PRINCIPLES OF THE TELEPHONE PART I SUBSCRIBER'S APPARATUS McGraw-Hill DookCompatry Electrical World The Engineering andMining Journal En5ineering Record Engineering News Railway Age G azettx? American Machinist Signal E,ngin* = Poo = - 012 am P- (6) The total current is equal to the sum of ii + i 2 + i 3 or 7 = 0.024 + 0.015 + 0.012 = 0.051 amp. (c) The joint resistance may be calculated in two ways. First by Ohm's law R E = -j 24 = or by the formula D it = R 1 XR 2 XR S = 1,000 X 1,600 X 2,000 = 3,200,000,000 RiXR 2 = 1,600,000 R! X Ra = 2,000,000 R 2 x Rz = 3,200,000 #1 X R* + Ri X R s + #2 X # 3 = 6,800,000 and _ 32,000 ~68~ _ 32,000 8,000 68 17 = 470.6 ohms, nearly. To calculate the joint resistance of any number of conductors connected in parallel, we proceed in exactly the same way. It is not necessary to show how any more formulas are calculated. A general rule will suffice. Rule. To find the joint resistance of any number of parallel conductors, 3 20 PRINCIPLES OF THE TELEPHONE divide the product of the resistances of all of the conductors by the sum of the products obtained by multiplying together all of the resistances less one. The same resistance must not appear in any partial product more than once. 33. Cells in Series. In the application of Ohm's law the electromotive force E must be the total electrical pressure in the circuit. It is, therefore, neces- sary to be able to calculate the pressure when cells are connected in series or in parallel. Cells are said to be connected in series when G the carbon electrode of one is con- ^J^^= =^r=/3>= ^==^=-- n a r L i. C a 6 / /? FIG. 8. -M/txz' c/Che FIG. 9. nected to the zinc electrode of the next and so on as shown in Fig. 8. The analogous diagram of tanks in series may help to show how the total pressure is calculated. The hydrostatic pressure at A is evidently the sum of the pressures due to the elevations of the water AB + BC + CD; that is, the sum of the pressures in the individual tanks. Similarly, the electrical pressure be- tween the terminals 1 and 2 is the sum of the pressures across the cells a, 6, and c. In general, if E is the pressure of one cell and n cells are connected in series, the total pressure is nE. 34. Cells in Parallel. Fig. 9 is a diagram of tanks and cells connected in parallel. It is evident that the hydrostatic pres- ELEMENTARY ELECTRICAL PRINCIPLES 21 sure exerted by the water in tank A is the same as that in B and C, since the height of the water is the same in each. The total pressure is equal to that of one tank. The three tanks could be replaced by one large tank, and as long as the water was maintained at the same height, the pressure at the orifice would be exactly the same in the two cases. When cells are connected in parallel the total pressure is equal to the pressure of one cell, and the three cells a, 6, and c, can be replaced by one large cell having the same cross-section of zinc and carbon as the three cells taken together. When tanks are connected in parallel it is evident that each supplies only a part of the current. The same principle holds with reference to cells connected in parallel each cell supplies only a part of the total current. The student can readily verify the law of pressures by connecting three cells in parallel and then connecting a voltmeter to terminals 1 and 2, Fig. 9, and comparing the voltmeter reading with the reading given when the voltmeter is connected to each cell separately. 35. Battery Resistance for Parallel Connections. The effect of connecting cells in parallel is to increase the current capacity and decrease the internal resistance. In so far as the internal resistance of one cell is concerned, it may be considered as a conductor whose resistance is r. Three cells in parallel will thus be the equivalent of three resistances in parallel. It has been shown that when three equal resistances are in parallel, the joint resistance is equal to one-third of the resistance of one wire. Accordingly, the joint internal resistance of a battery of m parallel cells is ra EXAMPLE Five cells each having an internal resistance of 1 ohm are connected in parallel. What is the joint resistance? Solution Since the resistances of the cells are the same, the joint resistance is K of 1 ohm = 0.2 ohm. QUESTIONS 1. Explain the steps necessary in order to send a telephone message from one point to another. 2. Name the main parts of a telephone instrument and give briefly the uses of each. 22 PRINCIPLES OF THE TELEPHONE 3. Of what use is a battery in telephone work? 4. Explain how a simple battery is made. . 5. What is a conductor? An insulator? Name the five best conductors of which you know. The five best insulators. 6. What are the elements of a battery? What is the electrolyte? 7. What is meant when an object is said to be "charged?" 8. What do you understand by electrical pressure? Compare it with water pressure. 9. What are the elements and electrolyte of the Le Clanche cell? Explain briefly how a dry cell is made. 10. What is electrical resistance? Upon what does the resistance of a conductor depend? 11. Which has the greater resistance, copper or silver? Iron or copper? Iron or lead? 12. Why is a unit of resistance necessary? What is this unit? 13. What is the unit of electrical pressure? By what other name is electrical pressure usually known? What is the voltage of a dry cell? 14. What causes an electrical current to flow? What conditions are necessary in order to have a current of electricity? 15. What is the unit of current? Define it in terms of volts and ohms. 16. What is the relation between volts, amperes, and ohms? 17. What is the rate of current flow through a coil of telephone wire having a resistance of 1 ohm, if the ends are connected to the terminals of a dry cell? If another coil of half the length is used, what will be the current in amperes? 18. Define the following: Circuit, open circuit, closed circuit, short cir- cuit, series circuit, parallel circuit, shunt. 19. Four resistances of 80, 95, 40, and 50 ohms are connected in series. What is the joint resistance? 20. Four series ringers of 80 ohms resistance each were connected in parallel. What was the joint resistance? 21. Four dry cells, each having an electromotive force of 1.4 volts and an internal resistance of 1 ohm, were connected in series to a circuit whose resistance was 10 ohms. What was the current in the circuit? 22. Suppose the cells mentioned in question 21 were connected in parallel to the same circuit, what current would flow? CHAPTER III BINOING POSTS , MAGNETIC PRINCIPLES 36. Receiver Action. An examination of a telephone receiver shows the working parts to consist of a permanent magnet on which coils of fine insulated wire are wound, and a thin iron disk mounted close to, but not touching, the poles of the magnet. This arrangement is shown in Fig. 10 for a single-pole receiver, which was the earliest type in use. This examination also shows the iron disk or diaphragm to be attracted by the per- manent magnet. Since the outer edge of the disk can not move, the disk will become slightly "dished," as the center is drawn in toward the magnet. When the receiver is not in use this pull will be steady, and there will be no movement of the disk. If the strength of the magnet be increased, how- ever, it will exert greater attraction for the disk, and the latter will be pulled closer to the magnet pole. On the other hand, if the force of the magnet's attraction be decreased the diaphragm will spring away from the pole, and return more nearly to its original shape. When the receiver is in use, the strength of the magnet is changed from time to time by the fluctuating line current which flows through the coil of the receiver. If these changes in the force of the magnet take place rapidly enough, the disk will vibrate at such a rate that sounds will be produced by it. In order to understand how an electric current flowing in the coil can change the strength of the magnet, it will be necessary to investigate a few of the relations existing between electricity and magnetism. 37. Magnetism.- The magnet, as first known, existed in the form of a certain iron ore known as magnetite (so named in 23 DIAPHRAGM' FIG. 10. 24 PRINCIPLES OF THE TELEPHONE honor of the city of Magnesia, where the ore having this pecu- liarity was discovered) which has the property of attracting pieces of iron. The strange force by which the particles of iron were attracted was likewise known as magnetism. These first magnets were natural magnets. It was found, somewhat later, that artificial magnets could be formed by subjecting pieces of iron to the influence of a magnetizing force. One of the early methods of producing such artificial magnets, was by stroking or rubbing a piece of iron with a piece of magnetic ore or natural magnet. There are at present other methods of producing artificial magnets. The first arti- ficial magnets were in the form of a bar as shown in Fig. lla. FIG. lla. FIG. lib. 38. Magnetic Substances. Iron in its various commercial forms, such as wrought iron, cast iron, steel, etc., is strongly magnetic and is known as a magnetic substance. Substances such as wood, glass, copper, etc., which can not be made to act as magnets, are known as nonmagnetic substances. 39. Magnetic Induction. When a magnetic substance is magnetized by coming into contact with a magnet, the substance is said to have been magnetized by induction, or the magnetism is said to be induced in the substance. 40. Experiment 1. Apparatus: Bar Magnet. Iron Filings. Wire Nails. (a) Dip the ends of the bar magnet into the iron filings, and note that the filings cling to the ends of the magnet. The parts to which filings cling are called the poles of the magnet. Those points to which no filings cling are known as neutral points. (6) Rub a knife blade or other piece of steel with one end of the magnet; always move the magnet in the same direction along the knife. Dip the end of the blade in the filings. Has the blade become magnetized? (c) Hold one end of the bar magnet against the head of a MAGNETIC PRINCIPLES 25 nail and dip the point of the nail into the iron filings. Note that a magnet pole has been developed on the point of the nail. (d) Try the last experiment with a piece of wood or short piece of copper wire in place of a nail, and note that there is no evidence of these substances becoming magnetized. 41. Magnetic Action. If a bar magnet be suspended by a fine thread attached to its center, the magnet will turn so that one end will point in a northerly direction and the other in a southerly direction, no matter what the original position of the magnet may be. The end of the magnet that will point toward the north is called the North pole (marked N.), and the other end is called the South pole (marked S.). A compass needle is merely a very light bar magnet. 