U G h/O WAR DEPARTMENT OFFICE OF THE CHIEF SIGNAL OFFICER, 1916 RADIOTELEGRAPHY UC-NI AL CORPS REVISED OCTOBER, 1916 WASHINGTON GOVERNMENT PRINTING OFFICE 1917 WAR DEPARTMENT OFFICE OF THE CHIEF SIGNAL OFFICER, 1916 RADIOTELEGRAPHY U. S. SIGNAL CORPS REVISED OCTOBER. 1916 WASHINGTON GOVERNMENT PRINTING OFFICE 1917 O ADDITIONAL COPIES OF THIS PUBLICATION MA" BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT 30 CENTS PER COPY TABLE OF CONTENTS. Page. Electric charges and static fields of force 5 Forces of attraction and repulsion 5 Currents and magnetic fields of force 7 Moving charges or currents 7 Direct and alternating currents 7 Static and magnetic fields near a wire 9 Charges with static lines 9 Currents with magnetic lines 9 Radiation of electromagnetic waves 10 Velocity of propagation 10 Currents in transmitting and receiving antennae 11 Measurement of potential by spark discharge 11 Needle and ball spark gaps 12 Systems of units 13 Electrostatic, electromagnetic, and practical systems 14 Definitions of inductance and capacity 15 Names of units 15 Conversion of units of one system to another ., 16 Mechanical and electrical oscillations 17 Oscillatory discharges; wave trains 17 Damped oscillations with spark gap 19 Undamped oscillations with arc and high-frequency alternator 19 Frequency 20 Resonance 21 Power circuits 22 Transformers; open and closed magnetic circuit types; oil and dry insula- tion 22 Alternators; revolving field and armature types; inductor type 24 Motor-generators 24 Rheostat and reactance; adj istment of power circuits by reactance 25 Key; relay and "break " type 26 Definitions of alternating current terms 27 Frequency and period ; frequency meter 27 Cycle and alternation 28 Amplitude 28 High-frequency circuits 28 Closed oscillating or primary circuit 28 Essential elements; connections to transformer secondary 29 Duration of wave train 30 Uniform spacing of wave trains and purity of note 30 Wave train or spark frequency; relation to alternator frequency 30 Multiple discharges 30 Advantages of high-spark frequency at transmitter 31 3 356385 4 TABLE OF CONTENTS. Page. Transmitting condensers 31 Function and types; brush discharge and its elimination; series- parallel connection; capacity 32 Transmitting inductances 34 Function and types, calibration curves; "skin" effect; change of resist- ance with frequency and diameter of wire; litzendraht inductances 34 .Spark gaps 40 Function and types; synchronous and nonsynchronous; quenched gap and its care 40 Connection of closed oscillating circuit to antenna circuit 43 Plain Marconi antenna; coupling, direct and inductive; close and loose coupling; oscillation transformer 44 Antenna? 47 Types; necessity of good insulation; radiation resistance; artificial antenna ; efficiency of radio set 49 Ground; necessity of surface ground; counterpoise 55 Wave length and frequency 56 Wave meter; indication of resonance by ammeter, wattmeter, detector, etc.; unipolar detector connection; fundamental wave length 59 Tuning of transmitting set 64 Mechanical illustration of coupling; single wave length with loose coupling; two wave lengths with close coupling; tuning without wave meter by maximum antenna current or potential; tuning with wave meter to single radiated wave length; objection to transmitters with double wave lengths 64 Theory of quenched spark transmitter 69 Opening of the primary circuit by quenching or stopping of spark; advan- tages of quenched spark transmitter; test of proper coupling by primary and secondary current 69 Receiving circuits 71 Direct and inductive coupling; untuned and tuned secondary circuits: changes of wave length with changes in coupling; changes in coupling with changes in transmitter damping; elimination of static and interfer- ence; selective circuits , 72 Detectors 79 Coherer; rectifiers; audion; advantages of high-spark frequencies at re- ceiver 79 Telephone receivers 83 High-resistance windings; with adjustable pole pieces, best value of shunt- ing condenser; group tuning 83 Calibration of receiving circuits 84 Use of buzzer with wave meter as source of oscillations 85 Signal Corps radio equipment 85 Fort Sam Houston set; 1-kilowatt Marconi 500-cycle quenched spark sets for Coast Artillery stations with instructions for installing and operating; Telefunken field wagon set; Signal Corps field pack set 86 Damping and measurement of logarithmic decrement 128 Definitions; use of decremeter and wave meter; formulas; resonance curve for computation of logarithmic decrement 128 RADIOTELEGRAPHY. ELECTRIC CHARGES AND STATIC FIELDS OF FORCE. Electrical phenomena may be grouped in two general classes, one of static electricity, when the electrical charges are at rest, and the other of dynamic or current electricity, when the charges are in mo- tion along a conductor. When an insulator, such as sealing wax, is rubbed with fur, or a glass tube with silk, it acquires the property of attracting light bodies near it, and is said to be charged. This action shows that forces exist in the adjacent space, and there is said to be an electrostatic, or, more briefly, a static field of force about the charged body. When two charged bodies are brought near together they may be either FIG. 1. attracted or repelled, depending on the nature of the two charges. If the rubbed glass is brought near particles touched and charged by the rubbed sealing wax they will be attracted to it, and similarly if the rubbed sealing wax is brought near particles charged by the rubbed glass they will be attracted; but two bodies, both of which have been charged by either the glass or the wax, will repel each other. Hence like charges repel each other and unlike charges at- tract. The names positive (glass) and negative (sealing wax) have been given, respectively, to these charges. By me^ns of a delicately suspended insulated body the static forces can be mapped out along directions in general perpendicular to the charged surfaces. In fig- ure 1 is shown in section the static field of force between a positively charged and a negatively charged body in which the direction of 5 6 RADIOTELEGRAPHY. the field at any point is indicated by the direction of the arrows at that point, and the intensity or strength of the field in any area is indicated by the number of lines in that area. It is seen that most of the lines are crowded together between the two as though there FIG. 2. was an actual pull along their length, thus suggesting attraction. Similarly in figure 2 are shown the static lines between two bodies with positive charges which are apparently driven apart, thus s FIG. 3. gesting repulsion. If both charges were negative the direction of the arrows would be reversed, but the static lines would have the same shape as before. In figure 3 are shown in elevation the static lines from a positively charged wire near the surface of the earth. If the EADIOTELEGRAPHY. 7 wire were negatively charged, the signs of the charges and the direc- tion of the arrows would be reversed. CURRENTS AND MAGNETIC FIELDS OF FORCE. If a wire connects a charged body with an uncharged or oppositely charged one, the static charge will flow through the wire from the charged to the uncharged body, or from the positively charged body to the negatively charged one, and become a current while so flowing, that is, a current is a moving charge or succession of charges. If the Fio. 4. same charge is continuously renewed there is a steady or direct cur- rent, often abbreviated as D. C. If the charges are continuously vary- ing in intensity and sign and the variations are periodic in character, there is an alternating current, or A. C. While the current is flowing in the wire it has been found that there exists around it a field of force of another kind. If a horizontal magnetic needle is brought near a vertical wire in which a direct cur- rent is flowing, the needle will be deflected and the direction in which it will point depends upon the direction in which the current is flow- ing. This action shows that magnetic forces exist in the adjacent 8 RADIOTELEGRAPHY. space, and the wire carrying the current is said to have a magnetic field about it. The lines of magnetic force may be mapped out with 5. iron filings or a magnetic compass. Thus, if the compass is moved in the direction indicated by the deflection of its needle it will trace M FIG. 6. out circles around the wire as a center and in planes perpendicular to it. RADIOTELEGRAPHS. 9 In figure 4 is shown a section of a wire, perpendicular to the paper and carrying a current downward through it, surrounded by circles, which by the direction of the arrows indicate the direction of the magnetic field at any point, and by the number of lines in any area_ indicate the intensity of the magnetic field in that area. -If the direction of the current in the wire were reversed so as to flow up through the paper, the direction of the arrows would have to be re- versed. Similarly, in figure 5 the wire is shown lying on the paper and the current flowing toward the top of the page, with the mag- FIG. 7. netic lines (appearing as dots) going down through the paper on the right of the wire and coming up through on the left. STATIC AND MAGNETIC FIELDS NEAR A WIRE. If a long wire is placed vertically, and positive and negative charges are alternately applied at the bottom and flow along the wire, there will be near the wire alternately opposite static fields, due to the charges; and at the same time alternately opposite magnetic fields, due to the alternating currents. Figure G shows in perspective the wire with a positive charge, surrounded by its vertical static field S and its horizontal magnetic field M, and figure 7 the wire with a nega- 10 RADIOTELEGRAPHS. tive charge and both its fields reversed in direction. Figure 8 show- both the static and magnetic lines as seen when projected on the plane below the wire where the magnetic lines are circles, as in figure 4, and the static lines are straight, being radial with respect to the circles. RADIATION OF ELECTROMAGNETIC WAVES. These two fields of force changing their direction and intensity with great rapidity and traveling outward from the wire in the medium called the et her with the velocity of light, 300,000,000 meters FIG. 8. or 186,000 miles per second, are the electromagnetic waves of radio- telegraphy. They spread simultaneously radially outward and up- ward from the vertical wire or antenna as it is called. The energy of the varying electric charges and currents is thus imparted to the medium, or is radiated. The two fields constituting the wave and their outward motion in radiation are shown in a general way in figure 9, where the electric RADIOTELEGRAPHY. 11 field is indicated as lines and the magnetic field as dots, this latter being necessary, as in figure 5, because the magnetic field is preperi- dicular to the plane of the paper. At great distances from the trans- mitting antenna the static lines become straight and perpendicular to the surface of the earth and the magnetic lines straight and parallel to the surface. These static and magnetic lines of force, moving with the velocity of light, sweep across the antenna at the receiving station. The vertical static lines in the wave are directed alternately upward and downward and produce in the antenna moving charges of alternately opposite signs; that is, an alternating current. At the same time the horizontal magnetic lines are directed alternately to the right and FIG. 9. left, and when cutting across the antenna produce an alternating current in it. The resultant current generated by these two fields gives an alternating current in the receiving antenna quite similar to that in the transmitting antenna, although of course much weaker. It is these alternating currents which produce the signals in the receiving apparatus. MEASUREMENT OF POTENTIAL BY SPARK DISCHARGE. If large charges of opposite signs are given to two insulated bodies close together, a spark will jump between them and the potential is said to be high. The distance between the points of two needles mounted in the same line may be used to measure this potential. The distance between two brass balls each 2 centimeters (about 25/32 12 RADTOTELEGRAPHY. inch) in diameter may also be used. It will be found that the needle points are more useful at low voltages, as from 5,000 to 15,000, and the brass balls more useful at the higher values. In figures 10 and 11 are given the voltage curves for the needle and the ball gaps. Thus, if the discharge occurs between needle points one-half of an incji SPARKING DISTANCE BETWEEN NEEDLE POINTS 50,000 1 4-5,000 40,000 35.000 30,000 25,000 O JO .20 .40 .60 .80 LOO 1.20 1-40 1.60 1.80 ^00 Sparking Distance in Inches FIG. 10. apart the potential is 15,000 volts. In Tables 1 and 2 are given the values from which the curves are plotted in which the potential is the maximum or peak value, and not the value which would be indi- cated on a high voltage voltmeter. The difference between these two readings is explained on page 28. RADIOTELEGRAPHY. 13 TABLE 1. Needle points. [Adapted from the table of the American Institute of Electrical Engineers.] Sparking distance Maximum potential m inches. m volts. 0. 15 5 > 00 m 20 6, 400 .30 9,300 40 12, 200 ' m 50 15, 000 ' ". 60 17, 700 . 70 20, 500 .*80 23,100 0. 90 25, 700 .. VJV .10 30, 700 .20 33, 000 .30 35, 300 .40 37,500 50 _ _ 39,700 .60 41,900 .70 43, 900 .80 45,800 ..90 47, 600 >. 00 49,500 The potential is the maximum or peak value. TABLE 2. Brass balls 2 centimeters in diameter. [Adapted from Prof. Fleming's book " The Principles of Electric Wave Telegraphy."] Sparking distance Maximum potential in inches. in volts. 0. 05 5, 700 . 10 10, 000 . 20 17, 700 . 30 25, 000 . 40 31, 700 . 50 36, 700 . 60 40, 600 . 70 44, 300 . 80 47, 700 . 90 50, 800 1. 00 53, 400 SYSTEMS OF UNITS, Inductances and capacities are essential elements in the circuits for generating and detecting electromagnetic waves. Their defini- tions and the units in which they are measured will be briefly given in the following paragraphs : A condenser is said to have capacity, which may be defined as its property of storing the energy of electric charges in the form of an electrostatic field, as mentioned on page 10. 14 KADIOTELEGRAPHY. A coil is said to have inductance, which may be defined as its property of storing the energy of electric currents in the form of a magnetic field, as mentioned on page 10. Capacity and inductance, as well as the other electrical quantities, can be measured in three different systems of units, the electrostatic, electromagnetic, and practical. From some points of view it is un- fortunate that three different systems have come into general use, SPARKING DISTANCE BETWEEN BALLS 2 CM. IN DIAMETER 50,000 45,000 40,000 .10 .29 .30 .40 .50 .60 .70 .80 .90 1.00 Sparking Distance in inches FIG. 11. but it is now impossible to abandon any one of them. The relations between the systems may be briefly explained as follows. The units of the electrostatic system may be considered as based on the value of a unit quantity or charge of electricity such that if two bodies are charged with it they will repel each other with a unit force when placed at a unit distance apart. If this charge flows along a wire it becomes a current, and if the unit charges RADIOTELEGRAPH Y. 15 are renewed at the rate of one every second the current so obtained is called a unit current in the electrostatic system. The units of the electromagnetic system may be considered as based on the value of a unit current of electricity such that its magnetic field will exert the same unit force as mentioned above on a body with a unit magnetic field when placed at a unit distance from a unit length of wire carrying this current. The current so defined is called the unit current in the electromagnetic system. The strength or intensity of these two unit currents is not the same ; in fact, it is very different, that of the current in the electro- magnetic system being 30,000,000,000 times stronger than the unit current in the electrostatic system. The units of the other electrical quantities, as capacity, inductance, resistance, etc., are likewise nearly all different in the two systems, in some cases the units being larger in one system than in the other, and vice versa. Owing to the incon- venient size of the units in the two previous systems, suitable frac- tions or multiples of these units have been chosen as the units of the practical system. The numerical relations between the units of the three systems are given in textbooks, so that only a few of the more useful ones will be included in the table below. It is sometimes convenient to abbreviate the words " electrostatic " and " electromagnetic " to " static " and " magnetic," as has been done in the table on the next page, and also to write more shortly E. S. and E. M. When capacity is measured in the practical system the units are the farad and the one-millionth part of a farad, called the micro- farad, and in the electrostatic system the unit is the centimeter. The relation between the two as shown in the table is as follows: Number of stat or centimeters = number of practical units or microfarads; thus, 1,000 C m 8 ,. 