opening of a circuit carrying current. It is impossible to guard against a spark in a dynamo room. The current in a dynamo is generated in the moving part or armature, and it is led to the circuit through an attachment to the armature called a "commutator" or "collector" (according to its design), the conductors of the circuit being electrically connected to the commutator or collector by stationary strips of copper or blocks of carbon called "brushes." Any defect in design or workmanship, improper adjustment, overloading of the machine, or even the presence of dust, may at any time cause "sparking" at the brushes. The cover specified in rule "d" is to protect the machine while not in service from dirt and moisture, as from leaky pipes, defective roofs, etc. It may also prove useful to prevent damage by water in the extinguishing of an incipient fire.

Care and Attendance.-As a variety of accidents may cause a dynamo to spark so badly as to throw off sparks to a considerable distance, or may even cause the burning out" of an armature, it needs no argument to show that an attendant ought always to be near. Any serious trouble on the circuit will usually be indicated in some manner in the dynamo room, and prompt inspection may discover a hazard in time to remove it without loss. The rule about oily waste should, of course, apply wherever waste is used. Its habit of indulging in spontaneous combustion, especially when it has a good chance to set fire to something else, is too well known to insurance men to call for comment.

Conductors.-Originally it was customary to place

electrical instruments and regulating and controlling devices either directly upon the wall of the dynamo room or, at best, to mount them upon a wooden board fastened to the wall. The board was called a "switch board." This name is now applied to any structure carrying instruments and regulating and controlling devices. In a dynamo room or station, all circuits from the dynamos are led to a main switch board, and thence the current is distributed by the necessary circuits to the lamps or motors. The switch board controls the entire output of the plant, and often carries many regulating and controlling devices and a large amount of complicated wiring. Many of the fires in the poorly constructed stations of the past have originated in the switch boards or their vicinity. It would seem evident that the wires to and from the switch board should be in sight and accessible; yet, in many plants, wires have been run from dynamo to switch board under wooden floors, and all the wires to and from the switch board have been crowded between the board and the wall in an inaccessible space. This has been done for appearance sake, it being easier for the constructor to conceal poor work than to do work in a mechanical and workmanlike manner. The unfortunate experience of insurance companies with central stations has been largely due to this kind of engineering. Rules "b" and "c" describe the safest possible kind of construction; i. e., such that current cannot leak from the wires even if their insulating covering is defective, and even if the wires become overheated, the heat cannot be communicated to any combustible material. With

such construction, a bare, red-hot wire would not set a fire.

As regards rule "d," no other rule for distances between wires can be laid down, but safe work can be done by having numerous insulating supports of proper design. We might add to the rule by saying: "Keep the conductors of opposite polarity as far apart as space and circumstances will permit." With conductors run upon glass or porcelain, it is more important to have the covering of the wire fire proof than moisture proof. Rule "e" is usually interpreted to mean that a high grade of insulated wire should be used, and that the insulation should have a flame-proof covering or braid. The best practice for a dry station is to use a fire-proof covering and to depend upon porcelain and glass for insulation in case of accidental dampness. It should be the engineer's aim to secure good insulation in any event, and to do it with the smallest possible amount of combustible material. In many cases, this means none at all. Where several dynamos are run in a power or incandescent lighting plant, it is common practice to lead all the circuits from the dynamos to one pair or set of conductors upon the switch board, and from these conductors to lead off all the lighting or power circuits. These main conductors are usually so large that flat copper bars are used instead of wires. These bars, as they carry the entire load of the plant, are called "bus bars." The bus bars are usually bolted fast in position, and, as there is no possibility of their getting out of place, it is perfectly safe to have them insulated only by their supports. The rule "g," con

g capacity," simply means that the ge enough so that they will not heat current that they may ever be called his subject is taken up more fully in