42. Experiment 2. Apparatus: Horseshoe Magnet. Two Bar Magnets. (a) Suspend a bar magnet by a fine thread attached to its center. When the magnet has come to rest, it will point north and south. Mark the end that points north, to indicate the N. pole. Suspend the second bar magnet in the same way and mark its N. pole. Bring the N. pole of the first bar magnet near the N. pole of the suspended one. Observe that the two poles repel each other. Now bring a N. and S. pole near each other and observe the strong attraction exerted between the two. (b) Using the suspended bar magnet as in the first part of the experiment, test the poles of the horseshoe magnet, and mark the N. pole. 43. Laws of Magnetic Attraction and Repulsion. If the N. pole of one bar magnet is brought near the S. pole of the other, a strong attraction is exerted between the two; but if the two N. or two S. poles are brought together, they repel each other; hence we can write two laws governing the action of one magnetic pole on another, as follows: (1) Like magnetic poles repel each other, and (2) unlike magnetic poles attract each other. The action of the compass in taking a north and south position can be understood, since investigation has proved that the earth is a gigantic magnet, having one magnetic pole near the earth's north pole, and the other magnetic pole near the earth's south pole. A horseshoe magnet is another common form of artificial 26 PRINCIPLES OF THE TELEPHONE magnet, one .of the ends being a N. and the other a S. pole, as shown in Fig. 116. 44. Experiment 3. Apparatus: Bar Magnet. Nail. Piece of Watch or Clock Spring about 3 in. long/ 1 (a) Repeat the third part of Experiment 1, and note that as long as the magnet and nail are in contact the point of the nail will hold a considerable quantity of the filings, but as soon as this contact is broken the nail loses the greater part of its magnetism and most of the filings drop. The nail, being of soft iron, is only a temporary magnet. FIG. 12a. FIG. 126. (b) Repeat the experiment using the piece of watch spring in place of the nails. Even after the contact between the spring and the magnet has been broken, the spring retains the greater part of its power to pick up the filings. The spring, being of hard steel, has become a permanent magnet. Try the bar magnet used in this experiment with a file to see whether or not it is of hard steel. 45. Permanent and Temporary Magnets. Artificial magnets which retain their magnetism a long time are known as perma- nent magnets. Wrought iron may be strongly magnetized, but as soon as the magnetizing force is removed it loses the greater part of its magnetism. Hard steel, when once magnetized, will retain its magnetic properties indefinitely. Both these forms of iron are made use of in telephone work; in some cases we use hard steel because we want the part to retain its magnetism, as in the magnets of a ringer, and in other cases we want to magnet- ize the part temporarily and then want the same part to lose its MAGNETIC PRINCIPLES 27 magnetism a moment later, as in the receiver diaphragm. The action of these parts will be explained later. 46. Experiment 4. Apparatus: Bar Magnet. Piece of Watch Spring. Be sure that the spring used in Experiment 3 is magnetized. After testing the spring by dipping in the filings, cut it into several short pieces, and test each piece for magnetic properties. A magnetic pole will exist at each end of each of the small pieces, and each will have the same power of attraction as the original magnet. 47. Magnetic Lines. The property by which a magnet will FIG. 13. FIG. 14. attract pieces of iron or other magnetic material has given rise to the conception of magnetic lines. Magnetic lines are the imaginary lines along which the forces of attraction and repulsion are exerted. The space surrounding a magnet in which these forces are exerted is known as the magnetic field. Each individual line forms a complete loop or circuit passing through the poles of a magnet, and the number of these lines which pass through the poles determine the strength of the mag- netic field. A bar magnet is surrounded by these lines which enter at one pole and leave through the other, as shown in Fig. 12a. Mag- netic lines are considered as passing out of the magnet at the N. and into the magnet at the S. pole. The distribution of magnetic lines under different conditions is shown in Figs. 12a and 126, 13, and 14. A magnet can not be produced with but one pole. If a bar magnet is broken into a number of small pieces, each piece will have a N. and a S. pole. 28 PRINCIPLES OF THE TELEPHONE 48. Experiment 5. Apparatus: Horseshoe Magnet. Bar Magnet. Iron Filings. Sheet of Smooth, Stiff Paper. (a) Lay the bar magnet on the table, and over it place a sheet of paper. Sprinkle iron filings over the paper. Tap the paper gently while sprinkling the filings and note that the arrangement of the filings is similar to the diagram of the magnetic field shown in Fig. 12a. (6) Repeat the above experiment using the horseshoe magnet instead of the bar magnet. 49. The Magnetic Circuit. The path of the magnetic lines is known as the magnetic circuit. Thus in Fig. 12a the magnetic circuit is made up of two parts, the steel of the magnet and the air through which the lines pass. A magnetic circuit is said to be closed when the circuit is composed entirely of magnetic substances, such as iron or steel. Whenever pieces of iron or steel are brought into a magnetic field, the lines pass through them very readily and they become magnetized. If the material is soft iron, when taken out of the field it will lose most of its magnetism, while if it is hard steel it will retain its magnetism, both substances behaving the same as if they had been in actual contact with a magnet. Magnetic lines tend to pass along the path offering the least resistance, the same as electric currents. Iron and steel offer the least resistance to the passage of magnetic lines, so are used for magnetic circuits whenever possible. Air offers from 1 to 10,000 times as much resistance to magnetic lines as iron, depending upon the degree of magnetization. Copper, glass, paper, and other nonmagnetic substances offer the same resist- ance to magnetic lines as air. Magnetic circuits through sub- stances other than iron are usually made short, so that the number of lines in the magnetic field will be as great as possible. The horseshoe magnet is stronger than the bar magnet of the same size because the magnetic circuit through air is shorter. That air does offer considerable resistance to magnetic lines can be seen from the fact that it is necessary to bring a magnet quite near a piece of iron before any attraction is noticed. The re- sistance which any substance offers to the passage of magnetic lines is known as reluctance. MAGNETIC PRINCIPLES 29 50. Electromagnetism. Any conductor carrying an electric current is surrounded by a magnetic field, as shown in Fig. 15. The dark spot in the center of the figure represents a cross- section of the wire. 51. Experiment 6. Apparatus: Two Feet of Bare Copper Wire. Dry Cells. Iron Filings. Compass. (a) Dip the wire in the iron filings and note that the filings do not stick to the wire. Now connect the ends of the wire to the terminals of the dry cells; place the wire in the filings and FIG. 15. observe that the filings stick to the wire, although it 'is not a magnetic substance. Disconnect one end of the wire and repeat the experiment. It is evident that .the magnetic field exists only as long as the current is flowing. (6) Place the wire over the compass so that it is parallel to the needle and close the circuit. Observe that the needle is deflected, showing the magnetic action of the current. Reverse the current through the wire by reversing the connections to the battery, and note that the needle is deflected in the opposite direction, showing the magnetic field has been reversed. 52. Solenoids. If a wire carrying a current be wound into a coil, as shown in Fig. 16, the magnetic lines surrounding each turn of the coil will be in the same direction as those of the other turns, and the result will be a magnetic field similar to that of a cylindrical bar magnet. A coil so arranged and carry- ing a current is called a solenoid. A solenoid behaves exactly like a bar magnet. At one end of the solenoid a N. pole exists, 30 PRINCIPLES OF THE TELEPHONE while at the other end a S. pole exists, depending upon the direc- tion of the current flowing in the wire. A reversal of the current will cause a reversal of the magnetic field. The strength of the magnetic field of any coil depends upon the number of turns in the coil and upon the current flowing in the coil, since the magnetic field of a solenoid is due to the added effect of all the turns in the coil. 53. Experiment 7. Apparatus: Solenoid. Dry Cells. Compass. (a) Connect the solenoid to the dry cell and by placing one end near the N. pole of the compass observe whether it attracts or repels the compass. FIG. 16. If it attracts the N. pole of the compass, what kind of a pole is it? If it repels? Mark the end with an N. or S. Now test the other end the same way and observe that it is of opposite polarity. Mark this end according to its polarity. (6) By reversing the connections at the battery, cause the current to flow in the opposite direction through the coils. Test as before with the compass and note that the pole which was marked N. in the first case has now become a S. pole, and the one which was marked S. has become a N. pole. 54. Electromagnets. If an iron core be placed in a solenoid, it becomes what is known as an electromagnet. Since magnetic lines pass through iron much more readily than through air, the same magnetizing force can produce a stronger field through iron than through air or some other nonmagnetic substance. MAGNETIC PRINCIPLES 31 Hence the purpose of the iron core is to increase the strength of the magnetic field of the coil without increasing the current or the number of turns in the coil. FIG. 17. The electromagnets used in telephone work are of three general forms, classified according to the form of the iron core. One form, the bar electromagnet, consists of a solenoid wound on a straight iron core, as shown in Fig. 17. An examination FIG. 18. shows that the magnetic circuit contains a long air gap. If this air gap be shortened, the number of lines and therefore the strength of the magnetic field will be increased without changing the current or coil in any way. 32 PRINCIPLES OF THE TELEPHONE 55. Horseshoe Electromagnet. One of the easiest ways of short- ening the air gap is to bend a bar electromagnet in the form of a horseshoe, as shown in Fig. 18. To facilitate manufacture, however, the core of the horseshoe electromagnet is usually made in three parts instead of being bent as shown. Fig. 19 shows the general form of commercial horseshoe magnets, consisting of two spools and the yoke joining their cores. Since such a magnet usually is arranged to attract an armature, the latter further decreases the air gap, as shown, to the short spaces between the armature and poles. 