An introduction to the study of electrical engineering

Front matter

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Contents

Cbaptsr. Pagb.

I. Historical Development of Electrical Engineering ... 17

II. Fundamental Electrical and Magnetic Quantities ... 51

Introduction

Pig. 9. Map of the transmission lines of the Ontario Power Company illustrating the case of transmission of power by electrical means.

interesting to note that power is being transmitted electrically a distance of two hundred miles or more.

Examples of Power Transmission. A trolley car climbing a grade furnishes a concrete illustration of mechanical power transmission. The power station furnishes current through the feeders and trolley wire and the rail return. In the motors which are mounted upon the car axles, the electric power is changed into mechanical form and drives the car against friction and grade resistances. The electric circuit, which includes the generator, line, and motors, acts merely as a connecting link between the engines and the driving gear of the car.

Another striking illustration of power distribution is furnished by the lines radiating from Niagara Falls. Five great power plants transform several hundred thousand horsepower. Part of this is used in the vicinity, but a large part is transmitted to Toronto, Lockport, Buffalo, Rochester, Syracuse, and other cities within a radius of two hundred miles, and the range is constantly extending. The electric power is used for any purpose that could be accomplished by power plants located at the points of demand.

2. Production of Light.

When electric current is forced through a circuit, heat is generated, and the amount produced depends upon the nature of material and upon its dimensions. It is evidently more difficult to pass a current through a long conductor of small cross-sectional area than through a short one of large area. In other words, the smaller conductor offers more resistance to the passage of the current than the large one. When the amount of heat generated in any part of a circuit is sufficiently great light is given off. This light is of two kinds, which are given the names incandescence and luminescence.

INTRODUCTION.

Incandescence is due to the high temperature of the substance^ and the intensity has a definite and well-known relation to the temperature without regard to the substance heated.

Luminescence, sometimes called " cold light," while depending somewhat upon the temperature, also involves a property possessed by some materials of generating light waves in greater quantity than can be accounted for by incandescence. This process is frequently named selective radiation.

The most common examples of incandescent light are furnished by the incandescent and the plain carbon arc lamps. In the former a filament of carbon or met al is heated in vacuum

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3. A carbon filament incandescent lamp in which electrical power is changed into heat and light.

by the current to a temperature of several thousand degrees centigrade. This temperature is carried as high as the material will allow, but it must always be kept far below the boiling point of the filament in order to minimize the gradual distillation of the material. In the arc lamps the current flows between two carbon points separated by a short distance (one-eighth to three-eighth inch) and connected by a stream of carbon vapor. The current in overcoming the resistance offered by the vapor path or arc heats the carbon tips or

ELECTRICAL ENGINEERING.

craters, so called from their resemblance in form to a volcano crater. At the crater the carbon boils, and, as carbon is prac-

Pig. 4. View and diagram of the carbon arc lamp in which light is produced by the heat generated in the arc.

tically the most refractory of conductors, the crater temperature is the highest which can be produced. A slight

INTRODUCTION.

amount of light also comes from the arc, but this is small compared with that thrown out from the craters.

Luminescent light sources are now attracting attention, and may in time displace the other forms. As a rule, they

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5. Vacuum tube lamp, which gives out light at low temperature and with small Itmiiiious intensity.

produce light more efficiently than the ordinary arc and incandescent lamps. An excellent non-electrical illustration of the principle is found in the Welsbach gas mantle. This mantle is composed of thorium oxide, which when heated to a moderate temperature gives o£F light efficiently. Among electrical lamps involving selective radiation the principal examples now coming into use are the vacuum tube, the mercury vapor tube, and the flaming arc. The first of these consists of a long tube, exhausted to a low pressure. The current passing through the rarefied gases produces a soft, white hght.

Fig. 6. Mercury vapor lamp, in which a greenish light is produced by an arc maintained through mercury vapor.

The mercury arc is produced in a tube of moderate length exhausted to a low pressure. The heat produced by the current maintains a supply of vapor from a reservoir of metallic mercury connected to the tube, and the vapor forms a conducting path for the current. The mercury arc gives off a greenish light at a high eflSciency. The flaming arc lamp produces the most highly efficient light now known, by means of substances added to the carbons, calcium being one of the best materials for this purpose. The calcium particles are maintained by the

INTRODUCTION. 9

arc at such a temperature as to render them highly luminous. The arc then becomes the source of Hght rather than the carbon tips as in the ordinary carbon arc. The carbon furnishes the vapor which acts as a carrier for the luminous substance.

3. Electrolysis.

Electrolysis is the name given to chemical decomposition produced by the electric current. When a current is passed through a salt in solution or through fused salt, between con-

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Fig. 7. Gas generator, in which the current decomposes water into oxygen and hydrogen at the two electrodes.

ducting terminals or electrodes immersed in the liquid, or electrolyte, decomposition of the salt usually occurs. There is a breaking up of the chemical compounds and a recombinlO

ELECTRICAL ENGINEERING.

ation of the elements. The decomposition is apparent in one or more of the following ways :

(a) Resolution of the liquid into gaseous compounds. (6) Decomposition of the base of the salt in solution, (c) Chemical changes in the electrodes.