6 mfd.- mfd. = 0.00111 mfd. Similarly 900,000 X number of microfarads^ number of centimeters. The unit of capacity in the electromagnetic system has received no name, but if a capacity is measured in the units of this system, they can be converted into those of the other systems by names of the table. When inductance is measured in the practical system the unit is the henry with its fractional parts, as the one-thousandth part, called the millihenry, and the one-millionth part, called the microhenry. Thus, 1/1,000 henry=l millihenry, and 1/1,000,000 henry=l micro- henry; 1 henry =1,000 millihenrys= 1,000.000 microhenrys. In the electromagnetic system the unit of inductance is the centimeter. It 16 EADIOTELEGRAPHY. is to be noted that the name of this unit is the same as that of the unit of capacity in the electrostatic system, an unfortunate choice which can not now be changed. The relation between the units of inductance of the two systems is as follows : Number of magnetic units or centimeters , - -000,000,000 = num p ' or henrys; and similarly 1, 000,000,000 X number of henrys=num- ber of centimeters; 1,000 cms.=l microhenry =1/1, 000, 000 lienry= .000,001 henry; 1,000,000 cms.=l millihenry =1/1, 000 henry=.001 henry; 1,000,000,000 cms.=l henry. Thus hem>y= hen 'T=0.002 henry. =.002X1,000,000 microhenrys=2,000 microhenrys. =.002X1,000 millihenrys=2 millihenrys. =.002X1,000,000,000 cms.=2,000,000 cms. The unit of inductance in the electrostatic system has received no name but can be converted into units of the other systems by the table. Table for changing some of the more common unit * from one system to a not In- r. CAPACITY. Electrostatip units (in cms.). ', Electromagnetic units (no name). Practical units (in mfd.)- To magnetic. To practical. To static. To practical. To static. To magnetic. Divide by 9X10> Divide by 900,000 Multiply bv 9X10> Multiply by 1X10" Multiply bv 900,000 Divide bv ixiois INDUCTANCE, Electrostatic units (no name). Electromagnetic units (cms.). Practical units (in henrys). To magnetic. To practical. To static. To practical. To static. To magnetic. Multiply by 9X1020 Multiply by 9X10H Divide by 9X10* Divide by 1X10 9 Divide by 9X10" Multiply by 1X109 CURRENT. Electrostatic units (no name). Electromagnetic units (no name). Practical units (in amperes). To magnetic. To practical. To static. To practical. To static. To magnetic. Divide by 3X1010 Divide by 3X109 Multiply by 3X10 10 Multiply bv 10 Multiply by 3X10 9 - Divide by 10 KADIOTELEGRAPHY. 17 Table for changing some of the more common units, etc. Continued. POTENTIAL. Electrostatic units (no name). Electromagnetic units (no name). Practical units (in volts). To magnetic. To practical. To static. To practical. To static. To magnetic. Multiply by 3X101 Multiply by Divide by 300 U 3XlOio Divide by 1X10 8 Divide by 300 Multiply by 1X10 RESISTANCE. Electrostatic units (no name). Electromagnetic units (no name). Practical units (in ohms). To magnetic. To practical. To static. To practical. To static. To magnetic. Multiply by 9X10* Multiply by 9X10" Divide by 9X10 20 Divide by 1X109 Divide by 9X10" Multiply by It will be noted that in many cases the units have received no name in some of the systems in which they are expressed, so that the name of the system must be given ; thus a current of 1 ampere is a cur- rent of 3,000,000,000 units of current in the electrostatic system, or 3.000,000.000 electrostatic units of current. Owing to the large numbers which must be used in converting units from one system to another it is usual to abbreviate as in alge- bra; thus, 3,000*000,000 is written 3X10 9 , where the number 9 in- dicates the number of times that the cipher or zero must be written after the number 3, and similarly 900,000,000,000,000,000,000 is writ- ten 9X10 20 . The table may be used to convert from one system to another, as follows: A potential of 2.5 units in the E. S. system is equal to 2.5 X BOO units in the practical system, or 750 volts; current of 1.0 ampere in the practical system is equal to 1.0-^-10 units of current in the E. M. system, or 0.1 unit in the E. M. system; an inductance of 1/500 henry is equal to 1/500 XlO 9 E. M. units of inductance or centi- meters, or 1/500X1,000,000,000=2,000,000 cms. MECHANICAL AND ELECTBICAL OSCILLATIONS. The following illustrations and explanations of oscillatory dis- charges and their occurrence in resonant circuits are introduced here so as to give a clear understanding of these most important principles. OSCILLATORY DISCHARGES. If a strip of steel is clamped at one end and the free end is pulled to one side and released, this end will not only return to its normal position but will swing past it, and returning it will swing past in 66536 17 2 18 RADIOTELEGKAPHY. the opposite direction, but not so far as before and will thus execute a series of oscillations, each of which takes place in the same length of time expressed in fractions of a second, which will gradually die down to zero, or are said to be damped. The free end returns to its normal position because of the elasticity of the metal, and swings beyond it because of its inertia. The energy stored up in the spring -I- o s o FIG. 12. in pulling it to one side is thus gradually wasted in friction, etc. In a similar way in electrical circuits we have to deal with capacit;/* which corresponds to the elasticity, and inductance, which corre- sponds to the inertia. If a condenser of considerable capacity C, such as a number of Ley den jars or condenser plates in parallel, is connected in a circuit with a coil L and spark gap S, as shown in figure 12, and the poten- FIG. 13. tial on the condenser gradually increased, quite a large charge may be stored in it before the potential rises high enough to cause a spark at the gap. When, however, the gap breaks down, the charge in the condenser discharges through the gap and the coil, and on account of the inductance (inertia) in the circuit it overshoots in the same way as the spring, then discharges in the opposite direction, etc., so RADIOTELEGRAPHY. 19 that the charge may oscillate many times back and forth across the gap before it is so used up in heat that not enough charge remains to jump across again. The charged condenser, as C of figures 12 and 17, is thus the immediate source of the energy of the electrical oscilla- tions. Its rapid oscillatory discharge through the gap S and the" inductance L takes place in the form of a series of decreasing oscilla- tions, called a train of damped oscillations or a damped wave train. In some circuits there may be 20, 30, or even more such oscillations in FIG. 14. a wave train. Figure 13 represents discharges in which the oscilla- tions die down quickly, and are said to be strongly damped or highly damped. Figure 14 represents discharges in which the oscillations die down gradually and are said to be feebly damped or slightly damped. Figure 15 represents, discharges in which the oscillations do not die down and are said to be undamped oscillations, con- tinuous oscillations, or sustained oscillations. These undamped oscil- FIG. 15. lations can not be generated by the discharge of a condenser through an ordinary spark gap, but may be developed by means of a special type of direct-current arc with metal or metal and carbon electrodes, as in the Poulsen or Federal system, or by special high-frequency alternators, as in the Fessenden or Goldschmidt system. One of these alternators having a speed of 20,000 revolutions per minute and giving 100,000 oscillations per second has been installed by the Signal Corps at the Bureau of Standards in Washington, D. C. This 20 BADIOTELEGRAPHY. machine and its driving motor are shown in figure 16. Both the arc and alternator methods of the generation of undamped oscillations are now in use. FREQUENCY. The rate of vibration of the steel spring or number of vibrations per second depends upon the weight, distribution, and elasticity of the metal. Similarly in the electrical circuit, when the condenser discharges across the gap and through the inductance, the rate of the electrical oscillations, or frequency in number of oscillations per second, depends upon the capacity of the condenser and the induc- tance of the coil. The larger the product of the capacity and indue- RADIOTELEGRAPH Y. 21 tance, the slower is the rate of the oscillations ; that is, the fewer the number of oscillations per second and the lower the frequency, and vice versa, the smaller the product of the capacity and inductance the more rapid is the rate of the oscillations; that is, the greater the number of oscillations per second and the higher the frequency. The formula for the number of oscillations per second is n = - 7=, where L is the inductance in circuit in henrys and C the capacity in farads ; thus, if C is 0.000,000,004 farad (0.004 microfarad) and L is 0.001 henry (1,000,000 cms. or 1 millihenry), then the oscillations are tak- ing place at the rate of about 79,600 per second. ~ 6.28 VO.OOl X 0^0,0007)04 6.28 VO.000,000,000,004 # RESONANCE. The principles of resonance can be illustrated by the steel spring, preferably in the form of two timing forks. If a loud note from FIG. 17. one tuning fork is sounded near another fork, the latter will be set in vibration slightly, even if the pitch of the note or number of vibrations per second is not the same as that which the latter itself would give. If, however, the note is of the same pitch, then each successive vibration of the prongs will be reenforced by air waves of the same frequency as its own, and stronger vibrations will be produced by this note than by any other. Under these conditions the two forks are said to be in resonance. Similarly if a circuit containing a coil 1, condenser c, and very small spark gap s, all in series, is brought near another circuit LCS, as shown in figure 17. in which oscillations are taking place, then small sparks may be seen passing across the gap s, of the first circuit, showing that cur- rents are being induced in it. If, however, adjustments are made in the number of the Leyden tubes in circuit or in the number of turns of inductance by means of the sliding contact, then generally the size and brightness of the sparks will be increased up to a certain 22 KADIOTELEGKAPHY. point, and any further changes in either the inductance or the ca- pacity will make the sparks smaller and fainter. At the adjust- ment which gives the largest and brightest sparks the induced oscil- lations are the strongest and of the same frequency in the two cir- cuits; that is, the two circuits are syntonized, or tuned, or are in resonance. POWER CIRCUITS. TRANSFORMERS. After each oscillatory discharge the charge in the condenser is renewed at regular intervals by an induction coil, or alternating cur- rent transformer. The former is but little used now, and will not be described here. The transformer is an apparatus for increasing the comparatively low voltage of an alternating current dynamo or generator to the high voltage necessary to cause the condenser charge to jump across the spark gap. The details of transformer construction are dsecribed in textbooks on electricity. It will suf- fice to say here that it con- sists of a primary winding of a comparatively few turns of heavy wire, wound on but insulated from a laminated iron or iron-wire core, which carries the cur- Primary FIG. 18. rent from the alternator; a secondary winding of many turns of finer wire wound in sections and well insulated from all other parts of the transformer, which delivers a smaller current, but at the necessarily higher voltage, to the condenser that is charged thereby. In general the transformer increases the alternator or primary voltage in the same proportion as the number of secondary turns is increased over the number of the primary turns. The voltage of the alternator impressed on the primary of the transformer is usually 110 or 220 volts; the voltage of the secondary which is impressed on the con- denser depends upon the size of the radio set and varies between, say, 10,000 and 30,000 volts. In the case of quenched spark sets a transformer is generally used in which by a proper choice of the capacity connected to its secondary circuit, the secondary voltage is increased by resonance to perhaps twice as many times as the ratio of the primary and secondary turns EADIOTELEGRAPHY. 23 Primary would indicate. Such a transformer is called a resonance trans- former. Transformers may be divided into two classes, depending on the type of the laminated core, whether with the open magnetic circuit, as shown in figure 18, or with the closed magnetic circuit, as shown in figure 19. These terms apply to the iron as a path for the mag- netic field. Thus in figure 19 it is seen that the magnetic lines M have a continuous path or circuit through the iron, or, as it is said, a closed magnetic circuit, whereas in figure 18 the path of the lines is partly through the iron and partly through the space outside, or, as it is said, an open magnetic circuit. In both figures the direction of the field as it exists at one instant is indicated by arrows, but it must be remembered that the field is continually reversing its direc- tion as the alternating current changes its direction. Both types of transformers are in general use, although it is probable that the closed magnetic type is now being used more than the other. There is no essential dif- ference in efficiency of operation. Prac- tical experience has shown, however, that in general it is not always possible to interchange trans- formers of the two types in any one set, particularly in quenched spark sets, where the alternator, transformer, and con- denser of the closed oscillating circuit, as shown in figure 73, must be designed as a whole to secure the best results. Transformers may be divided into two types, depending on the nature of the insulation, whether oil insulated or dry insulated. In the first the transformer is completely immersed in a suitable in- sulating oil, such as transil oil, in an iron tank provided with a cover to keep the oil from spilling, through which the terminals extend, strongly insulated, as with porcelain for example. In the second type strong insulating fabrics or materials are used around and between the windings which are saturated with a nonfluid insulating compound. In the higher voltage transformers of both types, the secondary coils are often heated in a vacuum to remove the air and moisture, dipped in an insulating varnish or compound, and baked until they are hard so as to protect the windings, exclude moisture, etc. The connections of the transformer, etc., are shown in figure 20 where A is the alternating current generator, K the telegraph key, FIG. 19. 24 EADIOTELEGRAPHY. T the transformer with primary and secondary windings, C the con- denser, S the spark gap, and L the inductance. There is no essential difference in operation of the two kinds of connections, the choice generally being made on account of some convenience of wiring. ALTERNATORS. The transformer receives its power from an alternating current generator, or alternator, as it is often called, which is either belt or chain driven from an engine or electric motor, or directly driven by electric motor, in which case the two machines are mounted on the same bedplate and the shafts connected by a flexible coupling, the set being called a motor-generator set. The two essential parts of an + C I 1 FIG. 20. alternator from an electrical point of view are the fields and the armature. A direct current is supplied to the former and an alter- nating current is delivered by the latter. Alternators are built in three general types, with revolving field, revolving armature, and of the inductor types, of which the last two are generally used in radio work. In the revolving armature type the fields are stationary and the armature rotates, its wires thus cutting the magnetic lines from the field windings and generating the alternating current which is brought out by brushes bearing on two collector rings, or slip rings, as they are called. In the inductor type both the field and the armature are stationary, the rotating part being simply an iron form Avith projecting pole pieces, the rotation of which carries the mag- RADIOTELEGRAPHY. 