Modern electrical construction

Front matter

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MODERN

Electrical Construction

Preface

In this volume an attempt is made to provide the beginner in electrical construction work with a reliable, practical guide ; one that is to tell him exactly how to install his work in accordance with the latest approved methods.

It is also intended to give such an elaboration of "safety rules" as shall make the book valuable to the finished workman as well. To this end the rules of the ''National Electrical Code" of the National Board of Fire Underwriters have been given in full, and used as a text in connection with which there is interspersed in the proper places a complete explanation of such work as the rules may apply to. This method of teaching and explaining practical electricity may at first glance seem somewhat haphazard, but it resembles very closely the actual method by which the most successful, practical workmen have learned the trade. It is thought that explanations pertaining directly to the work in hand will be more deeply considered and more likely to be fully comprehended than explanations necessarily more abstract.

It should be noted that, while the rules published in the "National Electrical Code" are standard and work done in conformity with them will be first-class, several of the larger cities have ordinances governing electrical work which conflict in some details with these rules. Workers in such cities should, therefore, provide themselves with copies of these ordinances (usually obtainable without charge), and compare them with the rules given in this work. It is necessary for the electrical worker at all times to keep himself posted, for safety rules are liable to change.

The tables concerning screws, nails, number of wires that can be used in conduit, etc., are especially prepared for this volume, and give to it particular value for practical men.

The Authors.

Preface To Second Edition

The favorable reception which the first edition of this work has received at the hands of electrical workers generally has induced the authors to prepare this, the Second Edition. Considerable new matter, notably a section on Theater Wiring, has been added. All the necessary alterations and additions have been made in the text to conform to the latest issue of the National Code, together with the required explanations and illustrations. Other sections have been extended and the whole work has been carefully gone over and revised wherever the progress of the art has made it desirable.

The Authors.

Preface To The Third Edition

In a work of this kind it is of prime importance that it be kept abreast of the times. Safety rules are liable to change, m fact must do so to adapt themselves to the steadily increasing number of new inventions and devices brought upon the market.

This woi^ having found sufficient favor in the eyes of many instructors to warrant its adoption as a text-book, is to be revised and new matter added as often as notable changes in methods of construction or safety rules make it appear desirable.

The latest additions and modifications of the "National

Electrical Code" are contained in an appendix which should

be consulted. New illustrations are inserted in the liody '>f

the work as needed.

The Authors.

CHAPTER I. The Electric Current

It is quite customary and convenient to speak of that agency by which electrical phenomena, such as heat, light, magnetism, and chemical action are produced as the electric current. In many ways this current is quite analogous to currents of air or water. Just as water tends to flow from a higher to a lower level, and air from a region of greater density or pressure to one of lesser density, so do currents of electricity flow from a region of high pressure to one of low pressure. Currents of electricity form no exception whatever to the general law of all action, which is along the lines of least resistance. It must not be understood, however, that electricity actually flows in or along a conductor, as water does in a pipe, and the analogy must not be carried too far, for the flow of water in pipes is influenced by many conditions which do not influence a flow of electricity at all, and vice versa; there are conditions surrounding conductors, which influence the flow of electricity which do not affect the flow of water.

Above all, let it be understood that electricity is not independent energy, any more than the belt which gives motion to a pulley is. In other words, it is not a prime mover, it is simply a medium which may be used for the transmission of energy, -just as the belt is used. To use electricity as a medium for the transmission of energy, it must be, we may say, compressed, or, to use a more properly technical expression, a difference of potential or pressure must be created in a system of conductors. This is very similar to the use of air for power transmission; this must also be compressed so that a difference of pressure exists within a system of piping.

It is the flow of electricity or air which takes place when switches or valves are operated and which tends to equalize this pressure, i. e., flow from high to lovv pressure, that does our work. The real energy, however, (so far as we are concerned), to which we must look for our initial motion in either case is derived from the coal which generates steam; or, in the case of water-driven machinery, the rays of the sun which evaporate water, allowing it to be carried to higher levels, from whence it flows downward over dams ar.d falls on its way back to the lowest level. In the battery, the real energy is that of chemical action, which is transformed into electrical energy.

The flow of current can take place only in a system, of conductors which usually, for convenience, are made in the form of wires. The current for practical purposes m.ay be considered as flowing along such wires only. It is not, however, necessary that these wires should be of any particular size, or consist all of the same material. In an electric battery, part of the circuit consists of the liquid contained within the battery; the rest being made up usually of wire. In an incandescent light circuit part of the circuit consists of the

ELECTRIC CURRENT. 9

lamp filament (usually carbon), while the balance of the circuit consists of copper wire.