56. The Ironclad Electromagnet. The ironclad or tubular electromagnet is shown in Fig. 20. In this type, the coil is wound FIBER PAPER CORE. COIL ARMATURE FlG. 19. on an iron core and is surrounded by a tubular shell. Such a magnet has the advantages of occupying small space, and of having its magnetic field confined strictly within the shell so that there are no stray lines to affect other apparatus which may be near. 57. Construction of Electromagnets. The coils for electro- magnets are usually wound in the form of spools. Such a spool may be entirely of fiber so that it can be removed from the core if desirable, or the fiber ends may be forced on the iron core as shown in Fig. 18. In the latter form several layers of paper are wrapped around the core to insulate it thoroughly from the coil. On this spool the insulated wire of the coil is wound in layers. Sometimes the layers are further insulated from each other by a thickness of paper. MAGNETIC PRINCIPLES 33 CORE 58. Magnet Wire. The insulated copper wire used in wind- ing coils for electromagnets is known as magnet wire. Most magnet wire is covered with either silk or cotton. Of the two, silk has the higher insulating properties and is used largely on very fine wire, as a cover- ing of silk is thinner than one of cotton. Cotton is used almost ex- clusively on the larger sizes. Silk or cotton insulated wire has either one or two layers of the insulating materials, and is known as single silk (or cotton) covered, and double silk (or cotton) covered. When two layers are used they are wound in opposite directions. As both silk and cotton absorb moisture readily, wire insulated with these materials is sometimes saturated with melted paraffine, shellac, var- nish, or some other insulating com- pound to make it waterproof. More often the coil is so treated after being wound. Enameled wire is a later develop- ment in the insulation of magnet wire. Enameled wire is made by- coating the wire with liquid enamel which is then baked on. The ad- vantages of this wire are that it is FIG. 20. waterproof, will stand high tem- peratures, and the covering of insulating material is very thin. 59. Magnetic Action of Receiver. If an electric current be sent through the coil of the receiver in such a direction that the magnetic lines set up by it are in the same direction as those of the permanent magnet, the strength of the magnet will be in- creased and the disk will be drawn closer to the pole. If a cur- rent be sent through the coil in the opposite direction, however, so that the magnetic lines due to the current oppose those of the magnet, the strength of the magnet will be decreased and the diaphragm will spring away from the pole. When a fluctuating current flows through the coil, the magnetic 34 PRINCIPLES OF THE TELEPHONE field of the coil will increase with increasing current and decrease as the current decreases, and these changes will cause changes in the strength of the field of the permanent magnet. Thus, whether or not the lines induced by the coil are in the same direction as those of the permanent magnet, there will be changes in the strength of the magnetic field whenever there are varia- tions of the current flowing in the coil. Hence the diaphragm will, vibrate in harmony with the changes of current. QUESTIONS 1. What is a magnet? What is magnetism? 2. What is an artificial magnet? How made? 3. What is the pole of a magnet? How many poles do magnets have? 4. What is a compass needle? Which is the N. pole of a magnet? 5. What is a magnetic substance? Name the most common magnetic substance. 6. What is meant by magnetic induction? Give an illustration of mag- netic induction. Can magnetism be induced in a bar of iron without having the bar come into contact with a magnet? 7. Give the law of attraction and repulsion between magnet poles. 8. What is a permanent magnet? What kind of material is usually used in permanent magnets? 9. What is a magnetic line? What is a magnetic circuit? 10. What is a magnetic field? Upon what does the strength of a magnet depend? 11. Which offers the greater resistance to magnetic lines: Air or steel? Steel or copper? Wrought iron or air? 12. Why is a horseshoe magnet stronger than a bar magnet of the same size? 13. If a telephone receiver is examined it will be noticed that there is a steady pull between the disk and the magnet. How can this be explained? 14. What would be the effect if the disk and magnet poles should be moved closer together? 15. What relation exists between an electric current and magnetism? 16. What is a solenoid? Explain how a solenoid is similar to a bar magnet. 17. What happens when the current stops flowing in a solenoid? When the current is reversed what happens? Upon what does the strength of a solenoid depend? 18. What is an electromagnet? Why is an iron core used? What are the common forms of electromagnets? Name the advantages of each. 19. How is magnet wire insulated? Name advantages of each kind of insulation. 20. Explain how a fluctuating electric current flowing in the receiver coil will affect the diaphragm. Explain fully. CHAPTER IV SOUND 60. Sound. Sound is produced by the vibration of some body, and is transmitted through space in the form of waves in the air; hence sound may be defined as wave motion in the air, capable of affecting the sense of hearing. If a stone be dropped into a pond of water, a disturbance is set up which spreads in the form of waves in ever-widening circles. If a tuning fork be started vibrating, sound is produced. The sound travels from the source of disturbance in the form of air waves. Investigation and experiment have shown that the air moves forward and backward in the direction in which the sound travels. At one instant the air in front of the fork is condensed, while that behind it is rarefied, and the next in- stant the air in front of the fork is rarefied while that behind it is compressed. The waves thus travel as a series of compressions and expansions. The sounds which issue from a telephone receiver are caused by the rapid vibration of the iron diaphragm. Air is not the only substance that will transmit sound waves, water, wood, iron, etc., being useful in this respect. The early telephone experiments mentioned in the first chapter depended upon the transmission of sound waves through a tightly stretched wire. That some material medium is necessary for the trans- mission of sound waves can be shown by placing an electric bell under the receiver of an air pump and exhausting the air. As the air is exhausted from the receiver, the sounds from the bell grow weaker and weaker until they cease entirely when the air has been all exhausted, although the bell may be seen in full operation all the time. Since vibrating bodies produce sound waves, it is to be ex- pected that sound waves are capable of causing certain bodies to vibrate when the waves come into contact with such bodies. This is shown by the fact that a person talking in a room where 4 35 36 PRINCIPLES OF THE TELEPHONE there is a piano will cause certain wires of the instrument to vibrate and thus give out sounds. Another proof is that a heavy clap of thunder will often cause the windows of a house to shake violently. In the telephone the sound waves of the voice are directed against the diaphragm of the transmitter, causing it to vibrate. 61. Velocity of Sound. It is well known that sound waves take a considerable amount of time to travel from one point to another. Experiments have shown that sound travels at the rate of about 1,090 ft. per second. In connection with this statement it is interesting to compare the speeds of electricity and light with that of sound. Electric waves and light travel with the' same speed, which is in round numbers 186,000 miles a second, or about 930,000 times as fast as sound, since sound travels about 1 mile in 5 sec. That light travels at a much higher speed than sound can be verified easily by watching a locomotive at a distance and observing how long a time is re- quired for the sound to reach the ear after one sees the steam issuing from the whistle. 62. Properties of Sound. The properties of sound depend upon three different quantities: pitch, loudness, and timbre or quality. 63. Pitch. Pitch is determined by the rate of vibration of the sounding body; that is, the number of vibrations per second determine whether the sounds given off will be "high" or "low," a high rate of vibration giving a higher pitched sound than a low rate of vibration. The short wires of a piano give off high-pitched sounds because their rate of vibration is rapid, and the longer bass strings which vibrate at a slower rate give off lower tones. 64. Loudness. Loudness of sound depends upon the distance through which the sounding body vibrates. The distance through which the vibrating body moves is called the ampli- tude. Thus when a piano key is struck a sharp blow, the amplitude of the string will be greater than when a light blow is given the key. In the former case, the sound is louder or stronger than in the second case, though the pitch is the same. Loudness depends upon the energy of the vibration. 65. Timbre or Quality. Quality is that property of sound not due to pitch or loudness, that enables us to tell one sound from another. For an example, a violin and piano may be sound- SOUND 37 ing the same note, yet a difference in quality can be detected. This difference is not due to pitch or loudness. The char- acteristics of the waves given out by the two strings are dif- ferent. This perhaps can be made clearer by considering a water wave. When such a wave is examined it is seen that many small waves surmount it. Similarly a string or other sounding body can start waves which consist of a fundamental wave and also small waves. These small waves are called overtones, and so change the wave form, and thus the quality of the sound, that we are able to tell one person's voice from another's, or to distinguish between the sounds of different musical instruments. 66. Transmission of Speech. If the sounds of speech were simply in the form of waves of a given pitch, they could be transmitted over the telephone lines by merely opening and closing the circuit at the transmitter the required number of times per second. For every time the circuit was opened or closed there would be change of current through the receiver and a corresponding magnetic action which would cause the diaphragm to move. The loudness of the sound, which depends upon the amount of movement of the receiver diaphragm, could be controlled by variations in the strength of the current. However, the vibrations due to the sound of the human voice are very complex, due to overtones and the variations of both pitch and loudness which take place hundreds of times a second. Hence to transmit such sounds is much more difficult than we might at first imagine, since the current flowing in the telephone circuit must vary with the slightest variation in the sounds to be transmitted, whether these variations be in timbre, pitch, or loudness. 