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the service is interrupted. It is more dangerous to operate power or lights from a trolley line than from a regular power or light circuit, for the following reasons: One side of the circuit is always connected with the earth. With other circuits every precaution is taken to keep both wires thoroughly insulated from the earth, but with the trolley circuit one side is grounded intentionally, permanently, and as well as possible. Where both wires of a circuit are insulated from the ground, to have trouble with earth connections it is necessary to have two connections with the earth, one on each wire at the same time. With a trolley circuit, a bad leak or short circuit is caused by one connection between the one wire and the earth at any time. With any circuit the electrical pressure is constantly striving to force current through the insulation: there are always minute leaks. When the pressure is doubled these minute leaks double and are much more likely to become rapidly worse. With a circuit having both wires insulated, only half the total pressure is acting to break down the insulation on each wire; but where one side of the circuit is grounded, the total pressure is acting to break down the insulation on the single wire. With a grounded circuit there is likely to be more trouble from lightning because there is a connection to the earth through motors and lamps. The discharge in passing through these is apt to cause damage. It is difficult satisfactorily to operate motors from the trolley-wire when a proper fuse is used, because the fluctuations in the pressure cause the motor often to take rushes of current that will melt the fuse unless it be too large for good protection.

The foregoing are the principal reasons why light and power from trolley lines are so strongly objected to by the underwriters. Incandescent lamps operated from railway wires would be prohibited by Rule 14 (<?) as well as by Rule 41, because with a 500 volt circuit it is necessary to connect five lamps in series and there is thus a multiple-series system.

Of course it does not make an insulated metallic circuit if two wires are run from the motor to the power-house, because the only change this makes is the connection of one wire to the earth at the dynamo instead of at the motor.

Figure 76 also illustrates the way in which the current in a trolley circuit returns to the generator. A part returns through the rails, and a part flows into the earth, and returns through buried pipes, moist earth, or any other conducting substance. This return current divides according to the conductivity of the different paths. (Page 38.) Since there is resistance it takes a part of the pressure of the dynamo to force this current from the car back to the dynamo; thus there will be a difference of pressure between any two points in the return circuit, say between A and B. If a gas-pipe should run near the point A and a water-pipe should run near the point B, and if these two pipes should touch each other somewhere, there might be dangerous arcing due to the current finding its way back through the pipes. This is one of the reasons for Rule 43. The other and greater reason

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Fio. 76. — Illustrating Street-railway Circuit and Paths taken by Current.

is, that if there are considerable differences of potential there is destructive action called "electrolysis" wherever the current leaves the pipe for the earth, as at C, Figure 76. The iron of the pipe is carried away into the earth toward the negative pole of the generator, just as in a silver-plating battery the silver is carried away from a silver plate toward the article to be plated which is attached to the negative pole of the battery.

Class P.

44. Storage or Primary Batteries:

a. When current for light and power is taken from primary or secondary batteries, the same general regulations must be observed as applied to similar apparatus fed from dynamo generators developing the same difference of potential.

6. All secondary batteries must be mounted on approved insulators.

[Section b. Insulators for mounting secondary batteries to be approved must be non-combustible, such as glass, or thoroughly vitrified and glazed porcelain.]

c. Special attention is directed to the rules (page 121) for rooms where acid fumes exist.

d. The use of any metal liable to corrosion must be avoided in connections of secondary batteries.

In a "primary battery" an electric current is produced by chemical action. If, for instance, a copper plate and a zinc plate are placed in a jar containing dilute sulphuric acid, chemical action produces an electrical pressure between the two plates, and if they be connected by a wire or other conductor forming a circuit, a current will flow. Batteries for operating the telegraph, call-bells, burglar-alarms, etc., are primary batteries.

One form of "secondary" or "storage battery" is made by placing lead plates and sulphuric acid in a suitable jar and then forcing a current .through the lead plates and acid. When the current passes, the lead plates are chemically changed, so that if after a time the battery be disconnected from the circuit by which current is furnished, it will act like a primary battery and will produce an electric current when connected to a circuit of wires or other conductors.

Batteries may be connected so that they will give a high electrical pressure or a large current, or both, and the same precautions are then to be taken with the wiring that would be taken if the current were coming from a dynamo. With primary batteries the pressure is ordinarily very low and the current very small, so that as far as danger from fire is concerned, usually little care need be taken.

Secondary batteries are mounted on insulators because salts formed by the chemical action frequently creep over the surface of the jars, and there would be leakage of the current if the jar were not properly insulated.

Acid fumes are often produced by batteries, and care must be taken that such action will not destroy the insulation or injure apparatus.

If the metal used about secondary batteries were to corrode and waste away, the carrying capacity of the

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