The flow of current is also said to have a certain direction ; that is, it is noticed that many of its effects are reversed when the terminals of the battery are reversed. Referring to Fig. 1, which shows a battery of three cells, the current flows from the copper element at bottom of jar 1, along the wire to the zinc element at top of jar 2, thence through the liquid to the copper element at bottom of jar 2, and from there to the zinc at top of jar 3, etc., and finally through the wire a back to the starting point. Within the battery the current flows from the zinc to the copper and the decomposition of the zinc generates the current. In the wire outside of the battery the current flows from the copper to the zinc as indicated by arrows. The combination of battery and wire is known as an electric circuit. The current will flow in this circuit only while it is complete, that is while each wire connects to its proper place as shown. If any wire is disconnected, the current flow will cease. Such a circuit is said to be open, but when all connections are properly made it is said to be closed.

Work can be obtained from a flow of current in many ways. If the current be forced to flow over a wire which is very small in proportion to the current carried, it will be heated thereby and finally melted if the current is excessive. This is how electric light is obtained.

If a wire carrying current be wound many times about an iron bar this bar becomes a magnet; that is, while the current is flowing around it, the bar has the power to attract other objects of iron or steel. The bar if made of well annealed iron will be a magnet while current is flowing around it, but will cease to be magnetic whenever the current flow ceases. Upon this fact the operation of electric bells, telegraph instruments and motors is based.

If a current of electricity flow through a properly arranged

''bath," one of the plates will be gradually consumed and the other increased in weight. This effect is made use of in electro-plating, etc. If the jar contains water slightly acidulated and the current flows through it, the water will be decomposed and oxygen and hydrogen gas will be formed. This and many kindred effects are daily used in thousands of chemical laboratories.

If a wire carrying an electric current be placed very close to another wire forming a closed circuit, a wave of current will ^ be induced in that wire every time the current in the other is made or broken, i. e., whenever it starts to flow or stops flowing. This fact forms the basis of the alternating current transformer.

All of these facts are used sometimes together, sometimes singly in measuring the electric current.

Conductors and Insulators.

Electrically speaking, all substances are divided into two classes. They are either conductors or insulators. By thi^ is not meant that some substances can carry no current at all, for, as a matter of fact, there is no such thing as either a perfect conductor or a pecfect insulator. A current of electricity can be forced through any substance, provided the pressure (E. M. F.) be made great enough, and there is no easier path open to the current. The two terms, conductor and insulator, are relative terms and must be understood simply to mean that the electrical resistance of a good conductor is infinitesimally small as compared to tliat of a good insulator. The lower the specific resistance of any substance, the better its conducting qualities ; the higher the specific resistance of any substance, the better will be its insulating qualities.

At the left is given a list of good conductors, in the order of their conductivity, the figures representing the relative con-

ELECTRO- MOTIVE-FORCE. 11

ductivity of these metals. A list of insulators is given at the right; all of these are more or less affected by moisture, losing their insulating qualities when wet.

Silver 100.0 Dry air. Fiber.

Copper 94.0 Rubber. Wood.

Gold 73.0 Paraffin. Shellac

Platinum 16.6 Slate.

Iron 15.5 Marble,

Tin 11.4 Glass.

Lead 7.6 Porcelain.

Bismuth 1.1 Mica.

Pressure or Electro-Motive Force.

Currents of electricity flow only in obedience to electrical pressure. This pressure is measured and expressed in volts, the unit of electrical pressure being the volt. If we speak of water or steam pressure, we speak of it in pounds, the pound being the unit of measurement. In speaking of electrical pressure we refer to it as of so many volts. There is no direct connection between the pound and the volt, but each in its place means about the same thing.

The volt is defined as that difference of potential (pressure) that must be maintained to force a current of one ampere through a resistance of one ohm.

If we have a resistance greater than one ohm and wish to send a current of one ampere through it, we can do so by increasing the pressure or voltage, as it is termed, accordingly. The current flowing in a circuit can also be reduced by reducing the voltage.

The ordinary incandescent lamps operate at about 110 volts pressure, although some are built for 220 volts. An electric bell requires about 2^ volts (a battery of 2 cells) for proper operation.

Resistance.

