Part 19 (1/2)

These are coils in which the turns are wound over each other but in such a way as to avoid in large part the ”capacity effects” which usually acco them for yourself but I doubt if the experience has one farther in the study of the mathematical theory of radio than this series of letters will carry you

TABLE III Circuit of Fig 112 Nuth with antenna of of turns mil-henries 00001 mf 00002 mf

14 004 120 170 20 007 160 220 28 012 210 290 36 018 250 360 44 025 300 420 56 038 370 520 75 060 460 650

In the secondary circuit there is only one capacity, that of the variable condenser If it has a range of values from about 000005 mf

to 00005 e of wave-lengths froe is 150 to 480 es are respectively 270 to 1220 and 270 to 670 For longer wave-lengths load with inductance Four tiths

[Footnote 11: If you can afford to buy, or if you can borrow, ae you should take the characteristic yourself]

LETTER 22

HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS

MY DEAR EXPERIMENTER:

This letter is to summarize the operations which must be performed in radio-telephone transmission and reception; and also to describe the circuit of an important commercial system

To transmit speech by radio three operations are necessary First, therecurrent; second, this current must be modulated, that is, varied in intensity in accordance with the human voice; and third, the modulated current must be supplied to an antenna For efficient operation, of course, the antenna must be tuned to the frequency which is to be transmitted There is also a fourth operation which is usually performed and that is amplification

Wherever the electrical effect is smaller than desired, or required for satisfactory transmission, vacuuive you an illustration later

Three operations are also essential in receiving First, an antenna y fro station There is then in the receiving antenna a current si antenna Second, the speech significance of this current must be detected, that is, the modulated current must be demodulated A current is then obtained which has the same wave form as the human voice which was the cause of the modulation at the distant station The third operation is performed by a telephone receiver which hborhood move back and forth in accordance with the detected current As you already know a fourth operation ive on their output sides currents of greater strength but of the same forms as they receive at their input ter equipment two or more of these operations may be performed by the same vacuuenerative circuit for receiving For exa set the vacuuenerative circuit for receiving continuous waves by the heterodyne h-frequency current and as a detector of the variations in current which occur because the locally-generated current does not keep in step with that generated at the trans station

Another exa si 120 which shows a sile tube perforeneration of the radio-frequency current and its modulation in accordance with the output of the carbon-button transmitter This audion is in a feed-back circuit, the oscillation frequency of which depends upon the condenser _C_ and the inductance _L_ The voice drives the diaphragm of the transmitter and thus varies the resistance of the carbon button This varies the current froh the pri voltage applied to the grid by the secondary _T_{2}_ The oscillating current in the plate circuit of the audion varies accordingly because it is dependent upon the grid voltage The condenser _C_{r}_ offers a low i _T_{2}_ of audio-frequency transfor 120]

In this case the tube is both generator and ”modulator” In some cases these operations are separately perfor set used in 1915 when the engineers of the Bell Telephone Systeton, D C, to Paris and Honolulu I shall not draw out completely the circuit of their apparatus but I shall describe it by using little squares to represent the parts responsible for each of the several operations

First there was a vacuuenerated a small current of the desired frequency Then there was a telephone trans through the primary of a transformer The e m f from the secondary of this transformer and the e m f from the radio-frequency oscillator were both irid of an audion which acted as a modulator The output of this audion was a radio-frequency current e audion ae antenna of the U S Navy Station at Arlington Fig 121 shows the syste 121]

The audion a in parallel When tubes are operated in parallel they are connected as shown in Fig 122 so that the sarids and the sarids vary in voltage there is a corresponding variation of current in the plate circuit of each tube The total change of the current in the plate-battery circuit is, then, the sues in all the plate-filaives a result equivalent to that of a single tube with a correspondingly larger plate and fila 122]

Parallel connection is necessary because a single tube would be overheated in delivering to the antenna the desired amount of power You remember that when the audion is operated as an amplifier the resistance to which it supplies current is made equal to its own internal resistance of _R_{p}_ That means that there is in the plate circuit just as much resistance inside the tube as outside Hence there is the sa the current through the tube as through the antenna circuit, if that is what the tube supplies ”Work per second” is power; the plate battery is spending energy in the tube at the sa it to the antenna where it is useful for radiation

[Illustration: Pl XI--Broadcasting Equipraph Coy expended in the tube appears as heat It is due to the blohich the electrons strike against the plate when they are drawn across from the filament These impacts set into more rapid motion the molecules of the plate; and the temperature of the tube rises There is a li the tube For that reason the heat produced inside it n of the tube and the ton experio, the tubes were cooled by blowing air on them from fans

We can find the power expended in the plate circuit of a tube bythe number of volts in its battery by the number of amperes which flows Suppose the battery is 250 volts and the current 002 amperes, then the power is 5 watts The ”watt” is the unit forpower Tubes are rated by the nuht ask, when you buy an audion, what is a safe rating for it The question will not be an i set since a detector is usually operated with such se as not to have expended in it an aerous to its life

In recent trans sets the tubes are used in parallel for the reasons I have just told, but a different eneration of the radio-frequency current is by large-powered tubes which are operated with high voltages in their plate circuits The output of these oscillators is supplied to the antenna The intensity of the oscillations of the current in these tubes is controlled by changing the voltage applied in their plate circuits You can see froth of the alternating current is changed accordingly It is the e which is particularly interesting

[Illustration: Fig 123]