67. Experiment 8. Apparatus: Telephone Receiver. Two Dry Cells. Copper Wire. Coarse File. (a) Connect one dry cell in series with the receiver as shown in Fig. 21. Attach one of the wires to the tang of the file. Draw the end of the other wire along the file so as to open and close the circuit repeatedly, in the meantime observing that the sound given off by the receiver is merely a series of clicks, which occur whenever the circuit is opened or closed. The pitch of the sound 38 PRINCIPLES OF THE TELEPHONE depends upon the rapidity with which the wire is drawn along the file. (6) Repeat the above with two cells in series, and note that the only change is that the sound produced by the receiver is louder than when one cell is used. This shows that the loudness of sounds depends upon the energy of the vibrations of the diaphragm. 68. Variable Resistance. With the telephone parts connected as shown in Fig. 22, a change in resistance in any of the parts causes a change in the current flowing in the circuit. Hence, instead of opening and closing the circuit to send variable cur- FIG. 21. rents over the line, the resistance of the transmitter is changed from time to time by the sound waves of the voice. This variable resistance is obtained by the use of. carbon. Carbon is found in a number of well-known forms, such as charcoal, graphite, lampblack, etc. Hard carbon, similar to arc-lamp carbon, is used in telephone transmitters. The property of carbon which makes it suitable for this work is that the electrical resistance of a contact made of this substance can be regulated by the pressure applied. This resistance, which depends in a large measure upon the closeness of contact of the carbon parts, is decreased when the pressure is increased, and increased when the pressure is reduced. Such a contact is very sensitive, the slightest variation in pressure causing a change in SOUND 39 its resistance. In the transmitter the changes in pressure neces- sary to cause variations in the electrical resistance of the carbon parts are produced by the vibrations of the diaphragm, and since the diaphragm is very sensitive and responds to the slightest FIG. 22. variations of pitch, loudness, and quality of the sound waves of the voice, the pressure on the carbon parts varies according to the characteristics of these sound waves. CHAPTER V TRANSMITTERS 69. The Carbon Transmitter. In the earlier forms of trans- mitters such as the Edison and Blake, the variations in resist- ance were obtained through the action of the diaphragm on a single disk of carbon. The use of such instruments was limited by the fact that currents heavy enough to give the required transmission burned the surfaces of the carbon electrodes at the points of contact, soon destroying them. In order to provide a large number of points of contact, between which the current is divided, the granulated carbon type of transmitter was developed. This type is used at present to the exclusion of all others, and consists of two carbon disks, one stationary, the other movable and arranged to vibrate with the diaphragm, separated by a small quantity of granulated carbon. The greatest drawback to the early adoption of this type was the tendency of the granulated carbon to "pack" into a compact mass, which rendered the transmitter useless. In order to overcome this tendency the solid-back type was developed. 70. White Solid-back Transmitter. The White solid-back transmitter which has been the standard of the Bell companies for many years, is shown in section in Fig. 23. The case, A, is made of brass, having a heavy cover, B, to which is attached the hard- rubber mouthpiece M. The mouthpiece serves to collect the sound waves and concentrate them upon the diaphragm, D, which is a thin iron or aluminum disk having its edge covered with rubber, R. As shown in Fig. 24, two springs, S, and $', bear on the diaphragm. The short one holds the edge of the diaphragm firmly against the cover, B, and the long one rests on the dia- phragm to dampen its vibrations and render it less sensitive to outside noises. The electrodes, which are two polished carbon disks, E and E', are contained in a brass chamber consisting of two parts. The rear electrode, E, which is the larger of the two, is firmly secured within the brass cup, F. The cup, F, is attached to the 40 TRANSMITTERS 41 FIG. 23. FIG. 24. 42 PRINCIPLES OF THE TELEPHONE bridge, G, by means of the pin and set-screw. The front carbon is fastened to the stud, 0, the shank of which passes through the diaphragm and is held in place by two check nuts. A thin mica washer, M, is clamped between the head of the stud and the threaded ring, N, the outer edge of this washer being held be- tween the cap, H, and the cup, F. The center of the mica washer is therefore rigidly attached to the front electrode and partakes of its movements, while the outer edge is fastened to the rear electrode which is fixed. Any changes in relative position of the electrodes can take place only through the bending of the mica washer. In addition to holding the front electrode in its normal position, the mica washer closes the chamber containing the electrodes and keeps the granulated carbon with which this space is filled from falling out. The front electrode is insulated from the frame by the mica washer, and by the fiber lining, L, which keeps the granulated carbon away from the sides of the cup. Since one terminal is connected to the front electrode by the flexible connection, C (see Fig. 24) and the other to the frame of the transmitter, any current which passes through the instru- ment must flow through the granulated carbon. The operation of the solid-back transmitter is as follows: The sound waves of the voice of the person speaking cause vibration of the diaphragm, which, being rigidly connected to the front electrode, causes that to vibrate also, as the mica washer which holds it in place is very flexible. Since the back electrode is held stationary, the granulated carbon is subjected to varia- tions in pressure. As a result the current flowing through the transmitter is varied. 71. New Western Electric Transmitter. The new Western Electric transmitter, shown in Fig. 25, is a modified form of the White instrument. As in the White transmitter, . the front electrode is carried on a mica washer and is connected by a stud to the center of the diaphragm, and the rear electrode is fixed in the bottom of the electrode chamber. This chamber is attached to the back of a metal cup, S (which takes the place of the bridge in the White transmitter) by the threaded part, C. This not only holds the chamber in place, but also holds the outer edge of the mica washer firmly between the two parts. The metal cup and diaphragm are insulated from the shell of the transmitter at R, so that neither of the electrodes is TRANSMITTERS 43 FIG. 25. FIG. 26. 44 PRINCIPLES OF THE TELEPHONE connected to the exposed metal parts. Of the terminals, shown in the figure, TI is connected to the cup, and the other, T z , which is insulated from the cup, is connected to the front electrode by a flexible connection. 72. Kellogg Transmitter. The Kellogg Switchboard and Supply Co.'s transmitter is shown in section in Fig. 26. It will be immediately noticed that the chief difference between this instrument and those previously discussed is that the electrode cup is made a part of the diaphragm, D, and therefore partakes FIG. 27. of its movements. In order that the moving parts may not be too heavy to respond readily to the sound waves of the voice, the diaphragm is made of hard-drawn aluminum with the elec- trode chamber stamped in its center. The diaphragm, D, the front carbon disk, E, which is attached to the bottom of the chamber, the granulated carbon, C, and the mica washer, M, are the movable parts. The disk E' is stationary, as it is rigidly attached to the bridge, G. This bridge is a straight piece of hard-drawn brass. To prevent the transmitter's taking up outside noises' and being affected by mechanical vibration which might inter- TRANSMITTERS 45 fere with talking, the diaphragm rests on a soft pad, P. Two damping springs having cushioned tips have been provided as in the White instrument. The working parts of this trans- mitter are all insulated from the case. 73. Monarch Transmitter. The Monarch transmitter shown in Fig. 27 differs from those already studied in having both its electrodes mounted on flexible mica washers which support the carbon chamber. The rear electrode, which is attached to the bridge, is the only fixed part. The diaphragm is of aluminum and is separated from the case by an insulating ring. The flexible connection between one terminal and the front electrode is shown in the figure. The ^_____________, stud of the rear electrode, which is insulated from the bridge, is connected to the other terminal. 74. Operator's Transmitter. In Fig. 28 is shown a special form of transmitter for switchboard operators' use. As this instrument is provided with a plate which rests on the oper- ator's breast, the long curved mouth- piece is always in the proper position for use. The breast transmitter and watch-case receiver described in the next chapter make up the operator's set. In cases where the operator is compelled to leave the switch- board frequently to attend to other duties, as in small exchanges, many of the advantages of the breast transmitter are lost. In such cases a transmitter of the same form as the subscribers' instrument is suspended by adjustable cords in front of the operator. 75. Carbon Electrodes. The disks for use in transmitters are made of specially prepared hard carbon. The faces in contact with the granulated carbon are made as nearly true as possible, and are highly polished. The reverse sides are copper plated and then soldered to the backing plates of brass. When assembled, the electrodes must be parallel to each other if good results in operation are to be obtained. The granular carbon is very hard, uniform in size, and free from dust. As mentioned above, a great deal of trouble was caused by the packing of the granulated carbon in the earlier FIG. 28. 46 PRINCIPLES OF THE TELEPHONE transmitters, due to moisture, unevenness in size of carbon grains, and by wedging apart of the carbon disks. These difficulties were overcome by making the chamber containing the carbon grains waterproof; by making the grains of uniform size and hard enough not to crush in service; and by improve- ments in manufacture, so that the electrodes are always parallel to each other. Any transmitter can be packed by pulling the diaphragm forward so as to widely separate the electrodes. This allows the carbon granules to settle and wedge the electrodes apart. In the earlier types this could be done by placing the lips against the mouthpiece and drawing in the breath. In order to prevent this, modern mouthpieces are slotted at the base. According to a recent report of the American Telephone and Telegraph Co. there were designed, constructed, and in- stalled, during the 37 years from 1877 to 1914, 53 improved types and styles of telephone receivers and 73 types and styles of transmitters. These figures do not include hundreds of minor improvements made in both transmitters and receivers. QUESTIONS 1. How are sounds transmitted by the telephone? Does sound actually travel from one instrument to the other? 