We have seen that a flow of current always takes place along or in a conductor. Every conductor, no matter how large or small it may be, offers some resistance to this llow of current just as the water pipe offers more or less resistance to the flow of water. This resistance may be measured and expressed in ohms; the unit of electrical resistance being the ohm. The ohm is defined as that resistance which requires a difference of potential of one volt to send a current of one ampere through it. If we should desire to send a greater current through any resistance, we can do so by increasing the pressure, just as we can increase the flow of water in a pipe by increasing the pressure or head of water in the tank that supplies it. If the pressure is fixed we can S^ecrease the current by using a wire of greater resistance or increase it by using wires of lesser resistance.

The ohm is the resistance of a column of mercury 106.2 centimeters long (about Syi feet) and one square millimetre (about .0015 sq. in.), in cross-section, at the temperature of melting ice.

The resistance of a No. 14 copper wire about 380 feet long is equal to one ohm.

The resistance of all conductors increases directly as the length and decreases as the cross-section increases. In Figure 2 the resistance of the two bars of copper is exactly equal. Bar No. 1 having a cross-section of 4 square inches and being 4 feet long, while bar No. 2 has a cross-section of only 1 square inch and is only one foot long. If bar No. 1 were

OHMS LAW. 13

reduced to a cross-section of 1 square inch, it would become 16 feet long and would have a resistance 16 times as great as that of bar No. 2.

Current.

The electric current is the result of electrical pressure (volts) acting through a resistance, and is measured in amperes, the ampere being the unit of current strength. The ampere is defined as that current which will flow through a resistance of one ohm when a difference of potential or pressure of one volt is maintained at its terminals.

The ampere expresses only the rate of flow, not the quantity. Knowing the amperes if we would know the quantity, we must multiply by the time that the rate of flow continues. Ihe rate of flow is analogous to the speed of a train; unless we know how long the train is to maintain a certain speed, we have no idea how far it is going.

Quantity in electricity is measured in coulombs. The coulomb is the quantity of current delivered by a flow of one ampere in one second.

Ohm's Law.

Ohm's law expresses the relation of the three principal electrical units to each other and forms the basis of all electrical calculations.

This law states that in any electric circuit (with direct current) the current equals the electro-motive force divided by the resistance. The current, we have already seen, is the medium which does our work. Current flow, we see from this law, can be increased either by increasing the electro-motive force, or electric pressure, which causes the flow ; or by decreasing the resistance which tends to prevent current flow. Expressed in symbols it is this : I=E/R ; where I stands for current, E, for electro-motive force, and R for resistance. If, as an example, we have an electro-motive force (which we shall henceforth designate by the customary abbreviation, E. M. F.) of 110 volts and a resistance of 220 ohms, the resulting current will be 110 divided by 220 =>^ ampere, being approximately the current used in a 16 cp. incandescent lamp at 110 volts. Thus it will be seen that by a very simple calculation we can find the current flow in any conductor if we but know the E. M. F. and the resistance of that circuit.

This formula can also be used to find the E. M. F., if we know the value of current and the resistance, since E divided by R=I ; I times R must equal E. If the current and resistance are known, we need only to multiply them together to find the E. M. F.; IXR=E. Knowing the current and E. M. F., we can find the value of the resistance by dividing the E. M. F. by the current; E/I=R.

As a practical application of these formulas : If we wish to know how much current a certain E. M. F. can force through a certain resistance, we must divide the E. M. F. (volts) by the resistance (ohms.) If we wish to know what E. M. F. (volts) will be necessary to force a certain current (amperes) through a certain resistance, we need only multiply the current (amperes) to be obtained by the resistance in ohms. If we wish to know how much resistance (ohms) must be placed in a circuit to keep down the current flow to a certain limit, we need only divide the E. M. F. (volts) by the desired current (amperes) ; the result will be the value in ohms of the required resistance.

Power.

The power consumed or transmitted in an electric circuit equals the product of the volts and amperes; pressure and current.

POWER. 15

To find the power of a steam engine, we must know the pressure of the steam and the quantity used ; the power contained in the water of a dam depends upon its vokime and its head. The power we can obtain from the wind depends upon Its speed and the surface we expose to it which also measures the quantity.

All of these cases are analogous and similar. Power expresses the rate of doing work, thus the rate of work is the same whether we are lifting one pound at the rate of 100 feet per minute, or 100 pounds at the rate of one foot per mmute. The unit of electrical power is the watt. It is the power expended in an electric circuit when one ampere flows through a resistance of one ohm, or when a difference of potential of one volt is maintained m a circuit having a resistance of one ohm. In an electric light circuit, for instance, as far as the power is concerned, it is immaterial whether each lamp requires 110 volts and H ampere, or 55 volts and one ampere, or 220 volts and % ampere. The power (watts) expended in an electric circuit is always equal to the volts multiplied by the amperes; thus, one ampere at 1,000 volts IS equal to 100 amperes at 10 volts, or to 200 amperes at 5 volts. In any power transmission whenever the pressure (volts) is lowered, the current (amperes) must be increased or the power (watts) will fall off, and, oil the other hand, whenever the pressure is increased the current may be decreased.