25 netic lines from the fields in and out of the fixed armature, the wires of which thus cut the magnetic lines and generate the alternating current. In this type of machine there are no revolving wires or moving contacts of any kind. The moving part, as armature, field v or inductor, as the case may be, is called the rotor. The stationary part is called the stator. The alternator fields require a direct current for their energizing, which may be furnished either by an outside direct-current source, such as the direct-current mains that supply the power to run the direct-current motor of a motor-generator set, as shown in figure 73, or by an exciter, which is a small direct-current machine that may be mounted on the alternator shaft or may be a separate machine inde- pendently driven by any. convenient means as shown in figures 77 and 79. RHEOSTAT AND REACTANCE CONTROL. In order to control the power delivered to the transformer a vari- r able resistance or rheostat is sometimes inserted in series in the circuit of the alternator armature and transformer primary ; in other cases a variable inductance called a reactance or reactance regulator is used, consisting of coils of heavy wire, with taps brought out at different points, wound on a laminated iron core. The rheostat and the re- actance may serve similar but not necessarily the same purpose; thus- increasing the resistance in the rheostat always decreases the power delivered to the transformer, and increasing the reactance may do likewise. In these cases the rheostat or reactance may normally be cut out of circuit and introduced only as needed to cut down the power, as for example, when it is desired to decrease the range of a set so as not to cause interference at a distant station or when, as required by law, a ship station reduces its power as it comes within 15 miles of a naval or military station. Increasing the reactance does not always cut down the power; in fact, in some circuits of the quenched-spark type it may actually increase the power delivered to the transformer, and hence to the antenna, where it causes an increase in the antenna current. The reason for this is that there is a combined adjustment of the in- ductances in the transformer primary and secondary circuits and of the capacity of the closed circuit condenser which is best adapted for the charging of this condenser at regular intervals. In some cases more inductance is required than that in the alternator armature, and the transformer primary, and it is then added as a reactance in the primary circuit. In other cases the inductance may be added as a re- actance in the secondary circuit, where evidently the coil must be de- signed to withstand high potentials. In a few cases reactances are added in both circuits so as to secure the desired results. When 26 RADIOTELEGRAPHY. the best adjustments have been attained it is often found that the transformer primary current drops to a minimum value, the antenna current rises to a maximum, and at the same time the note of the spark is the clearest. KEYS. In the smaller sizes of radio sets the current from the alternator to the transformer can be controlled by ordinary types of Morse keys, with either silver or platinum contacts, without troublesome sticking, trailing, or arcing even at fast sending. In the larger sizes, how- ever, special means of cutting down the arc at the breaking of the circuit must be used, such as shunting the key by a resistance, con- denser, reactance, etc., so that the key does not break the whole cur- rent, as shown in figure 73. In this case, however, it must be remem- bered that, as these shunts always allow some current to flow through them, the high-tension and high-frequency circuits are alive and it may be dangerous to touch any of them. In the largest sets a relay key is generally furnished, which consists of an electromagnet the windings of which are in series with an ordinary Morse key and a source of direct current, and the armature of which carries the heavy contacts necessary to break the current in use. Such a key may be used to break a current of 50 or 60 amperes or more with- out injurious sparking. In some cases a single large key with con- tacts an inch or so in diameter and a handle a foot long has been used. Another type of key is coming into use, known as a "break key" which permits the receiving operator to break the transmitting operator as on a wire line. Among other ways this may be accom- plished by providing the ordinary key with an extra set of contacts which, just after the current has been broken in making a dot or dash, and just as the key handle comes up to its final position, auto- matically connects the receiving circuit to the antenna and ground without the necessit}^ of throwing a special switch. At any time that the receiving operator misses a word or desires to " break " the trans- mitting operator he holds his key down or calls " bk," and the trans- mitting operator with the telephones on his head and with his detector in adjustment will hear the call between the dots and dashes of his own sending and thus be broken. For most successful use both operators should be provided with break keys. It is essential that the receiving circuits in general and the detector in particular be protected from sparks from the transmitting circuits, and that the operators be not bothered by the sounds from their spark gaps or machinery. KADIOTELEGRAPHY. 27 DEFINITIONS OF ALTERNATING-CURRENT TERMS. For a proper understanding of some of the points on the following pages, definitions and explanations will be given of the more common terms in use in the practice of alternating currents. The frequency with which the charges in the condenser C of figure 20 are renewed by the transformer depends, among other things, upon the rate at which the voltage and current delivered by the alternator is varying. Figure 21 represents the manner in which these quantities vary, where the set of values ABODE, half of which is positive and half negative, is called a cycle of voltage or current, the symbol for which is often thus written ~ . The number of cycles per second is called the frequency and the letter " n " or " f " is often used as its symbol. In commercial alternators used in radio teleg- raphy the frequencies are generally 60, 120, 480, or 500 cycles per second ; that is, there are 60, 120, etc., complete sets of values, such as :B A A \ \ Time \ FIG. 21. ABODE of figure 21 per second, or n 60, 120, etc. Half a cycle, such as the set of values ABC or ODE of figure 21, which may be either positive or negative, is called an alternation. There are always twice as many alternations per second as there are cycles. The fre- quency of an alternating current is sometimes given in alternations per minute instead of cycles per second, thus a current of 60 cycles per second is of the same frequency as one of 7,200 alternations per minute. The time taken to complete one cycle is called the period, and the letter T is often used as its symbol, thus if there are 500 cycles per second, the time to complete one cycle is 1/500 second or 0.002 second; that is, T= T J Tr second or T= 0.002 second. Similary the time for one alternation of a current of the same frequency is 1/1,000 second or 0.001 second. The relation between the fre- quency in cycles per second and the period in fractions of a second is given by the formulae T = -vr or N = m. '28 KADIOTELEGRAPHY. The highest value of the current or voltage in any alternation, as at points B, D, etc., of figure 21 or the corresponding points in figures 13, 14, and 15, is called the amplitude or sometimes the peak of the curve. It will be noted that there is a similarity between the sustained oscillations as represented in figure 15 and the alternating current or voltage as represented in figure 21. The two curves have the same shape or form, being known in trigonometry as sine curves, but they differ in the greatly increased frequency of a hundred thousand or million per second in the radio circuits (the closed and open oscil- lating circuits), as compared with that of 60 to 500 per second in the power circuits (the alternator and transformer circuits). It is the general practice to speak of the number of oscillations or of cycles per second in radio circuits, but only of the number of cycles per second in power circuits. If the voltage or current varies as a sine curve, as in figures 15 and 21, the voltmeter or ammeter will not read the peak or ampli- tude value, because this value lasts for only a short part of the total time, but a fractional part, 0.707 = -r of the peak value. Similarly y ^ if the voltmeter or ammeter reading is given, the peak value or amplitude can be found by multiplying by 1.41 =-r=--- V 2 The frequency of the alternating current is sometimes indicated by a frequency meter, which in one type consists of a series of flat steel springs or reeds, each with a different period of mechanical vibration which is marked on it, the whole series covering a range of frequency of from, say, 470 to 530 vibrations per second. Behind the springs is an electromagnet carrying the alternating current, the frequency of which is to be measured. When the frequency of the electromagnetic impulses is the same as that of any one of the reeds it is set into vibration by resonance with these impulses, and the frequency of the current is then the same as that marked on the reed in vibration. HIGH-FREQUENCY CIRCUITS. CLOSED OSCILLATING OR PRIMARY CIRCUIT. The circuit of coil L, condenser C, and spark gap S, as shown in heavy lines in figure 20, is called the closed oscillating or primary circuit, as distinguished from the open, radiating, or secondary cir- cuit to be described later. These three elements are always connected in series to form the circuit, which is found in all spark excitation types of radio stations. There are two different methods of con- necting the transformer secondary leads to this circuit for the charg- ing of the condenser, one of which is shown in the upper part of RADIOTELEGEAPHY. 29 figure 20, where the condenser is seen to be directly across the trans- former secondary leads, and the other in the lower part where the spark gap is so connected. In this latter case the condenser is charged through the inductance L, but its resistance and inductance are so small as compared with that of the transformer secondary as to have no effect in the charging. There is no essential difference in the operation of the tw r o types of connections. The actions taking place in the closed circuit as a whole are as fol- lows: The condenser begins to get its charge at the beginning of each alternation, as at points A, C, E, etc., of figure 21, and reaches such a potential as to cause its discharge across the gap and through the inductance at the peaks of the curve, as at points B, D, etc. The condenser is, so to speak, a reservoir which is filled and discharged 1,000 times per second in a 500-cycle alternator set. In figure 22 the upper curve represents the 500-cycle alternating current delivered by FIG. 22. the transformer secondary to the condenser which is charged thereby ; the low r er curve represents the discharge of the condenser, produc- ing damped wave trains of perhaps 20 or 30 oscillations, each train lasting a few millionths or hundred thousandths of a second, as shown in figures 13 and 14. In order to be able to show the wave trains at all in figure 22 their duration must be shown much exag- gerated as compared with the intervals between them. Thus, if the period of each complete oscillation in the train were 500*000 second and there were 25 oscillations in the train, each train would persist f r soo, 5 ooo second, or go^ooo second, or the duration of each wave train is only one-twentieth of that between successive trains. It must be noted that although the transformer secondary is con- nected to the closed oscillating circuit, as shown in figure 20, it takes 30 RADIOTELEGRAPHY. no part in the oscillations of this circuit. The reason for this is that the period of the circuit of transformer secondary and closed circuit capacity is so long (in fractions of a second) on account of the large secondary inductance that the wave train in the closed oscillating circuit has been completed before the transformer secondary circuit has had time to complete a part of one of its own slow oscillations. The period or frequency of the oscillations of the closed circuit is thus independent of the transformer circuit. In the preceding example it has been assumed that there was one discharge in each alternation or two discharges per cycle; that is, 1,000 wave trains per second. In some cases, however, the circuit may be arranged so that there is a charge and discharge in every other alternation that is, only one discharge per cycle which, with a 500-cycle alternator, would give only 500 wave trains per second. In both cases, however, the wave trains are separated by equal inter- vals of time. When the wave trains are thus separated by equal intervals of time the note of the spark is said to be pure. In some cases, however, it is possible to charge the condenser two, three or even more times per alternation, and hence four, six, or even more times per cycle, and then it is said that these are multiple discharges. Under these circumstances the intervals of time between the wave trains will not in general be all equal and the note will not be pure. The pure note is often very desirable, although not always necessary in practical work. WAVE TRAIN OR SPARK FREQUENCY. The number of wave trains per second is called the wave-train frequency or the spark frequency. If the alternator frequency is 500 cycles per second and there is a discharge once in every alterna- tion, or 1,000 discharges per second, the spark frequency is 1,000 per second. It must be noted that in general the alternator frequency and the wave-train frequency are not the same ; in fact, they may be very different, as in the case of multiple discharges mentioned in the last paragraph. If the spark frequency is, say, 120 per second, as from a 60-cycle alternator, it is said to be low, but if it is 1,000 per second, as from a 500-cycle alternator, it is said to be high. There are certain advan- tages in a high spark frequency which appear both at the trans- mitting and at the receiving stations. If the closed circuit condenser is charged 1,000 times per second to a certain potential, it is evident that more energy will be required than if charged only 120 times, the formula for the energy being 1/2 C V 2 N, where C is the capacity, V the potential, and N the number of times per second. If the same amount of energy is available in the two cases that is, if 1/2 C V 2 N is constant the smaller the value of N the larger must be the value of RADIOTELEGRAPH Y. 31 V, other conditions being constant, and, vice versa, the larger the value of N the smaller may be the value of V. The earlier practice was to make N small, as 120 per second from a 60-cycle alternator, and V large, as 30,000 volts. The modern practice is to make N large, as 1.000 from a 500-cycle alternator, and V small, which in this ex- ample must be about 10,800 volts. It is evident, then, that the trans- former secondary and the closed oscillating circuit condenser do not need to be built to withstand the high voltages formerly used, and that, therefore, they may be lighter and more compact ; also that the oscillation transformer and antenna, to be described later, do not need the very high insulation which was formerly necessary. The advantages of the high spark frequency at the receiving station will be mentioned later under that heading. If suitable constants are used in the formula for the energy, it is possible to determine the capacity, peak voltage, etc., for any size of set. Let K. W. be the number of kilowatts that the trans- former secondary must deliver to the closed oscillating circuit con- denser ; M. F. the capacity of this condenser in microfarads ; V. the peak value of the voltage to which the condenser is charged and then discharged as the spark gap breaks down; and Cycles the number of cycles per second of the alternator in which there are two dis- charges per cycle, then T , W ^(M.F.)X(V 2 )X (Cycles) 10 9 Thus if M. F. is 0.012 mf.; Y. 18,250 volts, peak value; and the Cycles 500, with two discharges per cycle, then K. W. will be 2.0. As it is impossible to build a transformer with an efficiency of 100 per cent, it is evident that the armature of the alternator must de- liver a larger number of kilowatts to the primary of the transformer than is given by the above formula. The actual number will be found by dividing the secondary kilowatts by the efficiency of the transformer. Thus, if the efficiency were 93 per cent or 0.93, then the alternator armature output or the transformer primary input C\ r\ would be ^ = 2.15 K. W. By simple changes in the above formula it is evident that when any three of the quantities are known, the fourth can be found. TRANSMITTING CONDENSERS. A brief description of the three elements, condenser, coil, and spark gap, will be given. The functions of the condenser are, by virtue of its capacity, to store the charge delivered to it by the transformer secondary circuit until its potential reaches the desired value as determined by the spark gap, and then to discharge through the gap and the inductance. 