2. What are the parts of the telephone used in transmitting the sounds of speech? 3. Will a telephone work if the battery be removed? Why not? 4. What do you understand sound to be? How is sound produced? How transmitted from place to place? 5. How fast does sound travel through air? Compare the speed of sound with that of light. With electricity. 6. Upon what three things does the quality of sound depend? 7. What is pitch? Loudness? Timbre? 8. Explain how a telephone receiver produces sound. 9. Explain how sound waves can cause the transmitter diaphragm to move. 10. What are the characteristics of the waves set up by the sounds of speech? 11. Why can not the sounds of speech be transmitted by repeatedly open- ing and closing the telephone circuit as in the experiment? 12. Examine carefully as many different makes of transmitters as pos- sible. What differences do you find? 13. Explain how changes in transmitter resistance can cause the receiver to operate. How are these changes in resistance caused? 14. Why is carbon used in transmitters? TRANSMITTERS 47 15. Explain briefly the construction and action of the solid-back trans- mitter. 16. What is meant by packing of a transmitter? How is packing caused ? 17. Does the carbon transmitter ever open the battery circuit? Answer this question by studying the transmitters shown in Figs. 23 to 27. CHAPTER VI RECEIVERS AND INDUCTION COILS 76. The Receiver. The telephone receiver makes use of the fluctuating electric currents to reproduce the sound waves which caused these current variations at the transmitting end of the line. Receivers are electromagnetic in their action, as has been briefly explained in an earlier chapter. 77. Early Receivers. Early receivers were of the single-pole type; that is, the diaphragm was influenced by only one pole of the magnet. An early form of receiver is shown in Fig. 10, the parts being named. The operation of such a receiver is due to the magnetic action of the current flowing through the coil, which either weakens or strengthens the magnetic field of the permanent magnet, and thus causes the diaphragm to vibrate in unison with the changes of current strength. The magnetic circuit of this type of receiver contains a very long air path ; hence a considerable current is required to produce the required changes in magnetic force. Another serious objec- tion to this type of receiver is the ease with which the adjustment is disturbed, owing to the magnet being attached to the shell at the end farthest from the diaphragm. 78. Induced Electric Pressure. A further investigation of the relations existing between magnetism and electricity shows that when a wire is moved in a magnetic field so as to cut the magnetic lines, an electrical pressure is set up in the wire. The value of this pressure depends upon the rate of cutting the magnetic lines, or, in other words, the number of lines cut per second. A pressure generated by the relative movement of a conductor and a magnetic field, is called an induced pressure. The direction of the induced pressure depends upon the direc- tion of the cutting of magnetic lines. Hence a movement of a conductor in one direction through a magnetic field will cause a pressure in one direction, and a movement in the opposite direction will generate a pressure in the opposite direction. A pressure can be induced in a coil by changing the strength of the RECEIVERS AND INDUCTION COILS 49 magnetic field inside the coil. Since a magnetic line makes a complete loop or path, it is evident that if the number of lines inside a coil are changed, some lines must be cut by the coil during the change. Increasing the strength of the field inside a coil sets up a pressure in one direction, while decreasing the number of lines sets up an opposite pressure, because the lines are cut in opposite directions during these changes. The induced pressure will be maintained only so long as the relative motion of conductor and field is kept up, or while magnetic lines are being cut. In general we may say that whenever the magnetic field sur- rounding a conductor varies in intensity an electrical pressure will be set up in the conductor, and if the circuit be closed a current will flow. Induced pressure may be either direct or alternating, depend- ing upon whether the magnetic lines are cut continuously in one direction or the direction of cutting is reversed from time to time. 79. Direct Current. A direct current flows continuously in one direction, although its strength may vary from time to time. The flow of a current of electricity caused by the pressure of a battery is in one direction. Direct currents may be divided into two classes, continuous and pulsating. A continuous current is one the strength of which does not change materially from instant to instant. A pulsating current, however, is a direct current the strength of which may vary from time to time without change in the direc- tion of flow. Continuous currents are used for lighting and power purposes. Pulsating currents are made use of in telephone practice. 80. Alternating Currents. An alternating current is one which varies continuously in strength and changes direction periodically. 81. Experiment 9. Apparatus: Two Telephone Receivers. About 50 ft. of Annunciator Wire. Connect two telephone receivers by about 25 ft. of copper wire. Have a person in another room to assist you and see if sounds can be transmitted without using any batteries in the circuit. It will be seen from the above, since no battery or other source of power is used, that the only energy used in operating this 50 PRINCIPLES OF THE TELEPHONE telephone is that of the sound waves themselves. This energy is very small; hence the resultant current sent from one station to another is likewise small, and sounds can be transmitted only a short distance. It was early realized by those interested in the development of the telephone that if the telephone was to become of any commercial value, one capable of transmitting speech to a greater distance was necessary. 82. The Receiver as a Transmitter. Two receivers connected as shown in Fig. 29 formed the first practical telephone for the transmission of speech, and constituted Bell's invention. The operation of such a telephone is as follows: Suppose that A is the sending or transmitting station, and B the receiving station. The sound waves due to the sounds of speech strike the diaphragm at A and cause it to vibrate in unison FIG. 29. with the waves of sound. That is, every variation in the pitch, loudness, or timbre of the sounds affects the diaphragm. The vibrations of the diaphragm cause variations in the strength of the magnetic field, since every vibration causes a change in the length of the air gap between the disk and the pole of the magnet, and thus increases or decreases the number of magnetic lines which pass through the coil. Every time the magnetic field is disturbed, induced currents are set up in the- coil. These electrical currents flowing through the coil at B cause the diaphragm at B to vibrate in unison with that at A, and thus produce sound waves like those which cause the diaphragm at A to vibrate. The receiver seemed to be quite satisfactory, for it would work when large enough currents could be made to flow through it. Hence efforts were made to improve the transmitter, which re- sulted in the development of the carbon transmitter. We have observed that the carbon transmitter does not generate its own current, but merely controls the current from some outside source, such as batteries. 83. Bipolar Receiver. In the bipolar type of receiver the air gap is very much shorter than in the single-pole receiver, since RECEIVERS AND INDUCTION COILS 51 both poles are near the diaphragm. The working parts of the receiver are attached to the case near the diaphragm, or are arranged in an inner metallic case so that the adjustment is independent of the outer case. 84. Western Electric Receiver. Fig. 30 shows the con- struction of the Western Electric bipolar receiver. The shell is of hard rubber and is made in three parts. Two permanent bar magnets are employed, being fastened together so as to form a single horseshoe magnet. Two soft-iron pole pieces P and P' are attached to the ends of the magnet near the diaphragm. Each one of the soft-iron poles is surrounded by a coil of very fine insulated copper wire, marked M and M' in the figure. Im- mediately in front of the poles is placed the sheet-iron dia- phragm D which must not touch the pole pieces even when vibrating through its widest range. One of the magnet poles is N. and the other is S. The diaphragm forms a part of the magnetic circuit, and where the lines enter the diaphragm a S. pole is formed, and where the lines leave the diaphragm a N. pole is formed. Thus the diaphragm acts as an armature and by the attraction of the magnet is constantly bent or dished toward the pole pieces. The coils on the pole pieces are connected so that the mag- netic lines set up by a current passing through them will make one a N. pole and the other a S. pole. The currents flowing through the coils in one direction tend to strengthen the field of the permanent magnet, and currents flowing in the opposite direction tend to weaken the field of the permanent magnet. The diaphragm will spring away from the pole pieces when they are weakened, and when the current ceases the diaphragm will be drawn back toward the pole pieces. When the magnetic field set up by the coils assists the field of the magnet, the diaphragm will be drawn nearer to the pole pieces, and when the current stops the diaphragm will again spring back to its normal position. From this it will be seen that if a current flows first in one direction and then in another, or an alternating current flows in the receiver coil, the diaphragm will answer every impulse of current, no matter from which direction it comes. Alternating currents flow in circuits where an induction coil is used. If the receiver were not equipped with a permanent magnet, its magnetic field would be strengthened by a current flowing 52 PRINCIPLES OF THE TELEPHONE through the coil in either direction, and the diaphragm would be attracted or drawn in toward the pole whenever a current flowed. However, an alternating current flowing in the receiver FIG. 30. FIG. 30a. coil will alternately strengthen and weaken the field of a perma- nent magnet, in the first case drawing the diaphragm out of its FIG. 306. normal position, closer to the pole, and when the field is weak- ened, allowing it to spring farther away. Hence, when a perma- nent magnet is used, an alternating current is capable of pro- RECEIVERS AND INDUCTION COILS 53 ducing a greater vibration of the diaphragm than would be the case if a soft-iron core were used. The pitch is also an octave lower. The resistance of the coils M and M ' is usually about 60 ohms for the pair, or 30 ohms for each coil. The magnet is attached to the case by means of a threaded