The Thermionic Valve

New Zealand Listener, Volume 31, Issue 804, 17 December 1954, Page 24

The Thermionic Valve

This is the text of a broadcast given recently from YA and YZ stations

of the NZBS by

W. L.

HARRISON

Chief Engineer of the NZBS

HE thermionic valve, or vacuum tube, or electron tube as it is called in the United States and some other countries, is a most essential part of all modern radio communication equipment. Without it I -couldn’t be speaking to you tonight. Depending on its size your radio receiver contains some five to ten thermionic valves, and broadcasting stations use many of them in their studio and transmitting equipment. For-example, Station 2YA ‘uses neatly a hundred valves. They .vary in size and weight from a few ounces up to 250 pounds for the biggest transmitting ones. The most common form of thermionic valve is a glass envelope or bulb containing a metal filament which can be heated, or made to glow brightly, by passing an electric current through it. This is very similar to the ordinary electric light bulb. But unlike the electric light bulb the thermionic valve has several other metal parts inside it besides the filament. In its simple form the thermionic valve has this filament surrounded by a metal coil or mesh called the grid, and this in turn is surrounded by a metal cylinder called the plate or anode. Connections to these elements are brought out from the in-

side of the glass bulb through special _ seals, and then all the air within the bulb is evacuated to a high degree of vacuum and the glass is then sealed off with a gas flame. When they are connected to radio circuits -usually containing coils, condensers and resistancés — thermionic valves of this kind can perform three important functions. The first is in amplifying or magnifying very ‘small electric currents of the kind generated by a. studio microphone or gramo-

phone pickup. In general these are about a few millionths of a volt; they are amplified up to several volts. Secondly, they can generate the very high frequency currents needed for communication, without using the wires that are generally used for ordinary telephone and power systems. Hence the origin of the term "wireless." For example, our hydro-electric power stations generate power at a frequency of 50 cycles per second, and this must be transmitted along wires, but the transmitting valves of Station 2YA gen-

erate electric power at over 500 kilocycles per second, or half a million cycles per second, and it can then be transmitted through space without ‘wires, Thirdly, the thermionic valve can rectify, or detect, in your radio receiver, the radio currents picked up by your receiving aerial, It enables these high frequency currents to be converted into such a form that they can energise the loud speaker or headphones, and you can hear them physically with your ear.

Today the thermionic valve is the most vital part of all radio broadcasting and other radio communication equipment. But it wasn’t always so. In the early days of wireless communication-at the end of the last century and the beginning of this century-one of the biggest handicaps in communicating between countries and with ships at sea was the poor sensitivity of the receiving devices then in use. Two of the most common of these devices were the coherer, in which a tube of iron filings cohered or stuck together when radio currents were

passed through them, ind le crystal detector, which was a small piece of mineral-often galena or iron pyriteswhich a fine wire touched lightly. This wire rather strangely was called the "cat-whisker," probably facetiously, but the name stuck permanently even though the wire didn’t, as some early listeners who struggled with these things will well remember. It was in those early days that Professor J. A. Fleming, who was closely interested in the new science of wireless telegraphy, was searching for a better means of detecting electro-magnetic or wireless waves. Professor Fleming was Professor of Electrical Engineering in the University of London, and was scientific adviser to the Marconi Wireless Telegraph Company. Some years earlier in association with Thomas Edison and the Edison Electric Light Company of London he had carried out some experiments on the passage of electric currents through rarefied gases in evacuated glass bulbs. He had noticed that if two electrodes were separated in a partial vacuum and one of them, such as a carbon filament, was heated, a current could be made to flow across the space between the electrodes in one direction but not in the other. Fleming remembered this interesting phenomenon and decided to give it a trial for the detection of radio waves. He took one of his old experimental bulbs from his junk box and connected it up to receiver circuit using a galvanometer as an indicator. At the other side of the room he pressed the key of his small spark transmitter and the galvanometer needle immediately de-flected-the. experiment worked magnificently. And so the first thermionic valve in its application to wireless communication was born. Fleming took out a patent on this form of detector on November 16, 1904-just 50 years ago. What a vast field of scientific application this has opened up-long distance radio communication, radio broadcasting, television, carrier telephony, radio navigational aids, radar and y other highly complicated electronic techniques, In those early days nothing was known of electrons, but Fleming assumed that the current was conveyed by particles of negatively charged carbon.thrown off from the incandescent filament, and subsequent events show that he wasn’t far wrong. The Marconi Company adopted the invention at once and the "Fleming Oscillation Valve," as it was called, was very soon in production. In January, 1907, an American inventor, Dr. Lee de Forest, who had been following Flemings work closely, fitted a third electrode in the form of a screen or grid between the filament and the plate. He then found that by varying the potential on this grid in relation to the potential of the filament he could control the current flowing between the plate and filament. And what was more, quite small variations of grid potential pro duced quite large changes in plate current. He thus discovered that the thermionic valve could be used in this way as an amplifier as well as a detector. It is this amplifying action of the thermionic valve that is its greatest use today. Marconi challenged the de Forest patent application and after several im-

portant legal actions it was ruled that the Fleming valve was fundamental, and that de Forest’s addition of the grid was merely ‘an improvement on the basic idea. However,’ there was still more to come in the development of the thermionic valve. In 1913 almost simultaneously, it was found independently by Meissner in Germany, Franklin and Round in England, and Armstrong in the United States, that by coupling the grid and plate circuits of the three-elec-trode valve — now called a triode — it could itself be made a generator of continuous oscillations of .a high frequency. It was thus able to replace other cumbersome, expensive and_ inefficient devices then being used for radio telegraph transmission. What is more, its oscillations were found to be steady, pure in form and easily modulated, and the door was then suddenly opened to the development of wireless telephony, broacasting and television as we know it today. All this has happened in the brief space of 50 years. It was only as recently as 1927 that the first "Australian Beam" circuit, as it was called, was put into operation between England and Australia, and this was the first really successful long-distance radio communication system. It used valve receivers and high power valve transmitters. As far as radio broadcasting was concerned, experiments at Station KDKA, Pittsburgh, U.S.A., in 1921 showed the great possibilities of this new medium and led to regular broadcasting in the United States of America and England in 1922. This was followed ten years later by the opening of the Empire Shortwave Service, and in 1936 by a regular television service in London from Alexandra Palace. In the early investigations in the use of radio valves more than a little credit is due to radio amateurs. Since the earliest days of radio-or wireless as it was then known-there has been a band of amateur workers who have made radio communication their enthusiastic hobby, and in many cases they have equalled professional workers in their standard of knowledge. Their main goal was to communicate over long distances with simple, inexpensive equipment, and by the end of 1924 the longest distance on the earth’s surface had been coveted by amateur transmissions. On October 18 in that year Mr. Goyder, otf Mill Hill School, London, exchanged messages with Mr. Frank Bell, of Waihemo, near Dunedin, New Zealand, using thermionic valve equipment costing just a few pounds. This was in sharp contrast to the results being obtained by the very costly spark and: are installations then being used by commercial companies. Today there are many other applications of the thermionic valve besides radio communication; but radio and television receivers together use more thermionic valves than any other equipment, Last year in the United States of America alone thirteen’ million radio and television receiving sets were manufactured; they required something like one hundred million thermionic valves to fit them out. When Fleming first reported the result of his original experiments to the Marconi Company in 1904, he said, "I have not mentioned this to anyone yet as it may become very useful." Just how useful his invention has become should now be clear to us all. Luckily, Fleming lived to see much of it himself, for he died in England as recently as 1945 at the age of 96. :