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ure of the system and taking only such current as the resistance of the device allows. (Fig. 20.)

The pressure between the mains of a multiple system -except alternating circuits-is usually below 300 volts, and is consequently "low," but there are often multiple circuits on which motors are operated, that have a pressure of 500 volts.

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(Page 36.)

EARTH

EARTH

FIG. 21.-A Form of Ground-detecting Device.

"Detecting" or "indicating devices" are arranged to operate by the leakage current. A simple form that will serve as an illustration, is as follows:

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Suppose that in Figure 21, A and D are the main wires of a multiple system, and that l and l' are two incandescent lamps connected-in between the mains, but in series. They will not burn brightly, because instead of each lamp's getting the full pressure, each gets only one-half. At B suppose there occurs an accidental connection with the earth. If now there be a per

manent ground-connection at C, the current that passes through 7 does not all go through l', but can partly, at least, leak through the earth, up to B and thence to D. As a result, less current than before the accidental connection occurred will go through l' and more than before through l. This will, of course, make 7 brighter and 'less bright. Thus when the left-hand lamp burns brighter, there is a leakage to the ground on the right-hand wire, and when the right-hand lamp burns brighter, there is a leakage on the left-hand wire. A lamp may also be put in the wire leading to the ground. This lamp will then light up more or less, according to the amount of the leakage.

MOTORS.

8. Motors:

a. Must be wired under the same precautions as with a current of the same volume and potential for lighting. The motor and resistance-box must be protected by a doublepole cut-out and controlled by a double-pole switch, except in cases where one-quarter horse-power or less is used on low-tension circuit, a single-pole switch will be accepted.

b. Must be thoroughly insulated, mounted on filled dry wood, be raised at least eight inches above the surrounding floor, be provided with pans to prevent oil from soaking into the floor, and must be kept clean.

c. Must be covered with a waterproof cover when not in use, and, if deemed necessary by the Inspector, be enclosed in an approved case.

[Section c. From the nature of the question, the decision as to what is an approved case must be left to the Inspector to determine in each instance.]

9. Resistance-boxes:

a. Must be equipped with metal or other non-combustible frames.

b. Must be placed on the switch-board, or at a distance of a foot from combustible material, or separated therefrom by a non-inflammable, non-absorptive, insulating material.

[Section a. The word "frame" in this section relates to the entire case and surrounding of the rheostat, and not alone to the upholding supports.]

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"Cut-out" here means fusible cut-out. Generally speaking, a cut-out is any device that makes a break in the circuit and "cuts out" any lamp, motor, or other piece of apparatus. The term is, however, becoming commonly applied to fusible cut-outs only. The fusible cut-out ordinarily consists of a block or box of porcelain arranged so that a "fuse," or piece of easily fusible metal, will form a part of the circuit. When the current becomes too great, the fuse melts, because of the heat that the current gener

FIG. 22.-Double-pole
Cut-out.

ates in it, and the circuit is thus broken. This fuse is supposed to be so proportioned that it will melt before the copper conductors can become unduly heated by an excessive current. The fusible cut-out is frequently called a "fuse-block."

A "double-pole" device is one that acts in each of the two poles, two wires, or two sides of a circuit. It is required that cut-outs and switches be double-pole so that the cutting-off will be complete and certain.

Suppose that, as in Figure 23, the fuse is singlepole, that is, acting on one wire of the circuit only. A heavy current flowing through the circuit will, of course, melt the fuse and the flow be stopped. But suppose that the fuse is just inside the wall of the building, and that the wire A outside the wall comes into connection with a metal column or with some other conductor connected with the earth. Suppose, moreover, that there is a connection with the ground on account of poor insulation at the motor, or that the wire comes in contact with a gas-pipe somewhere along BC. (Page 59.) Then the earth will be a by-path or "shunt" round the fuse and will make a closed circuit even though the single-pole switch be open. In this case a large part of the wire will not be protected by the fuse at all. A double-pole cut-out or doublepole switch will, however, cut off all the circuit beyond it, as may be seen by comparing Figures 23 and 24.

The "switch" is a device that operates mechanically, and is simply an arrangement by means of which one can pull out of circuit, metal that bridges over a gap. An air-space or some other insulation is left, over which the current cannot pass.

When the circuit is broken, there is an arc at the point of separation. If the circuit be broken at only

B

FIG. 23. Showing Single-pole Cut-out FIG. 24.-Double-pole Cut-out and Single-pole Switch. and Double-pole Switch.

one point, the arc will be greater than if there are several points of separation. With a double-pole switch having two breaks in each pole, there is thus much less arcing upon opening, than if the switch has a single-pole with one or two breaks. With a small motor running on a low-potential circuit, the current is small and the pressure low, so that the arc formed

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