of attributing to electrical causes all fires for which no other cause was readily apparent. Today, however, the production of light and the transmission of power are without question accomplished more safely as well as more conveniently and economically by electricity than by any other means.

In judging electrical work as affecting the fire hazard, the fireprotection engineer and underwriters cannot undertake to do for the assured, the work which lies within the province of the electrical engineer on whom properly falls the responsibility of designing installations and choosing the methods and materials to be used to accomplish the results desired. In electrical, as in other engineering, considerations of first cost, economy of operation and maintenance, efficiency of machinery and appliances, depreciation, and reliability are of prime importance. It is the proper function of the fire-protection engineer to act as critic of the plans and their execution in order that other considerations involved shall not be allowed to dictate methods which do not afford suitable and reasonably safe fire conditions. Electrical engineering is far too complicated a profession to be fully mastered by the fire-protection engineer or by an inspector, but the general principles of safe electrical work can be mastered and applied without presuming to encroach on the work of either the consulting or the installing engineers.

Architects are now very generally prepared to recognize the necessity of providing in their plans for the electrical installations which in more elaborate buildings must be given very careful attention if adequate provision is to be made for electric wires and appliances. It is in the smaller and less carefully planned structures that the electric work is most likely to be left to the installers to be put in as best it may. A well considered plan adapted to the type of building and the uses to be made of current is an essential to successful and safe work. Makeshifts are usually dangerous. For this reason, also, it is often much more difficult to secure proper electrical work in wiring old buildings than new ones.

~ Electrical contractors are usually willing to do good work for a fair price and most poor work is due to an attempt to secure business at too low rates in order to meet competition. Since a faulty or dangerous electrical installation may perform its appointed work, and since electrical defects are not easily discovered, often develop

ing after some time, only thoroughly good work from the start affords real protection against disastrous failure from causes which, after a fire, may be very difficult to assign but which might have been easily foreseen and removed.

Electrical inspectors either of the underwriters or of the municipality should be fully acquainted with approved methods, rules, reasons for rules, and must have a great range of practical knowledge which only actual field experience can give; and such inspectors must be specialists of a high order. The fire-protection engineer must refer to such specialists the technical details while undertaking himself to be a competent judge of general methods and principles.

Electricity as a Cause of Fires. Electricity may in general be the cause of a fire in either one or both of the following ways:

First: By causing wires, cables or other conductors to be overheated by the passage of the "electric current," thereby setting fire to the insulations on such conductors or to nearby combustible materials. All conductors are heated by any current however small, but if the conductor is badly overloaded it may become red- or even white-hot. Hence it becomes necessary to prescribe the safe carrying-capacity of wires, and all conducting parts of electric appliances must be properly proportioned. Such parts as are peculiarly liable to become heated or which must be heated to perform their proper function must be suitably protected and separated under all circumstances from materials which might be ignited.

Fuses, circuit breakers, and other automatic appliances have to be installed to afford protection in case of accidents which may result in conductors or other devices being overheated by abnormally large currents. Such protective devices may themselves become dangerous and the rules, therefore, prescribe carefully how they must be constructed and installed. Overheating of conductors is thus one of the two general ways in which electricity may cause a fire and provisions against such accidents, therefore, form an important part of the rules.

Second: Electricity may form "arcs." An arc is the visible evidence that the current is passing from one conductor to another or across a break or gap through the air or over the surface of an insulating material. An arc always causes heat and if any appreciable current passes, the arc will be very hot and if continued is capable

of melting the adjacent metal at the gap or setting fire to any combustible matter near by. It, therefore, becomes necessary to prescribe rules, the observance of which tends to lessen the chance of accidental arcs being established, and to so locate all appliances, the operation of which produces arcs, that no harm will result. Switches, circuit breakers, and fuses are all "arcing" devices. The problem of insulation, spacing between conductors and excellence in materials, methods of installation and workmanship, all have to do chiefly with the prevention of accidental or dangerous arcs.

From the viewpoint of fire hazard, it is well to treat all conductors, however well insulated, as bare, and to proceed to furnish adequate protection against grounds or short-circuits on that basis.

In judging of installations it must constantly be kept in mind that conditions are liable to become worse rather than better after the wiring and appliances have been in use for some time. Requirements are, therefore, made to anticipate in part such deterioration as is inevitable in even the best equipments, and which may be surprisingly rapid when inferior materials are put in by careless workmen and used and abused by those having little or no understanding of electrical affairs and no appreciation of the hazards involved.

