"off," so that it shall be known definitely, without regard to anything else, whether the current is turned on the wiring of the building or not. The marking also increases the probability that the switch will be thrown in case of necessity, even by one unfamiliar with its working.

Freedom from moisture is required, so that there will not be leaks between different parts of the switch, or between the parts and the ground. The non-combustible case prevents damage that might be caused by overheating or arcing in a defective switch.

Though the form of switch specifically mentioned in Rule 12 (6) can be used only on series circuits, the rule applies generally to all circuits having a pressure of over 300 volts, whether series or multiple. In the case of a multiple circuit, such as a 500-volt motor-circuit, the switch must be of some such form as that shown in Figure 24, where the circuit is broken at once. In this case, it is the pressure that remains constant, and if a cross-connection were made, as in Figure 37, the dynamo would tend to maintain a constant pressure between the two sides of the circuit, and there would consequently be a tremendous flow of current through the connection of low resistance, that would be stopped only by something being melted or burned out.

Snap-switches are not allowed for use on high-potential circuits, because the air-spaces, which make the

breaks in the circuit when the switch is off, are very short and the arc that forms upon breaking the circuit is likely to bridge over these spaces, destroy the switch, and endanger property. Snap-switches are sometimes used to extinguish arc lamps by closing a by-path; as if, for instance, one were introduced in E in Figure 37. A switch of this kind does not perform the functions of a service-switch, because, although it forms a bypath of low resistance and so causes the lamps to be extinguished, it does not break the connection to the lamps as in Figure 38.

It is required that wires forming a high-potential circuit shall always be in sight, because, with hidden wires, the conditions are always uncertain and the danger of leakage, with its resulting effects, is much more serious with high potential than with low potential. Where wires are exposed to mechanical injury it is sometimes necessary to enclose them.

The kinds of insulated wire that are approved for interior conductors are the same as those given under Rule 10 (a), page 80.

The greater danger from leakage from high-potential circuits, is the reason for the requirement that all insulating-tubes, even in dry places, shall be of noncombustible material.

Soldered joints are, of course, quite as necessary for inside work as for that outside. (Page 84.)

LAMPS AND OTHER DEVICES.

13. Arc Lamps-In every case:

a. Must be carefully isolated from inflammable material.

b. Must be provided at all times with a glass globe surrounding the arc, securely fastened upon a closed base. No broken or cracked globes to be used.

c. Must be provided with an approved hand-switch, also an automatic switch, that will shunt the current around the carbons should they fail to feed properly.

[Section c. The hand-switch to be approved, if placed anywhere except on the lamp itself, must comply with requirements for switches on hanger-boards as laid down in Section g of Rule 13.]

d. Must be provided with reliable stops to prevent carbons from falling out in case the clamps become loose.

e. Must be carefully insulated from the circuit in all their exposed parts.

f. Must be provided with a wire netting around the globe, and an approved spark-arrester above to prevent escape of sparks, melted copper, or carbon, where readily inflammable material is in the vicinity of the lamps. It is recommended that plain carbons, not copper-plated, be used for lamps in such places.

[Section f. An approved spark-arrester is one which will so close the upper orifice of the globe that it will be impossible for any sparks thrown off by the carbons to escape.]

g. Hanger-boards must be so constructed that all wires and current-carrying devices thereon shall be exposed to view and thoroughly insulated by being mounted on a waterproof, non-combustible substance. All switches attached to the same must be so constructed that they shall be automatic in their action, not stopping between points when started, and preventing an arc between points under all cir

cumstances.

h. Where hanger-boards are not used, lamps to be hung from insulated supports other than their conductors.

The "arc lamp" makes practical use of the fact that when an air-space is introduced into a circuit, the current in passing through this space heats the ends of the conductors and the resulting gases to a very high degree of incandescence. (Page 33.) Carbon is used as the most suitable material to form the conductors on each side of the air-space. The arc lamp as used is made up principally of regulating apparatus that keeps the arc at the best length by feeding the carbons as they burn away, and maintains steadiness in the light. (Fig. 39.)

The "hand-switch," mentioned in Rule 13 (c), is a small switch that will close a by-path round the lamp and thus give a path other than the carbons for the passage of the current. The rules require it so that the lamp may be cut out in case it does not work properly. (Fig. 39.) The "automatic switch" serves the same purpose, but is a part of the regulating apparatus

of the lamp itself and cannot be moved by hand if the lamp fail to operate.

The carbons are held in place by "clamps." (Fig. 39.) Should the clamp holding the lower carbon be

come loose, the carbon, with its intensely hot end, will drop to the floor unless there be a "stop" of some kind to hold it. (Fig. 39.)

The "wire netting" required around the globe when

H