There is a flange on one end to prevent the tube from sliding through the hole. (Fig. 14.)

The "bus-bars," or omnibus bars, are the main conductors from which the smaller wires lead off to the

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FIG. 11.-Porcelain Cleat. FIG. 12.- Porcelain Knob. FIG. 13. - Petticoat Insulator. FIG. 14.- Insulating-tube.

different circuits in the building or city.

The bus

bars are placed on the switch-board, usually at the back. (Fig. 15.)

The "carrying capacity," or safe carrying capacity, of a conductor is the amount of current it will carry without heating to an unsafe temperature. A certain temperature is fixed upon, and there is determined

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