Instead of muhiplying vohs by amperes, we can find the power m an electric light circuit by multiplying the current by Itself and then by the resistance; or the E. M. F. by itself and divide by the resistance.

Thus knowing the volts and the amperes, we use the formula E X I=W. Knowing only the amperes and the ohms, we may use the formula, P X R = W; and lastly

MODERN ELECTRICAL CONSTRUCTION.

knowing only the volts and ohms, we use the formula, EVR = W.

In the above E stands for E. M. F., or volts; I for current or amperes ; and R for resistance or ohms.

Divided Circuits.

Currents of electricity always flow along the paths of least resistance just as currents of water do. Water, it is well known, will not flow over the top of a mill dam while there is an opening alongside of it through which it can flow. If a barrel of water be provided with two openings, one large opening and one small, a much larger quantity will flow out through the large opening than through the small. This is because the resistance to the flow of water of the large opening is so much less than the resistance of the small opening.

An electric current will act in just the same way; the conductor having the lesser resistance will carry the greater current. If we know the resistances of the different paths open to a certain current we can determine to a nicety how much current will flow in each. In Figure 3, which represents diagramatically a battery of two cells and an electric circuit, the resistance of the two paths, a and b, is equal to

DIVIDED CIRCUITS. 17

10 ohms each, and the current will divide equally between them. If the resistance of a were 5 ohms, and that of b, 10 ohms, two-thirds of the total current would pass through a and the one-third through b.

In all such divided circuits, the current is always inversely proportional to the resistance and the simplest way to find the current in each is to add the resistances of the two circuits; for instance as above, 5 plus 10 equals 15; now 5/15 of this current will flow through the 10 ohms and 10/15 of the current will flow through the 5 ohms.

To determine the combined resistance of the two wires, a and b, we need simply to consider them as made into one wire. If they are both alike, they would, if made into one wire, be twice as large as either one is at present, and would then have only one-half as much resistance as either one had before; for the resistance of any conductor increases directly as its length, and decreases as the cross-section increases. The combined resistances of any two conductors can be found by multiplying their two resistances together and dividing this product by their sum. Thus, again taking the value of a and b as 10 ohms each, 10X10 equals 100, this divided by 10 plus 10 equals 5, which is the combined resistance of the two.

If we have a large number of branch circuits as shown in Figure 4, which represents diagramatically an incandescent

GnunuB electric light circuit of 12 lights (which is equal to 12 separate circuits, since each lamp really forms a circuit by itself), we can find the joint resistance of the 12 by proceeding as before; that is, multiplying together the resistance of the first and second lamp and dividing by the sum of these resistances ; next take the result so obtained (which is the combined resistance of the first two lamps) and with it multiply the resistance of the third lamp and divide by the sum as before. By repeating this operation and always treating the joint resistances already found as one circuit, the joint resistance of any number of such circuits can be found. Another and a very much quicker way consists in using the following formula : The joint resistance of any number of parallel circuits is equal to the reciprocal of the sum of the reciprocals. The reciprocal of any number is 1 divided by that number. If we have three circuits, having respectively 10, 20, and 30 ohms resistance, we proceed in the following way: The reciprocal of 10 is 1/10, of 20, 1/20, etc., the joint resistance, therefore, is 1/10 plus 1/20 plus 1/30 equals 11/60, and 1 divided by this number which is 5 5/11.

These methods are only necessary when the resistances are of different values. When all of them are alike, as is usual with incandescent lights, the resistance of one lamp needs only to be divided by the number of lamps to find the joint resistance. Thus, supposing each of the 12 lamps to have a resistance of 220 ohms, the joint resistance of the circuit would be 220/12=181/3.

CHAPTER II. Electric Bells

We are now in a position to apply the electrical laws we have just discussed practically, and for this purpose may take up electric bells and bell circuits.

Figure 5 shows an electric bell, push button and battery, all connected up and complete. The action of the bell when fully connected is as follows: Pressing the push button closes the circuit and current at once flows from the carbon pole marked + through the push button to the binding post A on the bell frame, thence along the fine wire W to the iron frame-work supporting the armature, B. This framework is in electrical connection with B. The armature, B, is provided with contact spring S, which normally rests against the adjusting screw, C. The current now passes from the contact spring to the adjusting screw and from it to the wire wound on the magnets, M, around the many turns of wire to the binding post, D, and back to the zinc pole of the battery marked — .

