RADIO TOPICS.

New Zealand Herald, Volume LXI, Issue 18887, 9 December 1924, Page 6

RADIO TOPICS.

BY THERMION.

VALVE MANUFACTURE.

The thermionic valve which has been the means of making !ong-distanct) - wireless possible is, in appearance, a simple device and few realise that remarkable scientific apparatus and correspondingly involved processes arc necessary for its manufacture.

Manufacturing problems commencc with the raw materials used as all metals contain impurities which must be removed before they are suitable for incorporation in tho valve. The chief difficulty is the presence of gaseous matter and such occluded gases would, if allowed to remain, gradually come out of the metal and, destroying the high vacuum of the valve, render it " soft." Pure nickel is used for the plate as it can be made free of all occluded gas. The grid is made of a hard, silver-coloured metal, molybdenum, and this is chosen as it will remain rigid at high temperatures. Any other metal -• would gradually sag when subjected to the heat from the filament. The filament is made of tungsten which, after being obtained in a powdered form from its ore, is pressed into sticks. These are converted by a chemical process into slugs of pure metal. The slugs are worked while hot into wires finally reduced to a diameter of about 1 -1000 th inch by hot drawing through a series of diamond dies. In the modern dull emitter valve the tungsten is either impregnated with or coated with a rare and costly substance, thorium, which is grey in colour, is heavier than lead, and has radioactive properties. It is used as being one of the most efficient emitters of electrons known.

The elements of the valve are supported by pure nickel wires welded to copperclad iron wires and they are placed in a heated silica tube through which a stream of hydrogen gas is passing. This process removes, oxygen and other gases from the heated nickel, the plate of the valve being similarly treated. The copper-clad welds of the mounting wires and then sealed into a glass stem and the grid, plate, and filament are welded into position by an electric arc. The next step is to weld a small flat*pieco of metallic magnesium on the outer sides of the plate.

EVACUATING THE VALVE. The completed mount is placed on an automatic scaling machine and a moulded glass bulb is dropped over it, the machine fusing the glass of tho mount and the bulb together. At tho top of the bulb, where the tip is seen in the finished product, is a glass tube .through which are drawn the gases in the process of evacuation for which tho valve is now ready. Such a high degree of vacuum is required, the highest known to science, in the " hard" valve, that specialised apparatus has been devised for the purpose. Oil pumps remove most of tho gas and a Langmuir mercury-vapour pump is employed for the final stages. With the valve in position and evacuation commenced, a heavy castiron oven heated very hot by internal gas jets is lowered over it. Many valves undergo this process simultaneously and they arc heated almost to the caving.in point so that occluded gases in the glajw walls ,are driven out.. When a certain stage is reached, a spiral' of copper wire carrying a largo high-frequency current is lowered over tho valve and the effect of this is to so excite the atoms 01 the nickel plate that its temperature rises rapidly. When the nickel is glowing red all remaining gases are driven out and removed by the pumps. When tho . temperature rcaches 1650 degrees Fahrenheit the magnesium welded on to the plate explodes into a silvery vapour that adheres to the walls of tho tube and, as magnesium exerts an attraction for oxygen and other gases, its vapourised particles " eat up" all tho gas still in the valve. . It is the magnesium that gives tho silvery appearance to the modern valve. Other substances are sometimes used for the same purpose and in orange-coloured valves red phosphorus is used. In valves of the WD-type a white patch of cryolite seen on the stem gives a similar effect. The tubes are then sealed off and com nected to the pronged base. All valves are subjected to a, process of ageing which brings the thorium of the filament into an active condition. This is done simply by lighting the filaments with a voltage applied to the plate. After being tested the'valve is ready for tho market.

NOTES AND HINTS. r Headers are reminded that a special transmission for children will be broadcasted from IYB on Saturday evening commencing at 7.15 p.m. Mr. F. D. Bell, 4A A recently copied a message from the United States Airship Shenandoah, the first foreign aircraft to bo heard in New Zealand. Mr. Bell is not sure of some of tho words. 'Iho messago was as follows: —"From U.S.S. Shenandoah, propose ( ?) take this opportunity to thank you for the splendid service you have rendered on our first flight across tho continent ( ?), and hope that you will bo able to render the same effective service on our return trip. All letters about pur radio work can bo addressed to tho Bureau of 'Aeronautics at Washington. We will be very grateful if you will help out in determining our ranges by writing these letters or cards. (Signed) U.S.S. Shenandoah."] A professor of astronomy recently reached the conclusion that New York is an unsafe place to live, says Tho Nautical Gazette, We read: "He contends that thero is something defective about tho rocks which form the foundation for the skyscrapers of Manhattan, with the result that some day New York will bo wrecked by an earthquake besido which the Japanese disaster will pale into insignificance; The professor announced his opinions in front of a microphone. _ An Italian liner was on her way to New York. The radio operator evidently an Italian, got the story slightly mixed, and the announcement was made that an earthquake had swept New York out of existence. Tho passengers passed a feverish time and tho pilot, when he came on board, had some difficulty in convincing them that 1-nvirinss was going on as usual in New York."

ewdrivers with long blades are extremely handy when making adjustments in an inacccssable corner of a set, but frequently arc the cause of accidental short circuiting of the. high tension battery with the possibility of burning out valves, unless these are first removed from their sockets, through the bare blade accidently coming in contact with tho wires. This may bo prevented by wrapping insulating tape round all except the extreme point of the valve.

By means of ordinary receiving valves, instruments for recording earthquakes can be made more sensitive than hitherto and earth movements as small as a five-hun-dredth of a billionth of an inch can be measured. In this manner the slightest tremor of tho earth from very remote parts of the world can be detected. Observing the expressions of intense'surprise that came over the faces of a number r.f toddlers who were being entertained recently at a radio concert at her residence in Sydney, Mrs. Harrington, wife of Mr. John Harrington, principal of tho wellknown photographic firm, quickly conceived a scheme by which numerous children have since contributed £135 toward the purchase of a receiving set for the children of the Roy a] Prince Alfred Hospital. Harringtons, Limited, subsidised the fund and a seven-valve set capable of operating up to a dozen loud speakers placed throughout the childrens' wards is now almost ready for installation.