As an example of the production of gases from a liquid take the case of the oxy-hydrogen generator. Two jars are inverted in a tank containing a sulphuric acid solution. Inside

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Pio. 8. Refinixig tank for copper, which is deposited by the current from a copper sulphate solution.

the jars are platinum or lead electrodes between which the current flows. The decomposition of the solution results in the liberation of oxygen at the positive plate or anode through which the current enters, and hydrogen at the negative plate or cathode. From the jars the gases are piped to storage tanks or gas holders.

In electrolytic refining of copper occurs an excellent illustration of the decomposition of the base of a salt. The copper

INTRODUCTION.

to be refined is to be cast into slabs which are suspended in a copper sulphate solution. In the same tanks are thin sheets of copper which form the negative plates and upon which is deposited the copper dissolved by the current from the anode slabs. This method of refining produces practically pure copper.

The storage battery is the most important application to engineering practice of the electroljrtic changes in electrodes.

Fio. 9. Storage cell, in which energy is stored in chemical form through oxidation and reduction of lead electrically.

In the ordinary lead type of cell with sulphuric acid solution as electrolyte, the energy supplied by the current in electrical form reduces the active material on the negative plates to spongy lead and oxidizes that on the positive plates to lead peroxide. The energy thus stored may be partially recovered by connecting the positive and negative terminals of the cell through an electric circuit.

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ELECTRICAL ENGINEERING.

4* Transmission of Intelligence.

The electric current furnishes the only convenient means for transmitting speech or signals to a considerable distance. This is accomplished by varying the duration of the current as in the telegraph, or by varjdng the intensity of the current as in the telephone. A simple telegraph circuit comprises a

Fig. io. Simple telegraph circuit showing, R. relay ; K. key; Sw. switx:h ; C, contact ; Bl., line battery ; Bs.. local battery ; S, sounder.

battery, sounder, and key, all connected in series. The key closes the circuit for long and short intervals, known as " dashes " and " dots," and combinations of these signals represent the letters of the alphabet. The sounder comprises a small electro-magnet which attracts a plate of iron, or armature, when energized by the current. The armature is carried .by a lever pivoted at one end. The free end of the lever plays between upper and lower limit stops, producing two characteristic sounds. The intervals between the two

INTRODUCTION.

sounds indicate the duration of current in the magnet coils. In the simple telephone circuit there are the transmitter, the battery, and the receiver. The transmitter comprises a diaphragm against which the speaker talks. Connected to the diaphragm is a carbon contact, the pressure upon which is varied by the vibration of the disk under the action of the sound waves. The variable pressure upon the carbon contact varies the electrical resistance and therefore the amount of current in the circuit. The receiver contains an iron dia-

' Fio. II. Simple telephone circuit showing T, transmitter ; A, B, line wires ; C, condenser ; M, ringer ; P, S, transformer or induction coils ; H, switch ; R, receiver.

phragm placed near the end of an electric magnet. The coil of this magnet carries the varying current from the transmitter» and the current reproduces the sound waves, impressed upon the transmitter, by attracting the receiver diaphragm with varying force. When applied to the requirements of actual business numerous other devices are necessarily used in connection with those described, but the essential principles are the same.

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5. Production of Heat

The production of heat has already been referred to in connection with light, but there are other applications of electrically produced heat, such as cooking, car warming, etc., which

Pig. 12. Electric car heater, used for car warming, producing heat by the flow of current through a high resistance.

are finding an increasing field of usefulness. Electrical heating devices are of simple construction, consisting merely of a wire of high resistance surrounded with insulating material and imbedded in the body to be heated. This method of heating is ver>'' efficient as all of the electrical energy is turned

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Pig. 13. Carborundum furnace, an important application of electric heating. Shows the arrangement of coke and sawdust core.

into heat. Such applications of electrical heating as have been

described are of trifling importance as compared with those

INTRODUCTION. 15

found in electro-metallurgy. By means of the electric arc and the passage of current through high resistance paths in masses of material to be heated it is possible to produce reactions not. otherwise practicable. For example, the abrasive material known as "carborundum" is produced by heating a mass of carbon and sawdust to high temperature with the current. Artificial graphite is similarly produced, and new discoveries in this line are being constantly made. Aluminum reduction, while essentially an electrolytic process, involves the use of electric heating. The aluminum oxide which is reduced by electrolytic action is carried in a mass of fused cryolite which is maintained in a fluid condition by the heat produced by the current.

Summary.

In the preceding paragraphs a general survey of the uses of electricity in every-day Ufe has been given with a view to preparing the way for a more detailed study. As these applications are viewed as a whole, one important fact stands out above all others. This is that every use to which electricity is put involves a transformation of energy. In the first illustration it is from mechanical to electrical form and back again. In producing light the energy is largely lost in heat, but incident^y some light energy is generated. In electrolysis the energy takes the chemical form from which it can be restored to mechanical energy in many cases. The transmission of intelligence involves the transformation to sound energy in the instances which have been cited, although, as in the case of light, only a small proportion is so transformed. Finally, in heat generation all of the electrical energy disappears as such and reappears in thermal form from which it can be recovered only by a roundabout and inefficient process. All of this goes to show that the electric current offers a conventent means for transmitting and tranforming energy, and nothing else. The study of electrical engineering, therefore, is that of the means used for economically producing these transformations. It is essential that the student have this fact continually before him, and that all so-called generators, motors, etc., be viewed merely as energy transformers. Energy cannot be "generated"; it can only be transformed.