32 RADIOTELEGRAPH Y. An ideal condenser would be one that was perfectly insulating, could not be punctured, and showed no heating or losses of any kind during charging and oscillatory discharging. There are several different types of transmitting condensers used in the Signal Corps radio stations, varying widely in capacity, size, voltage, etc., from the small mica ones of the field radio sets to the 4|-foot jars or compressed-air types in the permanent stations. All types consist essentially of two conducting surfaces, as tin or copper foil, separated by an insulator or dielectric, as it is often called, which can withstand without puncturing the high voltage required to break down the spark gap. Probably the most efficient condenser is the compressed-air type, which consists of a large number of cir- cular metal plates mounted on two sets of supports with a small air space between each plate, the top plate and every alternate plate being connected together as one set and the remaining plates as the other set. The whole is contained in an air-tight tank, one set of plates being connected to the tank as one terminal and the other set to a terminal brought out through the cover in a porcelain insulator sealed air-tight by a lead gasket. Air is then pumped into the tank until a pressure of about 240 pounds per square inch is reached, or about 16 atmospheres of 15 pounds per square inch, as shown by a pressure gauge on top of the tank. At this pressure it has been found that air has an insulating strength many times greater than at ordinary pressures. Condensers of this type will withstand a maximum or "peak" voltage of about 20,000 volts under service conditions. The most serious objection is the excessive weight, a tank of about 0.006-microfarad capacity weighing about 300 pounds. There are many types of condensers using glass as the dielectric, such as plates or jars covered w y ith foil or plated with copper. When these condensers are used at high potential, such as 25,000 volts or more, there is developed at the sharp edges of the foil or plating a discharge (sometimes called brush discharge}, which spreads out over the surface of the glass, is accompanied by a hissing sound and con- siderable heating of the glass close to the edges, and in a dark room shows a pink light at the edges. The puncturing of the glass and the breaking down of the condenser often takes place close to the edges, due probably to the brush discharge and the local heating of the glass. These discharges represent losses which, in part at least, can be prevented by covering the edges of the foil with an insulating coating, such as asphaltum, and more completely by immersing the condensers in an insulating oil, such as castor oil, etc. The capacity of these condensers and the voltage which they can withstand depend so much on the quality of glass, the manner in which it was annealed, its thickness, etc., that it is impracticable to give figures except for condensers that have actually been tested. EADIOTELEGRAPHY. 33 [ T FIG. 23. The capacity of one glass plate about T 3 e inch thick and with the foil 15 inches square is about 0.0020 to 0.0025 microfarad. The capac- ity of a jar with glass -J inch thick, 4f inches in diameter, and height of foil of 10 inches is about 0.002 M. F. In the case of a good grade of plate glass about -fg inch thick, free from scratches, bubbles, etc., a potential of 20,000 volts, peak value, can be safely used. In figure 23 is shown a closed oscillating circuit with three con- denser jars connected in parallel; that is, the three outside coatings are connected to- g-ether as one ter- minal and the three inside coatings as the other, and with a potential of 20,000 volts between the terminals. When condensers are thus connected in paral- lel the total capacity is the sum of all the capacities; if the con- densers are all of equal capacity, the total capacity is the capacity of any one condenser multiplied by the number. Thus in figure 23 if each condenser were a jar of capacity 0.002 M. F., the total capacity would be 0.006 M. F., or three times 0.002 M. F. If the condensers break down at this potential or if higher poten- tials, such as 30,000 volts, are to be used, two banks, each of three jars in parallel should be connected in series, as shown in figure 24. It is to be noted that this connection requires twice as many jars as be- fore, but if the total potential is 30,000 volts, the po- tential across each FIG. 24. J ar i g now on ly 15,000 volts instead of 20,000 as before. Whenever condensers are connected in series, the total capacity is always reduced ; if two equal condensers are so connected, the total capacity is one-half the capacity of either; if three equal condensers are so connected, the total capacity is one- third, etc. As the connections shown in figure 24 rdeuce the capacity to one-half the desired value in figure 23, two banks each of six jars must be connected in series-parallel^ as shown in figure 25, thus requiring four times as many jars as the first circuit. 66536 17 3 t W5000 V.+I5000 VM I* 30000 Volts->| 34 RADIOTELEGRAPHY. Another type of condenser having some advantages is the Moscicki jar, which consists essentially of a glass tube or jar Avith 'inside and outside coatings, as in the other types, but at the edges of the coatings where the puncture usually takes place the glass is thickened to give increased strength, and at the same time the edges are covered with an insulating liquid to stop the brush discharge. The whole is con- tained in a brass tube to which the outside, coating is connected, the inside coating being brought out to a binding post through a sealed porcelain insulator. The case and the binding post thus become the two terminals. These tubes are made in two sizes, the larger of which is in more general use, has capacity of about 0.005 M. F., and is capa- ble of withstanding 20,000 volts. There are many other types of condensers using such dielectrics as mica, paper, and various molded insulating compounds. In a IV \v cases oil is used as the dielectric, in which case metal plates are mounted on insu- lating supports a short distance apart in tanks filled with a suitable insulat- ing oil, such as cas- tor oil, etc. I 1 U| 5000 V*l+f 5000 vJ k- 30 000 Volts ->| TRANSMITTING INDUCTANCES. FIG. 25. The function . of the inductance is to form one of the two elements, the condenser being the other, neces- sary for developing and maintaining the oscillations, and to serve as a mans of transferring energy from one circuit to another. An ideal coil would be one having the desired inductance but with a zero resistance to the oscillating currents. The inductance coil L, which has been shown in the various figures, may be any one of several different types, such as a helix of heavy copper wire, thin-walled copper tubing, or flat strips, or a flat spiral of copper ribbon, such as the linking coil of the early Signal Corps field radio sets, etc. These are generally provided with clips so as to be able to vary continuously the number of turns, and hence the inductance in circuit. In any single coil, the fewer the number of the turns the less will be the inductance, and vice versa, the larger the number of turns the greater will be the inductance. In some cases the coil may be provided with plugs and sockets to vary the inductance by steps and other means provided elsewhere in the circuit to get all adjustments between the steps. Curves showing how the inductance of a coil varies with the num- bers of the turns in circuit is called a calibration curve of the indue- KADIOTELEGEAPHY. 35 tance. In figure 26 is shown such a curve for a helix, with square turns wound with copper tubing about one-fourth inch in diameter, the length of each side being 21| inches and the spacing of the turns being 1 inch between centers. In figure 27, A and B, are shown two calibration curves of a flat spiral, similar to the one used in the field radio sets, in the first of which (A) the turns are counted from the outside inward, and in the second (B) they are counted from the inside outward. Thus it is seen that in using different numbers of turns in a flat spiral care must be taken to state how the turns are counted. The explanation of the difference between the two curves .150 .100 $' 1 5 ! 050 10 15 Number of turns FIG. 26. is that, other things being equal, the greater the diameter of the turn the larger will be the inductance; and hence the inductance will be the larger for a few turns in that curve in which the turns are counted from the outside inward. There is another useful type of inductance called the variometer, which consists essentially of two coils connected in series or parallel, as desired, one of which is movable with respect to the other. In some cases one coil is arranged to slide past the other in a plane parallel to its windings, as indicated in figure 28 ; in other cases one coil is rotated inside the windings of the other, as indicated in figure 36 RADIOTELEGRAPHY. 29. In the second type, when the coils are in the same plane and the windings are connected so that the current is circulating through .120 .110 .100 .090 0.080 ^.070 |.060 *.050 * -040 1 .030 |.020 ^ .010 2 46 8 10 12 14 16 18 20 22 24 26 28 30 Number of turns FIG. 27. them in the same direction, the two magnetic fields are helping each other and the inductance is a maximum ; if, now, one coil is rotated FIG. 28. through an angle of 180 degrees the two fields are opposing and the inductance is a minimum; for intermediate angles the inductance will have some intermediate value. The variometer thus has the RADIOTELEGRAPH Y. 3 7 advantage of giving a continuous change of inductance without moving clips or contacts, but has what may be under certain condi- tions the disadvantages of not giving zero inductance at its minimum position and of always having the resistance of all its wire in circuit. A variometer is generally used in connection with a helix or coil, variable only by steps, to give intermediate values of the inductance as mentioned above, and shown in figure 76. The earlier types of closed circuit inductance were wound with wire or tubing, the resistance of which to direct current was very low. Both theory and experiment have shown, however, that the resistance to high-frequency currents may be comparatively large. The explanation is that these high-frequency currents tend to travel almost wholly on the surface of the conductor and do not penetrate to any considerable distance into the wire. Thus a thin-walled tube will have practically the same resistance to high-frequency currents as a solid wire of the same diameter, the inside of the wire carrying no current at all. This tendency of the current to flow only on the outer surface is sometimes called the "skin effect" and the distance to which the current penetrates the thickness of the skin. The higher the fre- quency the more marked is the skin effect and the thinner is the skin ; in other words, the higher the frequency the larger will be the 38 RADIOTELEGRAPHY. resistance for the same size and length of wire. In figure 30 is given the curve showing the increase in resistance for Xo. copper wire, B. & S. gauge (about 325 mils in diameter), as the frequency changes from zero or a steady current up to 1,000,000 cycles per second. Thus at 500,000 cycles it is seen that the resistance has been increased about 22 times the D. C. value. The scale of such a curve will differ with the different sizes of wire, the increase being greater than here shown for wires larger than Xo. and less for smaller sizes. In fig- ure 31 is given the curve showing the increase in resistance for the various sizes of copper wire in the B. & S. gauge at a frequency of A/o. O yy/re, B. & S. Gauge A/umber of Times of Increase in Resistance r _' r\> ru GJ o $ 9UIOUOWOUIC ^ <1 ^ ^ / ^ / / / 01 234-567 Frequency Jn Hundred thousand cyc/es FIG. 30. 10 500,000 cycles per second. Thus a wire as small as Xo. 35, B. & S., has very nearly the same resistance at this frequency as at a steady current, or, in other words, the thickness of the skin at this fre- quency is about equal to the radius of the wire. In order to be able to include all sizes of wire at all frequencies it is evident that a large number of curves or an extensive table of resistance and frequency would be necessary. If a large number of wires, the diameter of which is such that the current just penetrates to the center at any given frequency, is used in parallel in the form of a compactly stranded wire or cable it is evident that all the copper is in use and that the current-carrying KADIOTELEGBAPHY. 39 surface of such a cable is very much greater than that of a solid wire of the same outside diameter, and hence the resistance is very much lower. Each wire must, however, be separately insulated, as other- wise the current will immediately seek the outer surfaces of the outer wires on account of the skin effect, and the resistance will not be much decreased from that of a solid wire. Such a stranded wire or cable, with its individual wires separately insulated, as with enamel, is sometimes called litzendraht, from the German word. The size of the insulated wire depends upon the frequencies at which it is to be used. If the highest frequency should be 500,000 cycles per second, then from figure 31 it is evident that there would be but little gain 40 35 30 25 20 15 10 1.0 Frequency 5OOOOO 5 10 15 20 25 Sizes of wires: B. & S. Gauqe FIG. 31. 30 35 40 in using a wire smaller than No. 34 or No. 35 on B. & S. gauge. The number of wires depends upon the current to be carried and the re- sistance desired. For small currents it is generally a multiple of 7, as TX^I or 49 wires, but for heavy currents the number may be in the hundreds or even in thousands. It is evidently impossible to get a continuously variable inductance by a sliding clip or contact on all the wires of a litzendraht coil, so that when such an inductance of low resistance is desired it is gen- erally made in the form of a variometer wound with litzendraht. Many modern sets, particularly those of the quenched-spark type of the Telefunken Co., use such coils. 40 RADIOTELEGRAPHY. The use of litzendraht is not confined to transmitting coils, but is also used in receiving sets to get low -resistance circuits. SPARK GAPS. The function of the gap is to serve as a trigger in starting the oscil- lations and to limit the potential applied to the condensers by the transformer secondary. An ideal gap would be one having an infinite resistance during the charging of the condensers and a zero resistance during each wave train of the discharge. The types of spark gaps in use differ nearly as much as the other parts of the closed-circuit elements. In small-sized sets the electrodes or terminals are generally made of zinc or brass, the sparkling sur- faces being either balls of one-half inch diameter or more, or else rounded surfaces. Sharp points are not used, as at small separations the potential required to break down the gap is too small to allow any considerable power to be used, and if the gap is opened to increase the potential and power the gap resistance becomes too high. As the power delivered to the transformer is increased it is soon found that the discharge at the gap becomes flaming in character and has a hissing sound, seeming to be more like an arc than a spark, and the gap terminals become very hot. The reason for this is, that owing to the great quantity of electricity discharged across the gap the resistance becomes so low that a high-potential alternating-current arc, which is almost a short circuit, is maintained at the transformer secondary terminals. This arc is formed in the heated air and the vapor of the metals forming the gap terminals. Experiment has shown that a blast of air across or through the gap will blow out the arc but not the spark. By thus removing the short circuit the condenser can be charged to the full potential of the secondary and the power of the set increased in some cases it may be nearly doubled. The air blast may be obtained from a blower or compressor driven, for example, by an electric motor or directly by the rotating of the gap terminals themselves, in which case it is known as a rotating gap. There are two general types of rotating gaps, in the first of which the rotation is simply a convenient means of giving the neces- sary ventilation and cooling. It is not necessary that it be provided with rotating terminals, although it may be so provided. In one of the early types used in the Signal Corps, shown in figure 32, a rotating disk is used between two fixed terminals. In this case the sparks shift from place to place on the edges of the disk as it turns, the ventila- tion being by means of fans on the face of the disk, which blow the air away from the gaps. As no attempt is made to secure any special time relation between the discharges and the alternator fre- quency this type of gap is often called a nonsynchronom yap. KADIOTELEGEAPHY. 