block which screws into the internal thread B. This arrangement allows of close adjustment of the distance between the pole faces and the disk, and as a result of the close coupling, changes in temperature do not readily affect this adjustment. - The latest type of this receiver no longer has exposed binding posts. This is shown in Fig. 30a and b. This receiver is the standard in use by the Bell Co. 85. The Kellogg Receiver. The Kellogg receiver with in- ternal binding posts for the wires, or cords, as they are com- monly called, is shown in section in Fig. 31. The shell, S, and FIG. 32. the cap, E, are of composition rubber, the shell consisting of a single piece. In order to make the cap stronger and less liable to split under hard usage, a perforated copper disk is molded into it. The diaphragm, D, is firmly clamped between the cap and the brass cup, C, to which the permanent magnets are at- tached. Therefore, the adjustment which is made between the diaphragm and the poles of the magnet at the time of manu- facture is permanent. The receiver can be completely taken apart, without breaking any connections, by removing the cap. Fig. 32 shows the re- ceiver with shell removed. The permanent magnets, P and P f , Fig. 31, are placed side by side with corresponding poles at opposite ends and bolted together at the rear end, holding the block of iron, H, firmly between them, in effect forming a U 54 PRINCIPLES OF THE TELEPHONE I magnet. At the diaphragm end the soft-iron pole pieces are attached. The two pole pieces are separated by a part of the brass cup, and are firmly clamped between the permanent mag- nets by the brass bolt, B. Brass being a nonmagnetic substance, as has been shown above, has no effect on the mag- netic field of the magnet. In order that no strain may be placed upon the bind- ing posts when the receiver is in use, the cord is firmly tied to the block, H. As receiver cords are a considerable source of annoyan.ce, it is interesting in connection with this receiver to note the F 33 method of fastening the metal tips to the flexible strands of the cords. Fig. 33 shows the details of making this connection. The cord tip is first wrapped tightly with wire, .and the strands are brought back over this and firmly held in place by a metal clamp, as FIG. 34. shown. The tip, which is turned and bored from a solid brass rod, is then soldered over this special clamp. The particular advantages of a receiver with internal binding posts are that the binding posts, being inside the case, are not RECEIVERS AND INDUCTION COILS 55 subject to injury; the cord at point of contact is not subject to damage; the user of the receiver cannot receive shocks from the same, since he can not touch the posts; and the receiver has a very neat appearance. 86. Operator's Receiver. The operator's receiver (or watch- case receiver, as it is often called on account of its shape) is shown in Figs. 34 and 35. This instrument is a double- pole receiver; hence the opera- tion is the same as that of the hand receivers described above. The permanent magnet consists of steel rings, P, which are cross- magnetized so that a N. pole ex- ists on one side and a S. pole on the other. The soft-iron pole pieces are clamped between the bottom ring and the case, as shown in the sectioned view. 87. Sensitiveness of Receivers. The sensitiveness of a re- ceiver depends upon the strength of the permanent magnet and upon the diameter and thickness of the diaphragm, and its distance from the magnet poles. A thin diaphragm responds very readily to currents of high frequency (or rapid vibration) and gives clear and sharp tones, while a thicker one is more rigid and responds readily only to those currents having low frequency. About )-{ oo in- is the average thickness of sheet iron used in receiver diaphragms, the diameter being about 2J in. For successful operation a thick diaphragm must be larger than a thin one. The chief objection to a very sensitive receiver is that it reproduces any disturbances which the line may have taken up as faithfully as it does the sound from the transmitting end. A very sensitive receiver, therefore, would be unsuitable for use on a grounded line or on a metallic circuit of poor con- struction. On a long metallic circuit of good construction, the simple weakening of the transmitter current may be compensated to some extent by using a sensitive and delicate receiver. To secure loudness, the receiver must be arranged so that the alternating currents from the line produce the largest possible movement of the diaphragm in both directions from its normal position. The strength of the permanent magnet must be de- signed with reference to the properties of the diaphragm and i r oo o. OO o FIG. 105. across the line anywhere between the two ends, or that a branch line may be run from some point on the main line and one or more telephones connected to the branch line, as shown in Fig. 106. FIG. 106. Telephone lines connected so that more than one subscriber can be called on the same line are known as party lines. 155. Classification of Party Lines. When telephone service is supplied to a few subscribers scattered over a comparatively 15 137 138 PRINCIPLES OF THE TELEPHONE large area, as in country districts, party lines are invariably used. Within cities, where all lines run to a central office, few party lines are used, and where they are used seldom more than four sub- scribers are connected to the same line. Party lines can then be classified in accordance with the number of subscribers con- nected to the same line, but it is undoubtedly preferable to classify them in accordance with the calling system used. We thus have two classes of party lines: namely, code ringing and selective ringing. 156. Code Ringing. The most simple party line system, and the one which was first used, employs the code system. In this system all ringers are bridged or connected across the line in parallel, and all must be of the same resistance in order that the ringing current may be equally divided between them. Any number of telephones, up to about 20, may be connected to the line, and as all the bells ring when ringing current is sent over the line, a code system of ringing is used. A code system with which everyone is more or less familiar consists of a system of short and long rings. Below is a code system for 14 stations, with their corresponding numbers. Station No. 1 - 11 - 21 - 31 - , \.t Zi ' O^ " ' " 3 - - 13 - - 23 - 4 - - 14 - 5 The central office is always given ring one. It will be noticed that the dashes which symbolize long rings represent tens, and that the short dashes represent units. Of course in the telephone directory only the number is given. This scheme of ringing is not often used in towns or cities, but is usually used on country party lines. On local battery party lines it frequently happens that sub- scribers fail to restore their receiver to the hooks or several parties may be listening at the same time. In either case, the receivers and induction coils connected across the line being of low resistance, the ringing current passes through them and not through the ringer coils, thus preventing the central opera- tor's calling the desired party. To remedy this condition and to permit the receiving of a call if receivers are left off the hooks, a condenser is often connected in the receiver circuit, Fig. 107. This condenser prevents the passage of the low-frequency ring- ing currents and causes them to pass through the ringer coils, but PARTY LINES 139 it does not offer any considerable opposition to the high-fre- quency voice currents. Hence, this condenser has the same use as in the central-battery system. It is the practice on code party lines for one subscriber to call another by giving the code ring without the call going through the central office. Some com- U FIG. 107. panies, however, desire to have all calls originating on magneto party lines come into the central office in order to have a record of all calls made on the line, and at the same time relieve the subscribers of the necessity of ringing parties by code. When FIG. 108. this is the case, the instrument is provided with a push button which may be used to connect the generator to ground, and thus use only one side of the line for signalling purposes. The operation of such a device will be readily understood from Fig. 108. When the switch is closed to ground the ringing-current 140 PRINCIPLES OF THE TELEPHONE circuit is through the sleeve side of the line to the drop at central, then to ground and back to ringer. The other bells on the line are not affected. Another plan is to have a direct- or pulsating-current generator in each subscriber's instrument. This current has no effect on the ringers of the instruments, but operates the signals at the central office. The pulsating-current generator is merely the ordinary magneto generator equipped with a commutator and a push-button switch for making connection with the commutator. When the line is connected to the commutator and the generator is turned, the current in the line flows continuously in one direction. It fluctuates in value, as shown in Fig. 45, but does not reverse in direction. Such a current will operate the drop at central, but will not operate the ringers of the other subscribers. FIG. 109. 157. Selective Ringing. In selective ringing the N ringers are so arranged that only the bell of the person wanted at the telephone is rung. Selective ringing is accomplished by two principal methods. One is by the use of a biased bell with pulsating ringing currents; and the other is by making use of bells which will respond only to a given frequency of an alternating current, this latter method being known as harmonic ringing. A common method of selective ringing, for use where only two parties are connected to a single line, is to connect one sub- scriber's ringer between one side of the line and ground, and the other subscriber's ringer between the opposite side of the line and ground, Fig. 109. In order to ring either party, then, it is only necessary for the central operator to send ringing current PARTY LINES 141 over that side of the line to which the desired subscriber is con- nected, which will ring his bell but will not call the other sub- scriber. The talking circuit is connected across the two sides of the line, as usual. In the four-party selective system, biased bells are used. A biased bell is a polarized ringer designed to operate with pulsating current ; that is, current which flows in one direction but is interrupted from time to time. The biased bell shown in Fig. 110 is essentially a polarized ringer with a spring attached to the armature in such a manner as to hold the clapper in the ex- FIG. 110. treme left or right position when no current is flowing. When the clapper is in the extreme left position, the right end of the armature is near the right core of the magnet. It is evident when the armature is in this position that it can be affected only by current flowing through the coils in such a direction as to cause the left core of the magnet to become a S. pole, when it will attract the left end of the armature and overcome the strength of the spring, causing the clapper to move to the right and strike the right gong. As soon as the current ceases to flow, the arma- ture will be returned to its original position by the spring, and the clapper caused to strike the left gong. The rapidity with which this operation is repeated will depend upon the frequency of 142 PRINCIPLES OF THE TELEPHONE the pulsations, or, in other words, the number of times the current is interrupted per second. In order to ring properly, the bell must be in selective adjustment; that is, the spring must be strong enough to pull the armature back to its original position during the time that no current is flowing; yet the spring must not be so strong that the force of the magnet can not overcome it. Ringers not in selective adjustment can be operated only by alternating currents. In ringing biased bells, selection between four stations on a party line may be had by connecting two biased bells, one of each polarity, between each wire and the ground, four bells in all, as shown in Fig. 111. When the pulsating TIP SIDE. FIG. 111. generator is connected so that current flows out along the tip side of the line, the ringer at A is operated. When the con- nections are reversed so that the current flows out through ground, it will operate the ringer at station B. In the same way the ringers at stations C and D may be operated by connecting alternately the positive or negative terminal of the generator to the line and the other terminal to ground. 