The following are the chief general requisites for a safe electrical installation: Excellence of material; simplicity in design so far as compatible with the results to be secured; ease of inspection and repairs of all wiring and appliances; thoroughly good mechanical execution of the work; the choice wherever possible of the more protected and safer forms of wiring; and the use of "approved fittings." No rules can take the place of good designing nor will perfunctory obedience of rules make a poorly executed job, safe. The architect, the owner, the manufacturer of devices and materials, the electrical contractor, the electricians who install and those who operate the plant-each and all must share the responsibility with the insurance and the municipal inspector.

Elementary Electrical Ideas and Terms. Electric power may be transmitted by either direct or alternating current.

Direct Current. Direct current is current of such character that what is usually called the "direction" of the current is always the same, or, more exactly, the magnetic effects of the current are not being reversed from instant to instant. If a small compass be

held near a wire carrying direct current, the needle may be caused to turn away from its natural north and south line. Thus in Fig. 1, if D is a direct-current generator connected to a wire from south to north, a compass needle over the wire, before the switch T is closed, will point along the wire; when the switch is closed, if current flows as shown by the small arrows on the wire, the needle will turn as shown. The amount it will turn is an indication of the amount of current, but the needle will remain stationary in its new position if the current is direct current. A battery gives direct current and so does a direct-current generator.

T

Alternating Current. If, however, the current came from an alternating-current generator or from a transformer supplied by such a generator, the needle would tend to swing very rapidly first to one side of the wire, and then to the other. This would also be the case if the connections on the direct-current machine, Fig. 1, were rapidly exchanged back and forth. Such reversals of current direction are made automatically by an alternating-current generator. The number of changes per second is called the frequency and 25, 60, and 135 are the commonest commercial frequencies of alternators. Evidently no

(a)

(b)

Fig. 1. Diagram of Simple Generator Circuit
Showing Effect on Magnetic Needles

compass needle could actually vibrate so fast, but it tends to do so, and the result is that the needle does not appear to be affected by the alternating current.

Both direct- and alternating-current systems are in common use for light and power. Street railways use at present chiefly directcurrent systems. Where power must be conveyed to considerable distances, alternating current is used because it is more economical under these conditions. Different motors and somewhat different arc lamps are required for direct and for alternating circuits. Incandescent lamps are the same for either system as are also most heating devices. Transformers can be used only on alternating-current systems.

The distinction between direct and alternating current is not one which has many important consequences in the safeguarding of electrical work. There are, however, a few important cases where different rules are established. It should be remembered that direct and alternating currents of the same strength produce the same heating effect in a given conductor. In some cases, however, with alternating currents an additional heat effect is produced in certain appliances by the magnetic action of iron cores, of coils, or other parts of the apparatus. In general, alternating current produces less severe and persistent arcs than direct current of the same strength and voltage. Furthermore, alternating-current motors in particular are somewhat less liable to emit sparks and, therefore, have a certain advantage. As a whole, however, no distinction may be made as regards fire hazard between direct- and alternating-current installations which should be made with the same care in workmanship and with the same precautions as to insulations, fuses, and all protective devices.

Current. Current is measured in amperes. It may be compared to the number of "gallons per minute" carried by a water pipe through which a stream is flowing. More "current" will, other conditions being equal, do more work, and will always cause more heat in the conducting wires and cables and in the appliances, lamps, heaters, resistances, motors and the like which use the current. Furthermore, the heating effect in conductors such as metals varies with "the square of the current," i. e., if one unit of current produces a certain amount of heat in a wire, twice as much current will cause four times as much heat in the same wire, three times the current will cause nine times the heat and so on. An instrument for measuring current is called an ammeter.

NOTE. The heat liberated is a measure of energy or power consumed in the wire. The temperature of the wire will not necessarily follow the rule given above, this being dependent not only upon the heat developed but also upon the surroundings of the wire as affecting the readiness with which the heat may be radiated or otherwise gotten rid of.

Voltage. Voltage or potential is measured in volts. Volts measure the propulsive force which causes "current" to flow through a conductor. It may be thought of as an electric pressure produced by the dynamo, battery, or other generator of electricity. Through