41 In the second type of rotating gap one set of electrodes is attached to the alternator shaft, preferably insulated from it, and thus rotates at the same speed as the armature; the other terminal is mounted so as to be capable of adjustment, both in the direction of rotation and in a radial direction. If the spacing of the revolving terminals is such that there are as many terminals pass the fixed terminal per second as there are alternations per second, and, further, if the ad- justments of potential, etc., are such that the discharge is at the peak of each alternation, then there will be as many sparks per second as there are alternations, and the gap is called a synchronous gap. In order to secure the correct adjustments of a synchronous gap the fixed terminal should be adjusted radially to give only a small clearance, as ^ inch or less, and then adjusted in the direction of rotation as follows: If the rotating terminals are watched by the light of the sparks themselves, they will appear either to be waver- ing back and forth or else to be nearly fixed in position. In the former case the discharge does not occur at the peak of the wave, but FIG. 32. perhaps before the peak in one alternation and after in the next, and hence the wavering appearance ; in the latter case the discharge is at the peak of the wave as shown by the apparent steadiness of position. At the same time that this correct adjustment is secured the note of the spark as heard either in the station itself or at a distant receiving station will become much clearer, the advantages of which will be mentioned later. As it is generally best not to have long leads from the spark gap to the other elements of the closed circuit, it may be necessary to have all of the closed circuit as well as the open circuit in the room with the alternator, in which case the operator and the receiving set should be in another room. In some cases it may be possible to mount the alternator and gap so that short leads can be brought out from the latter through well-insulated bushings into the next room, which should be sound proof, and thus all the circuits be contained in the same room with the operator for convenience and promptness in making changes in wave length and other adjustments, etc. 42 EADIOTELEGRAPHY. QUENCHED SPARK GAPS. Most modern sets use the quenched spark gap, a brief description of which will be given here and the theory of the quenched spark transmitter later. The gap is essentially a series gap consisting of a number of plates with small separations between the sparking sur- faces, which are inclosed in air-tight chambers formed between the plates themselves. In figure 33 is shown a section of a gap where P are the plates often made of copper, which, on account of good conductivity for heat, will carry off the heat of the spark; F are the flanges, which Fio. 33. help the cooling by exposing a large area to the air or to the air blast to be mentioned later; S are the sparking surfaces between which the sparks pass, which may be of the same copper stock as the rest of the plate or of heavy silver plate fastened in place at S; M the separators or insulating rings, also called gaskets, between the plates, often made of mica, about 0.010 inch thick (10 mils), the thickness of which determines the distances between the sparking surfaces. In some cases the separators are made of rubber or other insulating materials which are somewhat compressible, and then the Compressible GasAet bearing surfaces are often corrugated, as shown in figure 34, so that the material may be pressed down into the annular spaces. What- ever the type of separator, the gap as a whole must be put under strong mechanical pressure so that the air shall be excluded from the sparking surfaces, the reason for which seems to be that these sur- faces are roughened with free exposure to air, and an arc is formed at some point which behaves as a short circuit between the plates and lowers the efficiency of the gap. Gaps with mica separators should not be compressed as tightly as the others because the mica will be injured by the excessive pressure and the heat from the gap and will HADIOTELEGHAPHY. 43 soon crack and puncture. In order to keep the gap cool the flanges of the plates are generally blackened, as a black body will cool more quickly than a polished body, other things being equal. In the larger-sized sets it is necessary to cool the gap by means of a bloweF driven by a motor similar to the type used in blowing out the arc of an open gap. The potential between each plate of a gap assembled as above is about 1,000 volts. This may be measured by finding the potential across several gaps by means of the needle gap and the values in Table I and then dividing this potential by the number of the gaps. Under service conditions a quenched gap should be taken apart only when it is absolutely certain that trouble in the radio circuits has been located in the gap itself, as shown, for example, by one or two of the plates becoming much hotter than the others, or by an ac- tual puncture of a gasket or separator. The reason for not taking the gap apart frequently seems to be that after a certain time, depending on the amount of use, the oxygen of the air contained between the plates becomes inactive and there is no tendency of the sparks to roughen the sparking surfaces and form local arcs, but rather that these surfaces are worn smooth and kept bright by the sparking ac- tion. If, however, the gaps are continually being taken apart air will be admitted each time, and the gap may not give the results that otherwise would be attained. There are cases where quenched gaps have been used handling heavy traffic daily for six months or more without the necessity of being taken apart once during that time, and in one of the Signal Corps sets such a gap has now been in service for nearly three years without having a plate or gasket replaced or even the gap taken apart. If, however, it becomes necessary to clean the plates, they should be laid face down on fine emery cloth or paper on a flat surface and the roughness carefully smoothed off. When mica is used as a separator, the bearing surface is generally flush with the sparking surface, and particular care must be taken to keep the two plane and parallel as shown by a straightedge. Any irregularities on the bearing surface will admit air and injure the gap, no matter what pressure may be put on the plates. Almost all gaps are pro- vided with more plates than should be used under service conditions, the extra gaps being short-circuited by clips for that purpose, so that when any one gap becomes bad it can be temporarily cut out of cir- cuit without the necessity of taking the whole gap apart. CONNECTION OF CLOSED OSCILLATING OB PRIMARY CIRCUIT WITH ANTENNA CIRCUIT. In the original transmitting arrangement of Marconi the spark gap was inserted between the antenna and ground, the transformer second- ary terminals being connected, one to the antenna and the other to 44 RADIOTELEGRAPHY. the ground, as shown in figure 35. This circuit is often known as the plain Marconi antenna or aerial. As the antenna has both induc- tance and capacity it forms in this case the oscillating circuit, taking the place of the circuit CSL of figure 20. The values of the induc- tance and the capacity vary with the size, shape, etc., of the antenna ; thus for a small antenna, as on an artillery tug or in a portable field set, the capacity may be between 0.0006 and 0.0009 mf., and the induc- tance between 20,000 and 30,000 cms. or 0.02 and 0.03 millihenrys; and for a " T " or inverted " L " antenna on 180- foot masts, the capacity may be as large as 0.0015 or 0.0020 mf., and the inductance 30,000 to 60,000 cms. or 0.030 to 0.060 millihenrys. It is to be noted that this capacity is about the same as that of one jar described on O O FIG. 35. page 33. Only in the largest stations is the capacity of the antenna as large as 0.01 mf. From its position and shape the antenna circuit is often called the open or radiating circuit, as distinguished from the closed oscillating or primary circuit. It is a good radiator of the electrical energy imparted to it by the transformer, but its small capacity makes it impossible to store a large charge in it, and consequently at each discharge across the gap there is comparatively little energy available for radiation. For this and other reasons to be mentioned later this circuit is not now used in practical radiotelegraphy. COUPLING. By means of the arrangement shown in figure 36 a large charge may be stored in the condenser C, much larger than that which can be stored in the antenna of figure 35, and the discharge of this con- denser through the gap S and the inductance L will produce powerful oscillations in the closed oscillating or primary circuit. On account EADIOTELEGRAPHY. 45 of its position and shape, however, this closed oscillating circuit is a poor radiator of electrical energy. There are two general ways in which the energy of this circuit can be transferred to the antenna circuit; or, as it is said, two ways of coupling the circuits. One is shown in figure 37, where the ground and the antenna circuits are directly connected to the inductance coil of the closed circuit, and the circuits^ are said to be directly connected, directly coupled, or c&nductively coupled. From its position in the circuit the coil is often called the antenna coil or helix. The other is shown in figure 36, where a number of turns in the antenna coil L 2 , connected be- tween the antenna and ground, is brought near enough to a number of turns of the coil L t in the closed oscillating circuit to have oscil- lations induced in the antenna coil and circuit, and the circuits are said to be inductively coupled or connected. The two coils L! and L 2 FIG. 36. form an oscillation transformer, as it is usually called, the coil L t being the primary and coil L 2 the secondary. Hence the antenna circuit is sometimes called the secondary circuit as well as the open or radiating circuit, as previously mentioned. There is no essential difference in the operation or efficiency of the transfer of enegry in the two types of coupling, but rather that each may have advan- tages in certain cases. Thus the directly connected set is somewhat more compact and the inductively coupled set somewhat more easily adjusted under certain conditions. In direct connected sets when nearly the same turns are connected in both the primary and the secondary circuits that is, when most of the turns in use are common to both circuits, as shown in figure 38 the coupling is said to be close or tight. When only a compara- tively few turns are common to the two circuits, as shown in figure 46 KADIOTELEGRAPHY. 39, the coupling is said to be loose. Similarly in inductively con- nected sets, when most of the turns in use in the two circuits are near together, as when one coil is moved inside the other, as shown in figure 40, the coupling is close. When the turns in use are not near together, as shown in figure 36, the coupling is loose. In the case of inductively coupled sets it is evident that moving the coils of the oscillation transformer nearer together will tighten the coupling or make it closer, and, vice versa, moving the coils farther apart will loosen the coupling. If the turns in use in either circuit of a directly I ? PIG. 37. connected set are moved so as to have few or even no turns at all in common, as shown in figure 39, the coupling is loosened. The cou- pling may be made loose in other w r ays, one of which is illustrated in figure 41, where the coil L 1 .,, often known as a loading coil, is in- serted in the antenna circuit, thereby adding inductance not coupled with the primary circuit. Similarly in the case of inductively con- nected sets the coupling may be loosened by inserting the loading coil IA in the antenna circuit, as shown in figure 42. In both these cases it is to be noted that the result is practically the same as though the turns in use in the two circuits w^ere moved farther apart as a EADIOTELEGRAPHY. 47 whole. In both the directly connected and the inductively connected sets the coupling may also be loosened by inserting a loading coil in the primary circuit, as shown in one case in figure 43. By meansuiiL these loading coils a directly connected set can thus be made as loosely coupled for practical work as an inductively connected set. In such a circuit as that in figure 41, the coil which is common to both circuits and serves to transfer the energy from one to the other is sometimes called the coupling coil. At the present time most of the sets in use in the Signal Corps are loosely coupled and all of FIG. 38. the various methods of obtaining loose coupling here described are in use. each one having advantages in its particular radio set. ANTENNA. The open or radiating circuit has its own natural period of oscilla- tion expressed, as in the case of the closed circuit mentioned on page 19, in fractions of a second. The most energy can be delivered to it from the closed oscillating circuit when by adjusting the in- ductance or capacity, or both, of the latter the oscillations in it have 48 RADIOTELEGRAPHY. the same frequency as in the open circuit ; that is, until the two cir- cuits are in resonance. Then the strongest oscillations or the greatest current will be flowing in the antenna as shown by the maximum reading in a hot-wire ammeter of figures 38 to 43, inclusive. This ammeter is usually connected between the ground and the secondary of the oscillation transformer, but may be connected between the sec- ondary and the antenna. These powerful damped high-frequency oscillations in the antenna or open circuit produce corresponding periodic disturbances in the I I FIG. 39. surrounding medium, which spread outward in the form of electro- magnetic waves, as has already been explained. In general the higher the antenna, the greater the energy in the form of electromagnetic waves which it can radiate and receive ; in other w r ords, the greater the distance to which it can send and re- ceive signals. In most cases a large capacity is also desired, which can be secured by putting up a number of wires, but there is little EADIOTELEGRAPHY. 49 gain in capacity unless the wires are at least a foot apart. Ad- ditional capacity and increased efficiency in radiation can be se- cured by using a flat top or horizontal spread of wires at the top of the mast, which becomes, as it were, one plate of a condenser, the earth being the other plate, with the air as the insulator or dielectric. Antennae are often divided into three types, depending on the way in which the wires are arranged at the top, such as umbrella, inverted L, and T, where the names are sufficiently suggestive so as not to FIG. 40. require a description. The umbrella is best adapted for shore sta- tions having a single mast or tower with several acres of land around the station, and has been largely used by the Signal Corps. The inverted L and the T can be installed on shipboard or at shore stations, but require two masts or towers. In the case of the umbrella antenna, the wires extending outward from the mast should be kept as nearly horizontal as possible and as far away from tree GG536 17 4 50 RAD10TELEGRAPHY. tops, buildings, roofs, etc., as circumstances will permit. The distant ends are dead-ended at high-potential insulators attached to long guys carried out to stub masts or deadmen. These guys should have insulators inserted every 50 or 100 feet so as to prevent them from serving as extensions to the antenna wires and thereby bringing the antenna too near the ground. It is not necessary that the antenna wires be symmetrically arranged around the tower, it being far more important that advantage be taken of the configuration of the ground and that the outer ends be kept well elevated than that a symmetrical arrangement be made. This is shown in the plan of the Signal Corps FIG. 41. % radio installation at Fairbanks, Alaska, figure 44, where, on account of swampy land along the river near the station, a symmetrical ar- rangement is practically impossible. The antenna must be well insulated, particularly at the outer ends of the horizontal wires, as otherwise there will be leakage to ground in damp weather or rainy seasons, which will cause a serious loss in efficiency when the station is transmitting. High-tension insulators of electrose or porcelain are usually furnished for use at these points of the circuit. RADIOTELEGRAPHY. 