158. Harmonic Ringing. In a harmonic system alternating current of four different frequencies is used for ringing purposes, the bells being so arranged that each one will ring only when supplied with current at one of the four frequencies. In order that a bell may ring for a given frequency of current, its clapper must swing from one extreme position to the other during the period that the current reverses. Bells used in harmonic ringing PARTY LINES 143 have a spring which holds the clapper in its middle position when no current is flowing. In order that the ringers may operate at different frequencies, the strength of these springs and the weights FIG. 112. of the clappers are different. If the ringer is properly adjusted for the given frequency, a small ringing current will cause the clapper to vibrate violently enough to strike the gongs, in the FIG. 113. same manner that a very small force at the right time causes the pendulum of a clock to swing. Just as a considerable force is required to cause the pendulum of a clock to swing at any but 144 PRINCIPLES OF THE TELEPHONE its natural period, so it is necessary that a heavy ringing current be required to cause the tuned ringer to ring at any other than its natural frequency. The frequencies usually used for harmonic ringing are 16%, 33%, 50, and 66% cycles per second. Since two alternations are required to complete one cycle, the number of alternations per minute, corresponding to the above, are 2,000, 4,000, 6,000, and 8,000. (For example, 16% X 2 = 33% alternations per second; and 33% X 60 = 2,000 alternations per minute.) In Fig. 112 is shown a Western Electric ringer for harmonic party- line service; and the clapper rods for ringers operating at four different frequencies mentioned above are shown in Fig. 113. FIG. 114. Eight-party service may be given by connecting four harmonic ringers of different frequencies between each side of the line and ground, if such service be desired. The ordinary method, however, is to bridge the ringers directly across the line, as shown in Fig. 114, making only four stations on the line. 159. Extension Bells. Many times a telephone ringer can not be heard as far from the instrument as the subscriber desires, in which case an extension bell can be used. As the extension bell is always connected to the same line as the ringer of the telephone, the extension bell must be of the same resistance and have the same adjustment as the other ringers of the line. QUESTIONS 1. Into what classes are party lines divided? 2. What is meant by code ringing? PARTY LINES 145 3. What are the advantages and disadvantages of code ringing? 4. Of what use is a condenser in a receiver circuit, in party-line service? 5. What is meant by selective ringing? 6. In what way is a biased bell different from an ordinary polarized ringer? Explain its operation. 7. What kind of ringing current is used with biased bells? 8. Explain the connections and operation of a four-party line using biased bells. Show how each bell can be rung without ringing the others. Show connections by diagram. 9. What is meant by harmonic ringing? What kind of current is used for harmonic ringing? 10. How are harmonic ringers different from other ringers which have been discussed? What is the difference between ringers designed for different frequencies of ringing current? 11. Explain the operation and connections of four- and eight-party har- monic lines. CHAPTER XV INTERCOMMUNICATING TELEPHONE SYSTEMS 160. Definition. An intercommunicating telephone system is the arrangement of several sets of telephones such that any station can call any other station without the assistance of a central operator. Such systems are extensively used in factories, offices, apartment buildings, stores, and large private dwellings as they afford a ready means of communication between different departments. Telephone systems for intercommunication may be operated either by a local battery for the talking circuit and a magneto for signalling, or they may be operated entirely from a common battery. When the common battery type is used, two sets of batteries are invariably employed. The most simple system of the local battery type is one in which two telephone sets are connected by a single line. Such a system needs no further discussion. However, when more than two stations make up the system, the arrangement is more com- plex. Of course, all the instruments could be connected to a single party line, but this would necessitate code ringing. The usual arrangement of intercommunicating systems is to have a separate line run from each instrument to every other one of the system. For magneto ringing the circuits are quite simple and easily designed. Each station is provided with a panel upon which are mounted as many jacks as there are stations, and lines running from any one station connect the jacks into as many parallel groups as there are stations. At each station the ringer is bridged across one line. This line is designated at all other stations as belonging to the station at which the bells are bridged. The talking and ringing circuit at each station is pro- vided with a terminal plug which is used to make connection with the jack of any other station. Fig. 115 is a simplified dia- gram of such a system. When a person at station A wishes to call some one at station D, he inserts the plug into the jack con- nected to the J) line and turns the magneto. As the only ringer 146 INTERCOMMUNICATING TELEPHONE SYSTEMS 147 that is bridged across this line is at station D it is the only station that will hear the call. As soon as the person at station D inserts his plug in jack D, the talking circuit with station A is complete. Although such a system is extremely simple, owing to the con- venience of automatic signalling provided by the common battery system, the latter is displacing it. 161. Common Battery Interphone Systems. Most of the manufacturers of standard telephone apparatus also manufacture STATION A I STATION & STATION C STA TIO'N D iflnf ^ FIG. 115. intercommunicating telephone apparatus. In general the prin- ciples of operation of the different makes are the same, but each has some distinctive method of connection for ringing. At each station is a telephone set, either a wall set containing the keys and talking set, or a desk stand with a separate key box. Each wall set, or desk set key box has a series of buttons, each one numbered or lettered to indicate the line it controls. Typical C.B. intercommunicating sets are shown in Figs. 116, 117, and 118. A person at one station wishing to talk to one of the other stations presses the corresponding button down to the ringing position, and the desired station is signalled. When this button is pushed down, any other button in the set which might happen 148 PRINCIPLES OF THE TELEPHONE to be depressed is automatically restored, thus clearing the station of any previous connection. When the pressure is re- moved, the button comes back to a halfway or talking position, so that as soon as the called station receiver is removed the talking connections are complete. The wiring of an intercommunicating system appears com- plicated, but this is due to the multiplicity of wires at each telephone. As a matter of fact the circuits are quite simple. FIG. 116. Diagrams of the circuits of two stations involved when one calls the other of a Western Electric interphone system is shown in Fig. 119. The diagram shows that two sets of batteries are used, one for ringing and one for talking. 162. Western Electric Intercommunicating System. In the diagram shown the station at the left is supposed to be ringing the station at the right. In doing this the push button d is depressed as far as it will go. This closes both the ringing INTERCOMMUNICATING TELEPHONE SYSTEMS 149 FIG. 117. FIG. 118a. FIG. 1185. 150 PRINCIPLES OF THE TELEPHONE circuit at d, and the talking circuit at the lower contact. The ringing current then passes from the ringing battery to the bell c, which it rings, at the station called, through the back contacts STATION NO. G LlA/S' CALLINQ STATION N&4 FIG. 119. n of the switch hook at that station, over the wire s of the line and through the lower contact of the button d at the calling station, whence over the other wire t back to the ringing battery. TALKING STATIOH N0.4 AN3IV/r//Hr ST/lT/0/f #0. 6. FIG. 120. When button d is released, it springs part way back opening the circuit at 1 but leaving it closed at 2 and at the lower contact. This condition is shown in Fig. 120. As soon as the subscriber INTERCOMMUNICATING TELEPHONE SYSTEMS 151 152 PRINCIPLES OF THE TELEPHONE CIRCUIT DIAGRAM FORA FULL METALLIC SYSTEM RINGING BAT. STATION ; TALKING BATTERY RINGING BATTERY OUR SETS ARE WIRED FOR FULL METALLIC SYSTEMS. TO ADAPT THtM FOR COMMOH RETURN SYSTEMS-MAKE THE rctlCWlH* CMAHit* (0 STRAP TOGETHER THE TWO TALKING BATTERY TERMINALS (DO NOT counter THE TALKING BATTERY WIRES IN THE CABLE TO THESE TERMimAL*) (Z) STRAP TOGETHER THE LOWER TERMINALS OF ALL LINES. (3) CONNECT THE BLACK TKAHSPO&IYICM WIRE TO POSITIVE (*) RlNlf4CBATTUl TERMINAL*.. <4) CONNECT THE RED TRANSPOSITION WIRE TO THE UPPER TCRMINAUOF THE HOME STATION LINE. te)COMNECT THE NEGATWE(-) TALK I MS, BATTERY CABLE WIRE TO LOWL* LINE NO.I RINGING BAT. TALKING BAT. CIRCUIT DIAGRAM FOR A COMMON RETURN SYSTEM REOTKAN*) i^g ?