result is an arc just like that formed with the carbons in the arc lamp.

A leaking current is likely to cause these arcs, because when a current passes by means of a semiconductor, it often burns away small particles and is maintained by an arc from one conducting particle to another. When a current is once established it is not easily broken, and a distance that would have at first proved a complete barrier is easily bridged by the arc with its hot, semi-conducting gases. But leakages of this kind are not the only cause of destructive arcing. One wire may come in direct contact with the other wire of the circuit, or it may come in contact with a pipe or a bell-wire in accidental connection with the other side of the circuit; and in this case there may be a "short circuit" with a very heavy flow of current, or there may be a sputter, and a burning away of the metal in contact, which results in a long arc of intense heat.

Excessive currents in multiple systems are provided against, by the use of a small wire or strip of fusible alloy, which will melt and thus break the circuit before the current has had time dangerously to heat the copper wire. These fuses are supposed to be of sufficient length to make it impossible, when they melt, for the arc to hold across the gap in the circuit, and the fuse itself is enclosed in a non-combus

tible case that will not be injured by the momentary heat. In series circuits, where the current is of uniform strength, the fuse cannot be used; and even in a multiple system, if the fuse is not properly proportioned, a disastrous arc may form and be maintained.

The modern methods of wiring have accordingly been designed to prevent, as much as possible, the leakage of electricity and the formation of "short circuits"; to keep local the effects of these accidents when they happen; and to make it easy to repair defects. The underwriters now require all wires not encased in approved forms of moulding or in approved makes of conduit, to be supported wholly on non-combustible insulators. It is their intention that wires shall not even come in contact with anything else. They also have made stringent regulations with regard to the insulation that is to cover the copper wire. Manufacturers submit samples of their wire, and it is subjected to severe tests before the underwriters will approve its use. No wire is allowed in concealed work, or in places exposed to dampness, except of these brands that have been approved; but where the wire is entirely exposed and is in a perfectly dry place, a somewhat inferior insulation is allowed.

It was thought in the early days that wiring would be perfectly safe and satisfactory if it were simply

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buried in the plaster, or if it were strung through the spaces between floors and ceilings and in hollow walls. Though wire covered with a good insulation was often run in this way, a number of difficulties were soon met with. Carelessness in drawing the wire through confined places abraded or stripped the insulating covering; workmen sometimes pierced the insulation or even cut the wire, with nails and tools; and there proved to be alkalies and acids in the plaster that sooner or later broke down all resistance to electrical pressure. Moreover, these methods of wiring left no way to make repairs. In the event of a failure, there was no remedy but to tear the wire out of the plaster, or to pull it out of the spaces in the walls and ceilings, leaving no way to replace it. There are now two approved methods of running wires for low potential circuits; they may be supported on non-combustible insulators, or they may be run in conduits. Wires may be run between floors and ceilings, in the hollow spaces in walls, or in any place one wishes, if the conductors are supported wholly on non-combustible insulators, such as glass or porcelain; if the wire has an approved insulating covering; and if in passing through walls, floors, timbers, ceilings, etc., proper insulating bushings are used. Wiring by this method can be made safe and satisfactory if the work is done conscientiously. Carelessness may, however,

leave vulnerable places for the carpenter's nail; there is no certainty after the work is finished that it has been properly done; and it is difficult to replace wiring that from any cause becomes defective. The greater of these disadvantages may be overcome if the architect will make provisions for the free access to the wires as he sometimes does to pipes. Accessible wire ways on or in the walls should be provided for upright wires, and floors should be arranged so that any part of the horizontal wiring can easily be reached. When these precautions are taken, this method of wiring on insulators may be made safe, trustworthy, and convenient.

The greater part of the concealed wiring in the larger buildings is now done, however, in conduits. The building is first fitted with a system of waterproof, and to some extent fireproof, tubes, much as if a twin system of gas-pipes were being installed. These tubes lead from the mains to the branches, and from the branches to the lamps, and wherever a wire is to be connected to another wire, a "junction-box" is provided. The wires are then drawn through these tubes and the necessary connections made. The tubes are not depended upon for insulation, but are simply raceways for the wires, protecting their insulating covering from injury, and affording a confined channel through which new wires may, at any time, be drawn.

Aside from the accessibility, there is the advantage with this method, that though the conduit is installed at the time the building is erected, the actual wiring may be left until all carpenters and finishers have gone, and when there is no longer danger from carelessly driven nails or slashing tools. The space required for the conduits is less than that needed for porcelain insulators, which is of importance in the larger installations.

It seems probable that conduit, in some form, is the last stage in the development of concealed wiring. For some time it has been possible to get wire covered with an insulation of very good quality, and the trouble has been that this insulation was not protected from mechanical and chemical injury. A proper conduit will protect the insulation; it makes possible a more suitable time for running the wires; it gives compact construction and what may properly be called an electrical system; and it makes this system of wires capable of change or renewal at any time.

But with any method the electrical features should have consideration when the building is designed. This does not make necessary any great changes in construction or in arrangement, and a little attention given to desirable provisions makes the wiring installation a part of the whole building system rather than simply an adjunct; it also gives sys