51 The antenna wires are generally stranded, thus giving somewhat greater strength than a solid wire of the same weight. For perma- nent stations a phosphor-bronze or silicon-bronze wire is generally used consisting of seven strands of either No. 20 or No. 14 B. & S. gauge, and for the portable stations, such as the Signal Corps field- pack sets, an antenna cord made up of 42 phosphor-bronze wires stranded around a hempcord center. A very low resistance in the antenna wires is not as necessary as it might seem to be, as it has been shown by theory and /proven by experiment that the radiation PIG. 42. of electromagnetic waves introduces a resistance, sometimes called the radiation resistance, which in general is many times the high- frequency resistance of the wires themselves. This radiation resist- ance rarely falls below 2 ohms on a ship set and may be as high as 20 or 30 ohms in a shore station. When the antenna resistance is measured under service conditions it includes that of the wires at the given frequency, the resistance of the ground, and that due to the radiation of energy, the latter being generally the larger part. 52 RADIOTELEGRAPHY. A typical antenna resistance is shown in figure 45, where it is to be noted that the resistance is largest near the fundamental wave length of the antenna and is smallest at a wave length about one and one- half or two times the fundamental. It is at or near this point that many stations work most efficiently. ARTIFICIAL ANTENNA. In many cases it is convenient to make station tests without using the actual antenna, particularly where such use would cause unnec- essary interference. A local circuit of a coil L and condenser C having the same inductance and capacity as the antenna and called an artificial antenna is often used, thus serving the same purpose FIG. 43. as an artificial line or cable in telegraph tests. When a resistance R is inserted in this circuit to give the same current as actually flows in the antenna this resistance is approximately equal to the antenna resistance as mentioned on page 51. The circuit for making these measures is shown in figure 48, where the circuit of L, C, and R, which replaces the antenna when the switch is thrown to the right, is the artificial antenna. The antenna inductance L and capacity C can be easily measured with the help of a wave meter and thus a suitable coil and condenser selected for use in the artificial antenna which will then closely represent the actual antenna. First using the wave meter as de- scribed on pages 61, 62, and 63, measure the fundamental wave length of the antenna itself X t using the plain Marconi antenna circuit as shown in Fig. 35. Next insert a loading coil of known inductance EADIOTELEGKAPHY. 53 1, expressed, for example, in millihenrys, and measure the funda- mental of the loaded antenna X 2 . Then antenna inductance X 2 1 X 2 X 2 L= 2 * 2 millihenry and antenna capacity- = 0.000281 1nn 2 rmn * r A2 AI lOuOOuOxl microfarad. Thus let X t = 430 meters, X 2 = 980 meters, and 1 = 0. 145 millihenry. Hence T 185000x0.145 . ir , = 776000 millihenry = 0.0346 millihenry C = 0.000281 1^~ mf = 0.0015 mf. 54 RADIOTELEGRAPHY. EFFICIENCY OF RADIO SET. The antenna resistance, the radiation resistance, and the antenna current all change as the frequency or wave length changes. If at any one frequency or wave length the square of the antenna cur- rent in amperes is multiplied by the antenna resistance in ohms, the product, PR, is in watts, and represents the power delivered by the closed oscillating circuit to the antenna; that is, it is the antenna input, as it is sometimes called, or the watts in the antenna. If the number of watts delivered by the alternator is known, the efficiency from alternator to antenna can be found by finding the quotient of the watts in antenna divided by the watts from the ~, . watts in antenna alternator, thus efficiency - ^ tts from a i tern ator. length: Meters FIG. 45. In the early types of spark sets this value was as low as 10 or 20 per cent, whereas in modern quenched spark sets, it may be as high as 50 per cent or even higher. If a motor-generator set is used and the number of watts delivered to the motor is known, the over-all effi- ciency can similarly be found by dividing the antenna watts by the motor watts, thus over-all antenna watts. The percentage so motor watts. obtained will of course be lower than before, as it allows for losses in the motor-generator which were not considered in the previous case. The rating of the earlier radio sets was given as the output of the alternator, but in modern sets it is often given as the number of watts delivered to the antenna. In the latter case the artificial antenna RADIOTELEGRAPHY. 55 may be used and its inductance, capacity, resistance, together with the current and watts at a given wave length must then be specified. When steel towers are used they are generally heavily insulated at the base, but provided with switches for grounding when desired, as during lightning storms, etc. In some cases the station becomes more efficient in transmitting if the tower is grounded. In general, how- ever, the result of grounding can be told only by tests at the receiving station of the loudness of the signals, and not by the readings of the antenna hot-wire ammeter or other means at the transmitting sta- tion. The grounding of the tower generally makes it necessary to change the tuning of the transmitter, and there are corresponding changes in the reading of the antenna ammeter, but increases in its reading do not necessarily mean in- creases in the signals at the receiv- ing station, as part of this increase is due to increased flow of current through the tower to ground. It is for this reason that the results of grounding should always be tested at the receiver. GROUND. An efficient ground for a radio station is very different from that used at an ordinary telegraph sta- tion. The latter generally has a metal plate set deep in wet ground, but the former needs a large spread on the surface or just under it. Thus, instead of using a large copper plate or rods close together, a far better type of ground would be to use wires radiating out from the station, or to duplicate the umbrella or flat-top antenna system a short distance under the surface of the ground. The advantages of a surface ground may be understood when it is remembered that close to the station the magnetic and static fields are very intense, so that if they had to pass down through the earth to a .ground plate instead of being able to travel wholly on the surface, as shown in figure 9, there would be introduced an additional ground resistance and local earth currents would be caused, with corresponding losses. The use of a surface ground serves to reduce these losses to a minimum. It should be noted that the instantaneous values of the transmitting currents FIG. 46. 56 RADIOTELEGKAPHY. are very large and the frequencies very high, sometimes a million or more per second, so that considerable copper, such as stranded wires or copper strip, should be used both in the ground wires and in the leads connecting the set to them. Another type of ground connection which has been successfully used at permanent stations, and also in the portable field sets, is known as the counterpoise. In the permanent stations this consists of a set of bare horizontal radial or parallel wires, which are sup- ported by insulators on posts 7 feet or more above ground. A counterpoise of a fan type has been installed at Fort Sam Houston, Tex., in which bare wires, No. 10, B. & S. gauge, 190 feet long, ex- tend outward from the station under the antenna, being spaced 6 feet apart at the station and 20 feet at the distance ends. A counter- poise of the radial type has been installed at the Fairbanks (Alaska) station, as shown in figure 44, where the wires are bare hard-drawn copper No. 12, B. & S., about 210 feet long, and spread out in two arcs, each of 90 degrees. A counterpoise is particularly efficient in case the soil is very dry, as at Fort Sam Houston, and also where there is a heavy snowfall, as at Fairbanks. At the latter station both a ground and a counterpoise have been installed. In the case of the Signal Corps wagon -sets, radial counterpoise wires mounted on tem- porary poles, carried as a part of the set, were used at first, but now have been replaced by the same type as that of the pack sets, which consist of rubber-covered wires, each 100 feet long, laid out radially on the ground. Although not directly connected with the ground at all, these wires really constitute one plate of a condenser, the ground being the other. WAVE LENGTHS. Before describing the various receiving circuits and the theory of their operation, some of the terms applied to them and to the trans- mitting circuits will be defined. In the mechanical illustrations of damped oscillations and res- onance, by means of the steel spring and the tuning forks it was convenient to use both the frequency expressed in the number of oscillations per second and the period expressed in fractions of a second. The same terms were used in describing the electrical oscilla- tions in the radio circuits, and although this usage is entirely correct, it is somewhat more common to use the term wave length, which will be defined in the following paragraphs. At the end of one second of time after an electromagnetic wave has begun to radiate from an antenna, it will have reached a point 300,000,000 meters distant; that is, it is. said that its velocity is 300,000,000 meters per second, or, as it is often abbreviated, V= 300,000,000 meters. During this interval of time the direction RADIOTELEGRAPH Y. 57 of the magnetic and the static lines of the waves has been reversed very many times; in fact, as many times as the oscillations in the antenna have been reversed. Similarly in this interval of space both fields will be in the same direction at very many points, all separated by equal distances, as represented in figure 9. The distance between any two such points is called a wave length and is generally given in meters, the symbol for which is X. It is evident that the greater the number of times per second that the two fields have been reversed the shorter will be the distance in meters between the points where the fields are in the same direction ; that is, the shorter the wave length; and, vice versa, the fewer the number of times per second that the fields have been reversed the longer will be the distance between the points where the fields are in the same direction; that is, the longer will be the wave length. If N is the number of points in the distance 300,000,000 meters that the fields have the same direction, and if X is the wave length in meters, then we have the relation NXX=V. This is one of the fundamental relations in radiotelegraphy and is shown graphically in figure 47, where to secure simplicity only the static field is indi- cated, in which it is seen that the direction of the field is repeated N times in the distance V= 300.000 ,000 meters, which is traveled in one second of time. A short table of wave lengths and frequencies, computed from the equation NX^=V, is given below: Wave Frequency in length in oscillations- meters. per second. 100 __________________________________________ 3, (XXX 000 200 __________________________________________ 1, 500, 000 300 __________________________________________ 1, 000, 000 400 __________________________________________ 750, 000 500 __________________________________________ 600, 000 600 _________________________________ , ___________ 500 V 000 1, 000 ____________________ _ _____________________ 3001, 000 2, 000 __________________________________________ 150, 000 3, 000 __________________________________________ 100, 000 4, 000 __________________________________________ 75, 000 5, 000 ___________________________________________ 60, 000 6, 000 _____________________________________________ 50, 000 10, 000 ________________________ __________________ 30, 000 From this table and from the relation T= N gi ven on P a g e 2 7, it is seen that the shorter the wave length the higher is the fre- quency in number of oscillations per second and the shorter the pe- riod of each oscillation in fractions of a second ; and, vice versa, the longer the wave length the lower is the frequency in oscillations per 58 RADIOTELEGRAPHY. second and the longer the period of each oscillation in fractions of a second. CO (VI I ~f~r i i T ! I Ci 5 o k * limiting current, as it is often called. Thus, in the case of a spark frequency of 1,000 per second there will be 1,000 pulsations of current as in the lower curve of figure 67, and the telephone will be operated as though by a direct current interrupted 1,000 times per second. One of the earliest of the rectifying detectors that was used with a telephone was the electrolytic, but like the coherer it is not now used in practical work. Other kinds of detectors, sometimes called crystal or contact de- tectors, consist of various substances in light contact, such as steel- carborundum, steel-silicon, etc.; metallic contact on pyrite, galena, etc. ; zincite-chalcopyrite, silicon-arsenic, silicon-antimony, etc. These have all been patented, and some of them have received trade names. KADIOTELEGRAPHY. 81 such as " perikon " for zincite-chalcopyrite, " pyron " for metallic contact on pyrite, etc. In the case of the perikon, silicon-arsenic, silicon- antimony, etc., the materials are embedded in flat buttons of fusible alloy or solder on an. adjustable holder and held in light con- tact by a spring; in the steel-silicon, pyrite, galena, etc., contact is made by a light wire spring on a universal jointed holder. Most of these detectors are sensitive to the high-frequency oscil- lations without the application of an external electromotive force, as 66536 17 6 82 RADIOTELEGRAPHY. the steel-silicon, galena, etc., and the simplest circuit in this case is shown in figure 69, where D is the detector, T the telephones, and S a fixed condenser of about 0.003-microfarad capacity. Other de- tectors are more sensitive when a small electromotive force, as from a potentiometer, is applied to them as the per ikon, pyron, etc., and in this case the circuit is shown in figure 70, where D is the de- tector, T the telephones, S the condenser, generally fixed, but some- times variable by steps. Another type of detector called the " audion," shown in figure 71, consists essentially of a partially exhausted bulb in which have been sealed a metallic filament, F, two small grids, .G, one on each side of the filament, and two small plates, P, outside of each grid, the plates and grids being insulated from each other and the filament. "A "Battery FIG. 71. The filament is heated to incandescence by a storage battery, A, often called the "A battery," of about 6 volts, the current from which is regulated by means of a small rheostat, R. The plates, P, entirely insulated within the bulb, are connected to one terminal of the tele- phones, T, the other one of which is connected to a battery of small dry cells, B, often called the " B battery," of 30 to 50 volts, the number of which in circuit, and hence "the voltage, is controlled by a switch. The positive terminal of the B battery should always be connected to the plates, P, through the telephones. The terminals of the detector circuit are connected, one to the base of the filament and the other to the insulated wire grids, G, through a small stopping condenser, S. The action of the audion seems to be that of a relay. RADIOTELEGRAPH Y. 83 and its operation is as follows: Under the influence of the hot fila- ment the molecules of gas remaining in the bulb acquire the property of conducting a small current on the application of 30 to 50 volts in the direction of filament to plates, but not in the reverse direction, and if the telephone is connected in this circuit as shown, a small, steady current will flow through it. On the arrival of the high- frequency oscillations at the grids and the filament it is probable that they can flow only in one direction, and during their passage over part of the path of the telephone current they change its resistance, and hence the current in the telephones, and thus make audible sig- nals. For reasons previously given, the pitch of the note in the tele- phones is the same as that of the spark frequency at the transmit- ting station. A sensitive detector of a somewhat novel type is now coming into use, called the ticker, consisting essentially of fine steel or other wire resting with light contact in a groove on a rotating disk of brass or other suitable material. This detector can be used instead of D in the circuit shown in figure 69 in which the condenser S should now be about 0.01 mf. and the telephones of low resistance. TELEPHONES, The telephone receivers used in detector circuits are wound to a hiyh resistance, as 1,000 ohms or more for each one of a pair. The reason for this is as follows: The movements of the telephone dia- phragm are caused by the attraction of the telephone magnet, which increases as the product of the current in the telephone and the number of turns in the windings. As the current from the detector is very small, it is evident that a large number of turns must be used to secure the necessary attraction, and hence the telephone becomes one of high resistance. Every telephone diaphragm has a certain natural period of me- chanical vibration or pitch. When the incoming signals are of the same pitch that is, they are HI resonance with the period of the diaphragm these signals will be heard louder than others from transmitters of the same power but of different pitch. In some cases the natural pitch of a diaphragm may coincide with that of the sig- nals, and thus the telephone will be found to be very sensitive. The pitch of the diaphragm can, however, be changed by changing the distance between it and the magnet, and some types of telephones are supplied with adjustable pole pieces. By this means it is possible to tune the telephone to mechanical resonance with the spark fre- quency of the transmitter and often increase the loudness of the signals. 