WIRELESS NEWS.

Press, Volume LXI, Issue 18551, 28 November 1925, Page 10

WIRELESS NEWS.

| RADIO VACUUM TUBES. ! MANY USES. (BISCUIXT MJ.IJJJI SOS M XEZ MSES."") (By. "ELecbca,.") It has been said that without the development of electron tabes tibe present system of radio broadcasting would never have been possible. Certainly, the electron tabe is the essential part of our radio telephone broadcasting system as it has developed. Every broadoast transmitting station uses several electron tubes in order to mate available radio signals that may be picked up by receiving sets, the majority of which are likewise equipped with electron tubes. The tubes used in the transmitting stations vary in size from 2§ inches (by i-inch in diameter, to tubes more than a foot long and 8 inches in diameter. The higher pow«r stations are using tubes that must be continually cooled by water. These tubes are made almost entirely of metal, and are capable of producing several kilowatts of radio frequency power. The action of these large tubes does not differ from the action of smaller ones used in reseta. The difference is only a matter of size, construction, and power supplied. Three Functions. The electron tube is capable of performing two functions, amplification and detection of alternating current voltages. Its action as amplifier makes possible a third function, that is, generation or production of aJternaV ing current. This third function is used primarily in transmitting sets, althoagn some receiving seta make use of it also. A disturbance is started in the circuit associated with the input of ihd tube, which is amplified by the tube, given to the output circuit, which, (by virtue of some form of coupling, gives it back to the input, and the cyolo is repeated continuously. Because of this the disturbance is not allowed to die away, but is sustained, the power being supplied from external sources, such as batteries. Since the electron current has no inertia effects the action is instantaneous, and alternating currents which have frequencies from a fraction of a cycle a second to several million cycles per second can bo generated by the same tube simply by altering the circuit arrangements which determine the frequency of the set.

Frequencies Produced. In most broadcasting stations the frequencies of the currents produced are from 550,000 to 1,500,00 cycles per second. The oscillated tubes generate this carrier frequency which is capable of radiating waves into space. Other tubes take the feeble alternating currents from the microphone, which have frequencies from 20 to 10,000 cycles per second and amplify them. However, these currents cannot radiate, so another set of tubes impress these amplified voice currents on j the radio frequency generating tubes and modify the radio frequency current so that it carries into space not only the radio frequency but also lower frequency voice currents. When 'the radio frequency _ current is induced in the receiving aerial, the audio frequency cannot be heard _ unless the radio frequency is partially destroyed. This is done by using the detecting property of the tube. The detector tube utilising the second function of the tube, detection, takes the audio frequency off the carrier wave and makes it audible. This audible signal may not give a strong enough signal, so it is introduced into a tube adjusted to amplify. Thus the signal is strengthened up to any degree desired. On the other hand, it is often desirable to amplify the radio frequency signal before detection. This is also done by means of the electron tube. All of the different uses are j brought out by the correct arrangement of the circuit constants such as the inductance, capacity, grid voltage, grid leak, etc.

Sizes and Adjustments. The different possible sizes and adjustments of the various circuit constants make up the many hundred different types of receiving sets. Tip to very recently the development in electron tubes in the TJ.S.A. for radio use has been to develop general purpose tubes so that any tube can be used for any purpose in most,types of receiving sets with only small adjustments. In Engicnd, separate tubes are made for all the different purposes they are put to. The choice of a tube for a particular receiving set is usually a matter of filament power consumption and power output of the set. The use of the electron tube, however, is not, limited to radio alone. Electron tubes so arranged as to amplify telephone currents have been inserted in the long distance telephone linos, and have made it possible to extend the distance limit of the telephone, which was formerly about 1000 miles. Amplifying Properties. The amplifying property of the electron ,tube has also been used to aid those persons who have defective hearing. The sound waves that are strong enough to be heard by the normal person,: but not by the person with defective hearing, are received on a microphone and transferred into electrical vibrations. These are amplified by means of one or more electron tubes and then transferred back to sound of much greater intensity than the original by means of a telephone receiver, thus serving the same purpose as raising the intensity of the original A similar use is m public address systems where the normal voice" of a speaker is made andible to many thousands of people; that is, the sound of the voice is amplified many times by use of several electron tubes and projected by nieans of loud speakers which are capable of being heard many hundred feet away. The use of the proper tubes and apparatus gives faithful reproduction of the original voice. Heating Metals. Another entirely different use of the electron tube is in the heating and melting of metals. The tube is used to generate alternating current cf a high frequency, as in radio. This current, flowing in a coil surrounding the metal, induces in it similar currents, which, because of the resistance of the metal, increases its temperature to the degree desired. This makes it possible to heat metals that are contained in & vacuum or in some particular gas, and bo control the conditions very accurately. A Wizard Tube. Electron tube amplifiers make it possible for a large assembly of doctors simultaneously to hear the heart-beats of a patient. Electron tubes have made it possible to send pictures by wire and radio, and just recently there was produced at a distance the image of moving objects sent by radio, using electron tubes. Going still further afield, the electron tube has been used to measure the rate and method of growth of a plant root, which illustrates how.it has "been adopted in other fields far removed from radio". There has bean put into the,hands of the engineer and the scientist a wonderful tool which is capable of many diverse applications. New uses ax* being iournd over; day, and a tool.