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candescent lamps, but the rule applies only to circuits of 300 volts or over. The reasons for Rule 14 (c) will apply with more or less force to all series-multiple or multiple-series lighting, and the danger is considerably increased with decorative lamps because they are usually surrounded by inflammable material.

ALTERNATING SYSTEMS.

33. Converters:

CLASS D.

CONVERTERS OR TRANSFORMERS.

a. Must not be placed inside of any building, except the Central Station, unless by special permission of the Underwriters having jurisdiction.

b. Must not be placed in any but metallic or other non-combustible cases.

c. Must not be attached to the outside walls of buildings, unless separated therefrom by substantial insulating supports.

34. In those Cases where it may not be possible to exclude the Converters and Primary Wires entirely from the Building, the Following Precautions must be strictly observed:

Converters must be located at a point as near as possible to that at which the primary wires enter the building, and must be placed in a room or vault constructed of, or lined with, fire-resisting material, and used only for the purpose. They must be effectually insulated from the ground, and the room in which they are placed be practically air-tight,

except that it shall be thoroughly ventilated to the out-door air, if possible through a chimney or flue.

Any explanation of alternating currents and of the laws regulating their action requires so technical a discussion that a simple statement of important facts will probably leave clearer ideas than would an attempt at an exposition.

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With the alternating system the dynamo ordinarily generates a pressure of 1000 or 2000 volts. formers" 66 or converters are then used to change this pressure, at the place where the current is to be used, into a lower pressure of 50 or 100 volts. The pressure is high before the transformation is made and the current comparatively small; after the transformation the pressure is low and the current comparatively large. The available amount of energy is but little changed by this transformation or conversion; the current multiplied by the pressure (the number of watts) is virtually the same before as after.

This transformation is possible with alternating currents because of the principle called "induction." Suppose (Fig. 72) a coil of wire B to be lying near, but not touching, another coil A in which a current may be started either in the direction of the arrows C, C, or in the direction of the arrows D, D. Without discussing the hypotheses made to account for the

D

COIL B

COIL A

D

DYNAMO

FIG. 72.-Illustrating
Induced Currents.

phenomena, it may simply be stated that if a current be started in coil A, a current will also flow momentarily in coil B if its ends are joined; if they are not joined there will be only a momentary electrical pressure between the ends of the coil. If the current in coil A be now changed in direction, there will be a momentary current in coil B in the direction opposite to that in which first instance. These currents in B are what are called "induced currents." They last but a very short time, and if the current started in A be continued in the same direction, there is only a short rush of current in B at the starting of the current in A. To keep a current flowing in B it is consequently necessary to keep the current in A constantly alternating in direction. An alternating dynamo is so

constructed that it will do this.

it flowed in the

The coil A which is connected with the dynamo, and in which the current first flows, is called the "primary coil," and the coil B in which the induced current flows is called the "secondary coil." The coils in Figure 72 have each but one turn of wire. The relation between the number of turns in the primary coil and the number of turns in the secondary coil, determines the relative electrical pressures and the

relative currents in the two coils. Thus, if there are 1000 turns in the primary coil and 100 turns in the secondary, the ratio between the number of turns is 10 to 1 and the electrical pressure in the secondary coil will be one-tenth of that in the primary coil; but the current in the secondary coil will be 10 times that in the primary coil. If, for instance, the pressure at the terminals of the primary in the case supposed is 1000 volts and the current flowing is 10 ampères, then in the secondary the pressure will be 100 volts and the current will be 100 ampères. The rate of work in the two coils (the watts) will be the same; in the primary coil the number of watts will be 1000 x 10 = 10,000 and in the secondary the number of watts will be 100 × 100 10,000. There is simply a transformation from = high-pressure and small current to low-pressure and large current. The transformer has somewhat the effect that a reducing-valve has with gas when it allows a larger flow of gas at a lower pressure. It should be remembered, however, that with the electrical transformer there is no electrical connection between the two coils; there is a space between them, and the current in the primary coil with its highpressure is not free to pass to the secondary coil.

In practice the terminals of the primary coil are connected to the wires from the alternating dynamo,

as in Figure 73, and the lamps are then connected-in between the wires leading from the terminals of the secondary coil, just as they would be on any other low-potential circuit. The two

DO

M

M

P P

S

A

B

coils are surrounded by soft iron, since this greatly increases the effect and makes it possible to transform a large amount of energy in a comparatively small piece of apparatus. The two coils are compactly wound, thoroughly insulated, and placed near together, and iron plates are so formed that they will fill the space within and about the coils. The whole is then enclosed in a cast-iron case for protection against the weather and against mechanical injury.

D,

FIG. 73. Illustrating Transformer Connections. Dynamo. M, Mains. P. Primary wires. A, Primary coil. B, Secondary coil. S, Secondary wires. L, Lamps.

The great advantage of the alternating system is in the saving of copper in the street conductors. If, for instance, the rate of output of a station is 100,000 watts, by using the alternating current the volts. may be 1000 and the ampères 100 (1000 × 100 = 100,000), while if the direct current (one flowing constantly in one direction) were used and the pressure were everywhere the same as at the lamps, say

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