84 RADIOTELEGRAPHY. The fixed condenser is shunted across the telephone terminals in order to provide a complete circuit for the oscillations between the secondary condenser terminals without having to flow through the telephones, the high inductance of which in circuit would tend to choke back the oscillations and so possibly prevent their detection. It is evident that a very large condenser can not be used, as it would serve as such a low-irnpedance shunt for the pulsating currents from the detector that no current would flow through the telephone, and on the other hand a very small condenser can not be used, as it would not allow the oscillations to flow through it. The best value must then be determined by trial and it is found in practice to vary slightly with the spark or wave train frequency. With the high- resistance telephones in general use the capacity of the condenser is about 0.003 to 0.0035 mf. for low-frequency transmitters, as 60 cycles, and about 0.002 to 0.003 mf. for high frequencies, as 500 cycles. In some cases this condenser is variable by steps so as to be able to adjust B c tilt FIG. 72. to different spark frequencies or to group tuning, as it is sometimes called. By the use of such a variable condenser and of a telephone with adjustable pole pieces it is often possible to increase the loudness of signals and the selectivity of the circuits without making changes in the tuning. In some types of circuits the fixed condenser serves another pur- pose, as shown in figure 68, where it prevents the short circuiting of the battery by the coil, in which case it is often called the stopping or blocking condenser. CALIBRATING WAVE LENGTHS OF RECEIVING CIRCUITS BY MEANS OF THE WAVE METER. In the previous illustrations of the wave meter it was used to receive oscillations from a transmitter and to measure its wave lengths. It may, however, be used to send out oscillations of known wave lengths of comparatively feeble intensity like a miniature trans- mitter. Several types of circuits may be used to excite the meter, as RADIOTELEGRAPH Y. 85 a buzzer shown in figure 72, where A is a battery of not more than two dry cells, B is the buzzer, and L C is the meter. This circuit is sometimes known as the ~buzzer vwthod of excitation of the wave meter which thereby becomes a source of slightly damped oscilla- tions. The action of the buzzer circuit seems to be that at each spark at the buzzer contacts, the meter condenser is charged and then dis- charged through the inductance and thus sets up oscillations, inde- pendently of the charging circuit in a manner similar to that of a closed circuit as charged by the secondary of the A. C. transformer. If a circuit is brought near the coil L and loosely coupled with it the meter will induce in the circuit oscillations of the wave length or frequency corresponding to the setting of the wave meter con- denser. The circuits of a station receiver connected to the station antenna may be calibrated by this method. This circuit may be used in making many measurements and tests in radio work, such as inductance, capacity, sensitiveness of tele- phones, detector, etc. RADIO APPARATUS IN USE IN THE SIGNAL CORPS. The Signal Corps has installed 10 radio stations in Alaska, varying in size from 1 kilowatt at Petersburg, Wrangell, and Kotlik to 8 and 10 kilowatts at Fort Gibbon, Fort Egbert, Nulato, and Nome. Stations of from 3 to 5 kilowatts have been installed at St. Michael, Circle, and Fairbanks. In the Philippines stations have been installed at Manila, Fort William Mcinley, and in the coast defenses of Manila, including a set of 8 kilowatts at Corregidor. In the United States 1 or 2 kilowatt sets have been installed in several of the Coast Artillery districts ; 1-kilowatt set at Fort Wood ; 3-kilowatt set at Fort Riley ; an 8-kilowatt set at Fort Sam Houston ; sets of from 1 to 5 kilowatts on 14 transports and cable ships; and sets of from one-eighth to 2 kilowatts on the harbor boats assigned to Coast Artillery districts that have a shore station. All the Alaska and the Philippine stations except Corregidor have their generators driven by gasoline engines. The generators in the Artillery districts and on the harbor boats are nearly all driven by motors from local electric power. The Fort Wood station may be operated either from a gasoline engine or the local electric-light plant. The Fort Riley and Fort Leavenworth sets are operated directly from city power. Two types of portable field sets have been issued by the Signal Corps. The smaller size, known as a field radio pack set, is fur- nished to the Organized Militia as well as to the field companies, and is described on pages 104 to 127. The range of these sets under nor- 86 KADIOTELEGRAPHY. mal conditions is about 25 miles over land, but much greater over water. Thus one of the one-eighth kilowatt sets, with a 100-foot mast, at Habana has worked with the naval station at Key West, a distance of about 110 miles. The larger size of field sets, known as a wagon set, is described on pages 93 to 104. It is of 2-kilowatts output and is carried on a two-chest pintle wagon, one chest with the engine and generator and the other with the transmitting and the receiving apparatus. The range of these sets varies from 75 to 800 miles, depending on favorable weather conditions, time of day or night, character of the land between the sets, etc. FORT SAM HOUSTON STATION SET. The following description of the Fort Sam Houston station is given as an illustration of the type of .the 8 and 10 kilowatt sets in- stalled by the Signal Corps in Alaska and in the United States. Towers. These are of structural steel, about 200 feet high, 28 feet square at base, and 4 feet square at top. The towers are supported on concrete piers, each leg resting on a cribwork of timbers 12 inches square, painted with insulating compound for preservation and in- sulation. Timbers are bolted to the piers and to each other, the bolts from the towers not extending down into the concrete. The towers are about 300 feet apart. Antenna. The antenna is of the T type, the flat top part of which is composed of 7 wires, each 280 feet long and 4 feet apart. Both ends of these wires are insulated with 18-inch electrose in- sulators. The vertical w y ires, reaching from the center of the flat top to the station, are each 180 feet long, separated 4 feet, and at the bottom are joined together and carried as a single wire for about 10 feet into the station through a porcelain wall insulator. Counterpoise and ground. Connections are made to the water- pipe system as a ground, but the most dependence is placed on a counterpoise, described on page 56, which covers about half an acre of land. Poicer equipment. The alternator is belted to a single-phase, 60- cycle, 20-horsepower induction motor driven by electric power fur- nished from San Antonio. The motor can be automatically started by closing a switch on the operator's table. In places where such power is not available, as in Alaska, a Fairbanks & Morse 20-horse- power gasoline engine is generally used. The motor speed is 1,750 R. P. M., the diameter of its driving pulley is 12 in., the diameter of the driven pulley on the generator is 14| in., thus giving the normal generator speed of 1,500 R. P. M. This machine is of the inductor type, separately excited by a 1.5 kilowatt D. C. exciter on KADIOTELEGRAPHY. 87 the same shaft as the A. C. armature, and delivers the power of 8 kilowatts, at a frequency of 500 cycles, 150 volts, 65 amperes, with a power factor of about 82 per cent. Switchboard. The switchboard is mounted close to the operating table and contains the 500-cycle frequency meter, A. C. ammeter and voltmeter, the exciter D. C. ammeter and voltmeter, and generator field rheostat for the adjustment of the alternator voltage. The 500- cycle wattmeter and the antenna hot-wire ammeter are mounted else- where. Transformer. The transformer in use is of the Closed magnetic circuit type and oil immersed, as mentioned on page 23. The spare transformer is of the open magnetic circuit type with dry insulation, with a reactance in its primary circuit for the proper adjustment of these circuits, as mentioned on page 25. Key. The key is of the relay type, controlled by an ordinary Morse key, which uses the direct current from the exciter to operate the relay. The Morse-key contacts are shunted by a condenser to cut down the sparking. Condenser. The closed-circuit condenser consists of 26 Leyden jars, covered with copper foil, each of a capacity of 0.002 mf. im- mersed in oil to reduce the brush discharge, as mentioned on page 33. Inductance. The closed-circuit inductance is in the form of a helix wound with flat strip and adjustable only by steps for certain pre- determined wave lengths, contact being made on the step correspond- ing to the desired wave length and the secondary or open circuit tuned to resonance with the closed circuit. Spark gap. The gap is of the quenched type with plates of copper but with a heavy plate of silver for the sparking surface, as men- tioned on page 42. The separators are of mica. The gap is cooled by a blower driven by an electric motor taking power from the direct- current exciter. Open or radiating circuit. As this set is of the directly connected type, the closed-circuit inductance is included in the open circuit. The coupling is made loose by the use of antenna loading inductance, variable by steps for approximate resonance, and an antenna variom- eter for fine adjustment between these steps, as described on page 35. Receiving set. This is of a statically coupled type similar to that in the field radio pack chest but of larger size for use with longer wave lengths and provided with both tuned and untuned secondary circuits. Both galena and audion detectors are used, the latter partic- ularly for faint signals and distant stations. COAST ARTILLERY STATION SET. The following directions and instructions should be used in the installation and operation of the 1-kilowatt Marconi 500-cycle sets supplied by the Signal Corps for use in the Coast Artillery stations. 88 EADIOTELEGEAPHY. RADIOTELEGRAPHY. 89 Installation. Install the motor-generator in a level position, securely mounted on a solid foundation, preferably of concrete, Jill the bearings with oil, and take care that the oil rings are working properly. Connect the apparatus as shown in figures 73 and 74, locating the quenched gap, oscillation transformer, antenna induc- tance, and switchboard so as to be easily reached by the operator at BACH V/EW FIG. 74. the key. Locate the antenna ammeter where it can be easily seen from the operator's seat. Ground the middle points of the carbon- rod protective devices on some ground other than the one used for the antenna circuit. In the case of A. C. motor-driven sets, one- half microfarad condensers should be used as protective devices in addition to the carbon rods. 90 KADIOTELEGRAPHY. Operation. The generator may be driven either by a D. C. or an A. C. motor. In the case of the A. C. motor set, the machine starts as a repulsion motor, with the armature short-circuited through car- bon brushes on the commutator, and when nearly up to full speed the brushes are automatically lifted from the commutator, which is short-circuited at the same time. This change of connections con- verts the motor into an induction motor. In motors of this small size, start the machine by closing the main A. C. switch. No means is provided for the regulation of speed. In the case of the D. C'. motor set, start the machine by closing the switch of the automatic starter and adjust the speed by means of the motor-field rheostat until the frequency meter reads 500 cycles. Connect into circuit 8 gaps of the quenched gap. Close the switch to the generator Melds and adjust the generator voltage by means of the generator field rheo- stat until the A. C. voltmeter reads about *JOO volts. Make certain that the spark-gap blower is running, which should have been started when the generator field switch was closed. Set the switch of the primary of the oscillation transformer on the desired wave length. CAUTION: Never move the primary switch which controls the wave length when the key is closed. Pull out the handle of the secondary of the oscillation transformer 3 inches or more. Then close the gen- erator armature switch and press the key. Rotate the handle of the secondary of the oscillation transformer until the antenna ammeter shows a maximum reading. NOTE: It is intended that the ht Fixed coupling C" A condensers ^8^ Detector r Telephone cond. f \ ^ o nir 1^1 {V o j \ j j ^*\ j ^ \^/ \^/ ground- Telephone \ / LONG WAVE LENGTHS \ /, 500 TO 2400 METERS /Antenna e C3f / o g '""^^ 1 U, , , _ * o O | jfp^ coupling Detector^ ( ^ o condensers | Vo")^ b < r fi "" .5 H 'I d: O = Ti F&/T96//7rf TYPE C RECEIVING SET DIAGRAMMATIC CIRCUIT 126 RADIOTELEGRAPH Y. the number of adjustments for tuning from 4 to 3, and at the same time the set is much more rugged, as there are no moving parts. The values of the coupling condenser have also been so chosen as to make the set much more selective than the others; that is, it can receive signals from a station on one wave length and cut out signals from another station on a different wave length more completely than be- fore. In addition to the above advantages, the set as a whole has been found to be more efficient than the previous types. The type C receiving set consists of two statically coupled circuits, high-resistance telephones, stopping condenser, fine wire-galena de- tector, switch for short and long wave lengths, three dial switches for tuning, etc. The circuits are shown diagrammatically in figure 84. The primary circuit consists of: (1) The antenna, which when the control switch in the cover of the chest is thrown to the " Receive " position, is connected by a double plug with flexible wires to the bind- ing post on the set marked "A"; (2) two primary coils in series, one large and the other small, the number of turns in both of which is variable by means of the two dial switches marked " Primary ". On each coil there are contacts, to 24, for tuning to different wave lengths, the dial nearest to the binding post "A" being connected to the large primary for large changes in wave length and the other to the small one for small changes and fine tuning; (3) counterpoise which is connected to the binding post marked " C " through the double plug and control switch. There is no series condenser in the antenna circuit for the reception of wave lengths shorter than the fundamental wave length of the antenna, as in types A and B, as it has been found not to be generally useful. When comparatively short wave lengths are to be received, as from 300 to TOO meters, the double-pole double-throw switch on top of