^ FEPTRAMt-4 Vl__ ,mOHl*4^_ J POSOWHLM^ ^J STATION^ STATION*3 &%X~tr^C TALKING BATTERY RINGING BATTERY FIG. 1216. INTERCOMMUNICATING TELEPHONE SYSTEMS 153 at the station called takes the receiver off the hook he depresses the answering button K which operation connects the two transmitters TI and T 2 directly across the line which is composed of the two conductors s and r. The talking battery is also bridged across the line through the two windings x and y of a retardation coil. The function of this coil is to prevent inter- ference or cross-talk from other stations which might be con- nected together for conversation at the same time, as the same talking battery is used for all the telephones in the system. The receivers R\ and R% are each connected in a local circuit which includes the secondary of an induction coil at each station. The connection between the talking battery and the ringing battery is necessary to prevent cross-ringing, that is, the ringing FIG. 122. of a bell at a station other than the one called. Figs. 121a and 1216 show the arrangement of a typical Western Electric system. 163. The Kellogg Intercommunicating System. The Kellogg intercommunicating telephone which is shown in Fig. 118 operates on the same principle, but the connections differ some- what. Instead of employing the same button for connecting the circuit and ringing, separate buttons are provided. Thus to call a station the button corresponding to the station desired is depressed. This closes the ringing circuit but not the talking circuit for the closing of which a separate green button is pro- vided. In answering a call at any station all that is necessary is to press the red or home button and remove the receiver from the hook in the regular manner. 16 154 PRINCIPLES OF THE TELEPHONE 164. The Monarch Intercommunicating System. The Mon- arch intercommunicating system is also of the push button type, but a modification is made in the manner of connecting the battery to the talking and ringing circuits. The operation u u NORMAL- POSITION O O O O O TAUK1NG POSITIOH FIG. 123. O O O OO RINGING POSITION of the system will be readily understood from an examination of Figs. 122, 123, 124, and 125. Fig. 122 shows the method of wiring for two stations. Two batteries are employed, one for ringing and one for talking, as in the Western Electric system. The ringing circuit is permanently connected to the talking circuit at the sleeve side of /the line at S; from there it leads through the buz- zer or bell, the lower contacts of the hook switch to the ringing battery, and to the calling key, a, for station 2 at station 1. When this key is depressed the cir- cuit is closed through conductor Ri to S. To call station 2, the person calling de- presses the calling, key 2 which closes the calling circuit as shown at a, Fig. 123, completing the circuit and ringing the bell at station No. 2. When the calling key is released it springs back part way opening the ringing circuit at a, and when the person called at station No. 2 takes his receiver off the hook the ringing circuit is also opened at the hook switch. The talking circuit is controlled partly by the calling key at sta- tion No. 1 and also by the home key at station No. 2. The switch controlled by the home button is shown in Fig. 124. In the FIG. 124. INTERCOMMUNICATING TELEPHONE SYSTEMS 155 156 PRINCIPLES OF THE TELEPHONE normal position of the home button, the switch points at d are closed. This corresponds to the lower contact at d, Fig. 122. When the home button is depressed at the station called, the switch points at d } Fig. 124, are opened and those at g are closed. This corresponds to the point g at station 2, Fig. 122. Normally only one side of the battery is connected to the talking circuits. When one station wishes to communicate with another station, the station calling leaves his home button in the normal position, but the station called depresses his button, thus transferring the battery connection at his station to the other side, bridging the battery across the talking circuit through two retardation coils. The circuit is not complete, however, until the receiver is removed from the hook. A complete diagram of connections for ringing and talking between two stations is shown in Fig. 125. An examination of this diagram will make clear the operation of the system. QUESTIONS 1. (a) What is meant by an intercommunicating telephone system? (6) What is the difference between an intercommunicating system and a party line? 2. Show by diagram the connections between two stations for the Western Electric intercommunicating telephone system. 3. Explain the operation of the Western Electric system from your diagram. 4. Diagram the connections between two stations for the Monarch system. 6. Explain the operation of the Monarch system from your diagram. INDEX Action of a condenser, 93 Alternating currents, 49 American wire gage, 11 Ammeter, 15 Ampere, 15 Annealed copper wire, table of, 14 Arresters, carbon block, 122 lightning, 122 self-cleaning, 124 Artificial magnets, 24 horseshoe, 25 Automatic switch, 68 B Bar electromagnet, 31 Batteries, electric, 6 primary, 6 storage, 7 Battery, 4 resistance for parallel connec- tions, 21 Bell or ringer, 69 extension, 144 Bipolar receiver, 50 Bridging telephone, 78 connections of, 79 Brown and Sharpe gage, 11 Capacity of a condenser, 89 unit of, 89 Carbon block arresters, 122 electrodes, 45 transmitter, 40 Cells, dry, 8 in parallel, 20 in series, 20 Circuits, closed, 16 electric, 15 grounded, 17 Circuits, local battery, 78 magnetic, 28 of C. B. subscribers 7 telephones, 112 open, 16 series and parallel, 16 short, 16 signalling, 63 Circular mils, 11 Closed circuit, 16 Code ringing, 138 Coil, heat, 129 induction, 4, 59 retardation, 98 Common battery interphone sys- tem, 147 telephone, 87, 113 C. B. wall set, 100 desk set, 101 hotel set, 100 Complete telephone, 72 Condenser, 88 action of, 93 analogy for, 92 and ringer in series, 95 capacity of, 89 manufacture of, 90 Conductors, lightning, 120 and insulators, 8 Connections of bridging telephone, 79 Construction of electromagnets, 32 Current, alternating, 49 direct, 49 electric. 14 sneak, 128 D Direct current, 49 receiver, 57 Dry cells, 8 Dynamo, Faraday's, 63 157 158 INDEX E Electrical pressure, 7, 15, 48 unit of, 13 resistance, 9 unit of, 13 Electric batteries, 6 circuits, 15 current, 14 Electrodes, 6 carbon, 45 Electrolyte, 6 Electromagnet, 30 bar, 31 construction of, 32 horseshoe, 32 ironclad. 32 tubular, 32 Electromagnetism, 29 Entrance holes, 132 Excessive voltage, 117 Extension bells, 144 Farad, 89 Faraday's dynamo, 63 Faults, localizing, 110 in C. B. telephones, 113 in L. B. telephones, 110 on telephone apparatus, 107 Function of condenser in telephone circuit, 95 Fuses^ 126 enclosed, 126 G Gage, American wire, 11 Birmingham, 13 Brown and Sharpe, 11 New British Standard, 13 numbers, 11 Standard wire, 13 steel wire, 13 Generator, 4, 63 telephone, 66 Grounded circuit, 17 Ground wiring, 134 II Harmonic ringing, 142 Heat coil, 129 Heating effect of current, 117 Hook switch, 4, 72 Kellogg, 73 Stromberg-Carlson, 73 Western Electric, 72, 74 Horseshoe electromagnet, 32 magnet, 25 Impedance, 59 Induced electric pressure, 48 Induction coil, 4, 59, 96 electromagnetic, 58 magnetic, 24 mutual, 58 self-, 57 Inside wiring, 133 Installation, 132 Instruments, telephone, 3, 80 Insulators and conductors, 8 Intercommunicating telephone sys- tem, 146 definition of, 146 Kellogg, 153 Monarch, 154 Western Electric, 148 Interphone system, 147 Western electric, 148 Ironclad electromagnetic, 32 K Kellogg intercommunicating tele- phone, 153 receiver, 53 Laws of magnetic attraction and repulsion, 25 Leading-in wires, 132 Le Clanche cell, 7 Lightning arresters, 121 location of, 125 conductors, 120 INDEX 159 Lightning phenomena, 118 Lines, magnetic, 27 party, 137 telephone, 137 Local battery circuit, 78 systems, definitions of, 76 Localizing faults, 110 Locating faults in C. B. telephones, 113 Location, of lightning arresters, 125 of protector, 133 of telephone set, 135 M Magnetic action, 25 of receiver, 33 attraction and repulsion, law of, 25 circuit, 28 field, 27 induction, 24 lines, 27 substances, 24 Magnetism, 23 Magnetite, 23 Magnets, artificial, 24 horseshoe, 25 natural, 24 permanent, 26 temporary, 26 Magnet wire, 33 Manufacture of telephone con- denser, 90 Measurement of wire, 10 Microfarad, 89 Mil, circular, 11 Monarch intercommunicating sys- tem, 154 Mutual induction, 58 N Natural magnets, 24 Nonconductors, 9 Nonmagnetic substances, 24 O Ohm, 13 Open circuit, 16 Operator's receiver, 55 P Parallel cells, 20 circuits, 16 resistance of, 17 connections, battery resistance for, 21 Party line, code ringing, 138 harmonic ringing, 142 selective ringing, 140 lines, classification of, 137 Permanent magnets, 26 Power circuits, protection against, 125 Pressure and resistance of electric current, 15 electrical, 7 Primary batteries, 6 Properties of sound, 36 Protection, against power circuits, 125 against weak currents, 128 of telephone lines and appara- tus, 117 Protector, 127 location of, 133 Western Electric, 127 R Receiver, 4, 50 action, 23 and transmitter in series, 95 bipolar, 50 direct-current, 57 early, 48 Kellogg, 53 Monarch, 57 operator's, 55 sensitiveness of, 55 Western Electric, 51 Reluctance, 28 Resistance, unit of, 13 electrical, 9 of a parallel circuit, 17 of a series circuit, 17 Resistivity, 9 Retardation coil, 98 Ringer, 4, 69 Ringing code, 138 160 INDEX S Selective ringing, 140 Self-cleaning arresters, 124 Self-inductance, 58 induction, 57 Sensitiveness of receivers, 55 Series, cells, 20 circuits, 16 resistance of, 17 telephone, 76 Short circuit, 16 Shunt, 16 Side tone, 107 wiring, 112 Signalling circuits, 63 Sneak current, 128 Solenoids, 29 Sound, 35 loudness, 36 pitch, 36 properties of, 36 timbre or quality, 36 velocity of, 36 Sources of excessive voltage, 117 Speech, transmission of, 37 Subscribers' telephones, circuits of C. B., 112 Substation, 130 instruments, 111 faults in, 111 Switch, automatic, 68 hook, 72 T Table of annealed copper wire, 14 of resistivities, 9,10 of wire gages, 12 Telephone batteries, 7 bridging, 78 circuit, function of condenser in, 95 common battery, 87 generator, 66 instruments, 3, 80 Kellogg intercommunicating, 153 lines, 137 locating faults in C. B., 113 operation, 1 Telephone, protection of, 117 receiver, 23, 48 series, 76 set, 72 desk, 83 hotel, 82 location of, 135 wall, 80 subscribers', 112 systems, intercommunicating, 146 troubles, 106 localizing of, 106 Temporary magnets, 26 Tests for telephone troubles, 106 Tone, side, 107 Transmission of speech, 37 Transmitter, 3 carbon, 40 Kellogg, 44 Monarch, 45 new Western Electric, 42 operator's, 45 White solid-back, 40 Tubular electromagnet, 32 U Unit of electrical pressure, 13 of resistance, 13 Variable resistance, 38 Velocity of sound, 36 Volt, 13 Voltage, 117 Voltmeter, 13 W Weak currents, protection against, 128 Western Electric protector, 127 receiver, 51 system, 148 Wheatstone's bridge connection, 99 Wire gage, table of, 12 magnet, 33 measurement, 10 Wires leading-in, 132 Wiring, ground, 134 inside, 133 "--,,,,',, 1 t^o*^a*ty UNIVERSITY OF CALIFORNIA LIBRARY