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be run as nearly as possible in a straight line from the arresters to the earth connection.

d. Must be so constructed as not to maintain an arc after the discharge has passed.

A "lightning-arrester" would perhaps more properly be called a lightning diverter, since it does not arrest the lightning, but diverts it from the wiring by offering a comparatively easy path to the earth. If there

A

LINE

EARTH

FIG. 18.-Showing Principle of Lightning-arrester.

were not this easy path, the lightning would pierce the insulation in some other place, because of the great force urging a discharge. The lightning-arrester is made so that the connection with the earth is broken as soon as the lightning or "static discharge" has passed, for otherwise there would be an opportunity for the leakage of the dynamo current.

Figure 18 shows the principle of a lightning-arrester. At A is a very short air-space that is an effectual insulator as far as the normal electrical pressure on the line is concerned. When a lightning charge accumu

lates on the line there is a very strong tendency for it to break through the insulation to the earth, and the short air-space is easily sparked across. The air is heated somewhat by the discharge, and the normal electrical pressure on the line is sufficient to maintain. a constant leakage to the earth if there is not some arrangement that breaks the connection. This connection can be broken in several ways. Often the arrester is made so that after a discharge the air-space is increased until the electrical pressure can no longer force a current through it, and sometimes a strong magnet is directed across the space; it being a curious fact that a magnet will repel an arc and thus "blow it out."

"Each side" of the circuit means each wire of the circuit; the one going out and the one coming in.

An "earth" is a connection with the earth, or the special arrangements, such as buried metal, pipes, etc., that insure the electrical connection. A good earth is made by imbedding a copper or iron plate about two feet square in broken coke and burying the whole in moist earth. All connections should be soldered.

The wire gauge of Brown & Sharpe is the one by which copper wire is usually measured in this country, and it is commonly called the "B. & S. gauge."

The electric "arc" has been explained. (Pages 33, 47.) It is familiar in the arc lamp. A current of elec

tricity in being forced through an air-space meets a high resistance, and the energy required to force the current through this resistance is transformed into intense light and heat. The heated air and volatilized substances make of the space a conductor to some extent, but one with a comparatively high resistance.

Lightning discharge currents do not act the same as the currents from dynamos. They do not always take the path that would be taken by a dynamo current, but frequently jump across spaces of air rather than go round corners and bends in conductors. It is consequently important that the path from the lightning-arrester to the earth be straight, so that the lightning discharge will not cause damage on its way to the earth. Though arresters are made to break automatically the connection with the earth as soon as the discharge current has passed, a heavy discharge may prove too much for the arrester and the arc be long continued. For this reason, everything about the arrester should be non-combustible. Since parts of the arrester have good connection with the earth, special care must be taken to prevent leakage, and consequently the base on which the parts are mounted should be of a material that will not absorb moisture.

The two earths are required to sufficient connection with the earth.

assure good and It is bad prac

tice to connect to pipes in a building, because there is then no certainty that the discharge current will go to the earth without damage in some unlooked-for place.

7. Testing:

a. All series and alternating circuits must be tested every two hours while in operation, to discover any leakage to earth, abnormal in view of the potential and method of operation.

b. All multiple-arc low-potential systems (300 volts or less) must be provided with an indicating or detecting device, readily attachable, to afford easy means of testing where the station operates continuously.

c. Data obtained from all tests must be preserved for examination by insurance inspectors.

These rules on testing to be applied at such places as may be designated by the association having jurisdiction.

A "series circuit" is one in which everything in the circuit, dynamo, wire, lamps, etc., comes one after the other, "in series "; the same current going through each. (Fig. 19.)

The electrical pressure necessary to force the current through such a circuit is usually high, because each device requires a certain pressure, and the total pressure on the circuit must be the sum of the pressures required by the different devices. The high pressure, of course, makes leaks more probable and more disastrous. (Page 31.)

In "alternating circuits" the current alternates in direction. Such a circuit has peculiar properties that make it possible, where branches are taken off, to change

FIG. 19.-Series Circuit, showing Same Current passing
through Lamps and Motor.

the pressure with transformers to anything desired. With alternating circuits the pressure between the wires that leave the station is high, and this system is consequently classed with series systems. (Page 154.)

оро

FIG. 20. Multiple Circuit, showing Lamps and Motor working

with Constant Pressure.

In the "multiple-arc" system, or, better, the multiple system, there is a constant pressure between the main wires, and the different devices are connected-in between the wires, each device operating at the full press

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