the set should be thrown to the position marked " Short." This makes no changes in the primary circuit, but connects into circuit (1) the secondary coil with the dial switch marked " Secondary," with contacts to 24 for tuning to different wave lengths; (2) detector and telephones. Short wave signals should be picked up by adjustments of the large primary and the secondary dials and fine adjustments made later on the small primary dial. When longer wave lengths are to be received, as from 500 to 2,400 meters, the D-P D-T switch should be thrown to the " Long " posi- tion. This makes no changes in the primary circuit, but disconnects the secondary coil, which in this set is most useful only at short wave lengths, and connects the circuits as shown in the second print. As the secondary coil is not in circuit, only the two primary dials are effective in tuning. RADIO-TELEGRAPHY. ' 127 Long wave signals should be picked up only by adjustment of the large primary dial and fine adjustments made later only on the small primary dial. RECEIVING SET, TYPE D. This set is practically the duplicate of the type C, except that the number of studs in the three dials has been increased so as to give finer tuning. TRACTOR SETS. The Signal Corps has designed and built two sizes of automobile radio sets, or tractor sets, as they are called (a) a "divisional" tractor of 1 k. w. size; (ft) an "Army" tractor of 2 k. w. size. The 1 k. w. set, complete with supplies and detachment of seven men, weighs about 6,700 pounds, and on an average road is capable of making a speed of from 20 to 25 miles per hour. It carries a 60-foot sectional mast, which can be raised in a few minutes by means of guides on the roof of the tractor. The antenna is of the umbrella type, with 16 radiating wires each 75 feet long. The counterpoise is likewise of the umbrella type, laid on the ground with 8 w T ires, each 75 feet long. The transmitting set is of the quenched-spark type, with inductively coupled circuits adjusted to radiate waves of 600, 800, 1,000, and 1,200 meters. The receiving set is of the statically coupled type similar to that in use in the 1915 radio pack sets, but of larger size and capable of reception of much longer wave lengths. The 2 k. w. set, complete with supplies and detachment of eight men, weighs about 9,000 pounds, and on an average road is capable of making a speed of at least 15 miles per hour. It carries an 80- foot sectional mast, which is raised in a manner similar to that in the 1 k. w. set. The transmitting and receiving sets are likewise similar to those in the previous set, but capable of using much longer wave lengths. APPENDIX. DAMPING LOGARITHMIC DECREMENT. The oscillations in a wave train in a single circuit of coil and con- denser die down to zero, as shown in figure 13. Other things being equal, the higher the resistance the more rapid is the decrease in amplitude of each successive oscillation ; that is, the higher the damp- ing; and, vice versa, the lower the resistance the less rapid is this decrease and the smaller the damping. In every circuit in which the resistance is constant any amplitude in the train is a constant frac- tional part of the preceding amplitude. It is possible to compare the relative amplitudes of the oscillations in this way and thus to indicate the rate at which they decrease. For purely theoretical reasons, however, the measure of the damping has been taken as the natural logarithm, sometimes called naperian or hyperbolic logarithm, of the ratio of two successive amplitudes in the same direction. The symbol for this expression which is constant for a wave train is generally written 5. Thus Iog e j 1 = 5, where Ij A 2 is the amplitude of any oscillation as at B, in figure 13, I 2 the amplitude of the next oscillation in the same direction as at F ; and 5 is the logarithmic decrement, or simply decrement, the significance of which term will be given later. Although the amplitudes are both positive, the same formula applies when both amplitudes are nega- tive. In both cases the amplitudes are one complete oscillation apart and hence the decrement when so measured is called the decrement per complete oscillation. In a few cases the logarithm of the ratio of two successive amplitudes in. opposite directions is used, in which case the decrement is per half oscillation, and numerically it is one- half the decrement per complete oscillation. The decrement per complete oscillation is always used in practical work in this country. Natural logarithms are indicated by writing the letter e as a sub- script ; thus, log 2 where e is the base of the natural system of log- arithms, s being the number 2.71828. (In some cases in books on pure mathematics the subscript may be omitted.) No subscript is used with the common or ordinary logarithms, the base of which is 10. 128 EADIOTELEGKAPHY. 129 Tables of natural logarithms are sometimes used, although not convenient for most computations. The natural logarithm can, how- ever, be found by multiplying the common logarithms by 2.3026; thus, log 3.000 = 0.4771, Iog e 3.000 = 0.4771 X 2.3026 = 1.099, as would be found directly in a table of natural logarithms. The expression 5 = log e =p can be written 5 = log I t log I 2 , the A 2 logarithm of the fraction being the logarithm of the numerator minus the logarithm of the denominator. The expression can also be written log e I A d = log e I 2 , in which form it is seen that as d is con- stant for any one wave train, the natural logarithm of the amplitude of any oscillation can be obtained by subtracting the constant quan- tity 5 from the natural logarithm of the next preceding amplitude in the same direction. The term logarithmic decrement, or simply decrement, as mentioned above, thus receives its name from the fact that it is the constant quantity by which the logarithm of any ampli- tude must be decreased so as to give the logarithm of the next ampli- tude in the same direction. A simple illustration of the decrement is given in the table below, where in the first column are given the numerical values of the successive amplitudes in a wave train, beginning for convenience with a value of 10. Each amplitude is a constant fractional part, 0.818 approximately, of the preceding; in the second column is the common logarithm of the amplitudes; in the third column the natural logarithm; and in the fourth column the decrement 6=log e I 1 -ldg I 2 . Ampli- tudes. Common logarithm of Natural logarithms. Decrement or amplitudes. 10.00 1.0000 2. 3026 0.200 8.18 0. 9128 2. 1026 0.200 6.70 0. 8261 1.9026 0.200 5.49 0.7396 1. 7026 0.200 4.50 0.6532 1.5026 From this table it is seen that the decrement of this wave train is 0.20, which is very closely represented in figure 14. Similarly in figure 13 the decrement is 0.4 and in figure 15 in the case of un- damped oscillations it is zero. MEASUREMENT OF LOGARITHMIC DECREMENT. The subject of damping and its measurement in terms of the logarithmic decrement is one of the most technical parts of the 66536 17 9 130 EADIOTELEGRAPHY. subject of radiotelegraphy so that only a brief outline of the simplest cases can be given here. The logarithmic decrement can be measured either directly by a decremeter which is a modified form of a wave meter or by a wave meter if it is provided with a suitable means of indicating resonance. When a wave meter is adjusted to resonance with a circuit in which oscillations are taking place it will be found that the larger the resistance in the circuit the broader will be the tuning in the wave meter i. e., the greater will be the change that must be made in the wave-meter condenser to make any decrease in the wave-meter current from the value at resonance. Similarly the larger the resis- tance in the wave-meter circuit the broader will be the tuning. On the other hand the smaller the resistances in both the circuit and the wave meter the sharper will be the tuning. As has been pre- viously stated on page 128, the less the resistance in the circuit the less will be the damping, and hence the smaller the logarithmic decrement. Thus it is seen, in a general way, that there is a relation between the shape and breadth of the resonance curve and the decre- ment of the circuit under measurement. It has been shown by theory that if the resonance curve is taken by a wave meter under certain standard conditions, a simple formula can be used to find the logarithmic decrement of a circuit. For this purpose the wave meter should have a variable condenser witli a suitable scale, graduated from to 180 or to 90 degrees, with which there is furnished a calibration curve of the capacity of the condenser, and the wave lengths indicated by the meter; and ti hot- wire wattmeter with a suitable scale, connected as shown in figure 48. The wattmeter indicates the value I 2 R in fractions of a watt, where I 2 is the square of the current flowing in the wattmeter wire and R is its high-frequency resistance. This wire is generally made of a special alloy w T hich does not change its resistance appreciably with heating and hence the product PR, that is, the watts on the scale of the wattmeter, can be taken as relative values of I 2 , and of the squares of the currents in the wave-meter circuit. Thus if for two different currents the wattmeter scale deflections are 0.35X1/10 watt =0.035 watt and 0.0175 watt, the relative values of I 2 are 1 and |-. The logarithmic decrement of a circuit can be measured as follows : Couple the wave meter loosely with the circuit and adjust the vari- able condenser until resonance is obtained. Adjust the coupling slightly until the wattmeter needle is on some convenient scale divi- sion at or near full scale reading. Note this wattmeter reading. I R 2 and the condenser capacity, C R . Without changing the coupling ad- just the variable condenser toward the zero end of its scale; that is, for smaller values of capacity and for shorter wave lengths than at KADIOTELEGRAPH Y. 131 resonance until the wattmeter reading is reduced to one-half of its I 2 value at resonance. Note this reading, - = Ij 2 , and the condenser capacity, C t . Similarly, without changing the coupling, adjust_the variable condenser toward the 180 end of the scale, that is, for larger values of capacity and for longer wave lengths than at reso- nance until the wattmeter reading is again reduced to one-half its 1 2 value at resonance. Note this reading -|- = I 2 2 = I 1 2 and the con- denser capacity C 2 . From the readings taken at resonance and on both sides of resonance, the following formulas can be used to de- termine the desired decrement, in which 5 t and 8 2 are, respectively, the logarithmic decrements of the wave meter and the circuit under measurement; x= 3.1416; C R is the capacity of the condenser in microfarads or other convenient units, where resonance was ob- tained, and G! is the capacity, where the wattmeter current was re- duced to one-half its value at resonance on the short-wave length side of resonance, and C 2 is the corresponding capacity on the long- wave length side. The formula as usually written gives the sum of the two decrements, from which the decrement of the wave meter, which is given as a part of the calibration of the instrument, must be subtracted to give the desired decrement. Two measures of the decrement can be obtained from the above values; the first from the readings at the resonance point and one side of the resonance curve, and the second from the resonance point and the other side of the curve. For the capacity at resonance C R and that on the short-wave side Q: (J _ (J t = 0.14 Similarly for the capacity at resonance C R and that on the long-wave side C 2 : P _p p _p t> i p. ^ Jg >. V7 R c\ i i ^V. V/o C 2 C 2 As the resonance curve is not always symmetrical it is best to take the average of these two values for the average value of the sum of the decrements. Instead of computing two values and taking the average, the fol- lowing single formula, using the values on both sides of resonance, gives approximately the same value for the sum of the decrements: 132 RADIOTELEGRAPH Y. It will be noted that the values of I R 2 , I t 2 , and I 2 2 , do not appear in the formulas but rather C R , C 1? and C 2 , which however depend on the relative values of I R 2 , Ij 2 , and I 2 2 . The following numerical example will show the use of the formulas, the data being taken from the resonance curve of figure 85, where, as described on page 62, a single turn of wire had been inserted in the antenna of a quenched-spark set, the two circuits of which had been carefully tuned to resonance as described on page 60. .011 .001L>4 .016 .00123 .022 .00122 .028 .00121 .035 .00120 . 028 Resonance _ . 001195 .036 .00119 .026 .00118 .016 .00117 .011 .00116 . 007 _______________________________________________ 0. 00115 From the plot of the curve in figure 85 it is seen that at resonance I R 2 = 0.038 C R is 0.001195 mf.; and at Ii**-nf- C t is 0.001175 mf., and at I 2 2 = -7r C 2 is 0.001225 mf.; hence Similarly, Average value, 8 1 -|-&o=0.065 Using the single formula i 2 . The value of 8 X being given with the wave meter as 0.016, it is seen that 5 2 =0.066 ^=0.050 by both formulas, which is the logarithmic decrement per complete oscillation of the antenna circuit. In some cases it is convenient to be able to use wave lengths in meters, instead of capacities for the computation of the decrement. The corresponding formulas are, for the short-wave side of resonance RADIOTELEGRAPH Y. 133 and for the long- wave side, six o 2 AR a 00^2 AR 0i + o 2 = ZTT r - = 6.28 r - A 2 A 2 and for the single formula, using the measures on both sides of reso-~ nance, 5 1 + 5 2 = 7r^f^ = 3. in which X R , X and X 2 are the wave lengths in meters or other con- venient units corresponding to the capacities CR, C 15 and C 2 . There are other formulas for the sum of decrements, as in terms of the frequencies, etc., but as they are not in common use they will not be given here. The preceding formulas apply only in the case where I R 2 and I x 2 and I R 2 and I 2 2 are both in the proportion of 1 to J. If for any reason this relation is not true the full formulas, from which the preceding were obtained, must be used as follows: In general in using these last six formulas the complete resonance curve is drawn from the observations as shown in figure 84. In the third formula, in which values on both sides of the resonance curve are used, C and C 2 must be taken from the curve for the same value of I 2 ; and similarly in the sixth for X and X 2 for the same value of I 2 . In any of these formulas if I 2 or I 2 2 is made J I R 2 the expression under the square-root sign becomes equal to 1, and hence the sim- plified form previously given. Sometimes a hot-wire ammeter is furnished with the wave meter instead of a wattmeter, in which case the value of C R is obtained at the value I R . The values C and C 2 must be obtained when I t and I 2 are equal to 0.7 I R (more accurately 0.707 I R ). With these values of C R , C 15 and C 2 , or the corresponding values of X R , X 1? and X 2 , the simplified formulas for the sum of the decrements can be used as 134 RADIOTELEGRAPHY. 0.040 0-030 0.020 0-010 o-ooo Capacity in M.F. FIG. 85. RADIOTELEGRAPH Y. 135 above. If the ammeter readings are taken at the relative values of I R and 0.707 I R , the squares of these readings are in the necessary ratio of 1 to J. Measures of the logarithmic decrement can also be made without the use of the wave meter in certain special cases. If a single circuiT with high-frequency resistance R, inductance L, and capacity C is not coupled with any circuit, or very loosely coupled with a primary quenched-gap circuit, theory shows that its logarithmic decrement per complete oscillation can be computed from the formula g= ^ ^ where, if R is in ohms, L must be in henrys, and N is the frequency in oscillations per second. Thus, if the antenna whose decrement was measured by the wave meter above as being 0.050 should have a resistance of 6 ohms, an inductance of 200,000 cm. or 0.0002 henry, and should be oscillating at a frequency of 300,000 or a wave length of 1,000 meters, its decrement by the above formula would be n 2X300,000X0.0002 Or - 050 ' as found b ^ the wave meter ' GEORGE P. SCRIVEN, Brigadier General, Chief Signal Officer of the Army. o UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW Books not returned on time are subject to a fine of 50c per volume after the third day overdue, increasing to $1.00 per volume after the sixth day. Books not in demand may be renewed if application is made before expiration of loan period. ftUG 4 19" 4UG 281917 S-EP i NOV 8 1 917 .DEC 15 1917 DEC 22 191 JAN & FEB 14 1918 flPR 24 1918 . 13 1918 DFO 4 3 50m 7,'1G YC 64494 UNIVERSITY OF CALIFORNIA LIBRARY