The current circulating many times in the wire wound on the spools of M makes the iron cores magnetic so that they now attract the armature B. When this armature is attracted, it moves towards the magnets, M, and carries the small contact spring with it, thus breaking the connection between C and S.

This stops the current flow and the magnets, M, are at once demagnetized, thus releasing the armature B, which flies back and again clo.^es the circuit at CS, this causes the armature to be attracted again and once more the circuit is broken. In this way the armature is made to strike the gong continuously while the circuit is kept closed at the push button. When the button is released, the circuit is permanently open and the bell at rest.

In the figure there is shown only one cell, this, if a good form is selected, is sufficient for a new bell if the circuit is not long. When, however, the bell is used much the contact points are eaten away by the little sparks occurring every time the bell breaks the circuit. Dirt is also likely to gather on them and prevent good contact being made. Both of these factors add resistance to the circuit, and consequently lessen the current flow*

We have seen before that the current equals the E. M. F. divided by the resistance, and in order to obtain the necessary current flow to operate the bell, we may either clean the contact points to lessen the resistance, or increase the E. M. F. by adding another cell in series with the first.

ELECTRIC BELLS.

The latter expedient is by far the better, because it gives us a little surplus of power which is very useful to overcome variations in adjustment of the contact spring, loose contacts, dirt, etc. We should avoid using too many cells as well as not enough. If too many cells are used, there

Q

Of cczsi n_

* m will be much unnecessary damage done to contact points by the larger sparks.

If the circuit is very long, the great length of wire will also provide additional resistance. This can be overcome in two ways, by increasing the E. M. F. as above, or by using larger wires. We have already seen that the larger the wire, the less will be its resistance. It is common practice to use

No. 18 copper wire for all ordinary distances and for single bells. With large bell systems, it is customary to use No. 16 or 14 for the main wire, which leads to all of the bells and may be called upon to supply several bells at the same time. Figure 6 shows a diagram of such a system and in case the three push buttons are used at the same time, three times as much current will flow in the main or battery wire a as in either of the other wires.

We have seen before that currents of electricity divide among different circuits in the inverse ratio of their resistances. In other words, the circuit having the least resistance will carry the most current. If our bell system. Figure 6, be "grounded" at the two points x and y (i. e., bare wire in contact with metal parts of buildings which are connected together) the current instead of flowing through the longer circuit and the bell, will flow through the short circuit and leave it impossible to operate the bells. If the contacts, at X and y are poor, i. e., of high resistance, only a small part of the current will leak from one to the other. In such a case, the bells may work properly, but the battery will soon run down and there is a strong likelihood that one of the wires will be eaten away through electrolytic action. To prevent troubles of this kind, bell wires should be well insulated and kept away from pipes or metal parts of building. Damp places should also be avoided and special care is recommended for the battery wire a, Figure 6. For further information concerning diagrams, etc., of bell circuits the reader is referred to Wiring Diagrams and Descriptions by the authors of this work, Fred J. Drake & Co., Chicago.

Bell wires are usuaHy run along base boards, over picture mouldings, etc., in some cases they are also fished as explained further on. Batteries should be located in cool, dry places, where they are not liable to freeze, and where they are readily accessible as they must be kept nearly full of water and must be recharged from time to time.

The Telephone.

The principle and action of the Bell telephone can be best explained by reference to Figure 7. In this figure, A represents the transmitter, and B, the receiver. The essential parts of the transmitter are: the diaphragm, a; an electric circuit, containing a battery, h, and consisting of the wires, c, c^ and partly wound upon an iron core, d.

This electric circuit, it will be seen from the figure, connects with one pole to the diaphragm, a, and with the other to a small metal plate, e. Between the diaphragm, a (which is a plate of very thin iron), and the plate, e, there are many small pieces of carbon which complete the circuit. When now a party speaks into the mouthpiece of the transmitter, the sound waves cause the diaphragm, a, to vibrate; the rate of vibration and character of the vibrations being an exact duplication of the voice speaking into it. These vibrations cause the small pieces of carbon between the diaphragm and the back plate to be alternately compressed and allowed to expand. Now the resistance of these carbon pieces is decreased as they are tightly pressed together, and again increased when the pressure is released. Therefore the current of electricity flowing through them varies continuously while the diaphragm is in motion.