A Thermionic Wizard

Radio Record, Volume III, Issue 14, 18 October 1929, Page 4

A Thermionic Wizard

The Aladdin’s Lamp of Modern Science

pDA4z by day, the thermionic valve is demonstrating its marvellous ‘qualities in an ever-increasing field of specialised application. Owing to the fact that the electron stream, the "working principle" of the valve, possesses hardly any inertia, this invention is admittedly the most delicate and sensitive relay known. It is capable of responding freely and instan‘taneously to the slightest applied impulse, without any lag or any perceptible loss of energy, It is also unique as a generator of high-frequency oscillations, and it is equally steady and reliable when handling the few watts of a small transmitting station as when generating the thousands of watts radiated from a powerful one. Its remarkable qualities are thus conducive to the development of its potentialities extending far outside the field of wireless for which it was originally devised. Medicine is one instance in point. The virtues of electricity have long been recognised by medical science, both as a curative agent and as a means for pain alleviation. Thus the use of high-frequency currents, induced by a valve oscillator, is at present rapidly growing in favour in the medical world of to-day. ‘ The stethophone is a further illustration of the use of the valve in medical science. In this instrument an ordinary stethoscope is used in conjunction with a multi-stage valve, amplifier, thus magnifying sounds which would otherwise remain obscure and possibly unidentified. In this way the stethoscope represents a valuable asset in the modern art of diagnosis. — __ Finally, high-frequency currents p duced by valve generators are now being employed for cauterising purposes -a8 well as for actual surgical operations. By using currents of radiofrequency from .25 to 2 amps. in strength, it is possible to cut through bone and tissue in an absolutely antiseptic manner, and ‘without loss of blood. _. Another property of a valve which ig utilised in many different ways is its extreme sensitivity to any change in

external circuit conditions, especially when the set is on the "threshold" of oscillation. If, for instance, the reaction coil of a valve receiver is set just short of the oscillation point, the set can be made: to "howl" by placing the hand near the coils, or by passing a metal object through the open core of one of the inductance coils. By utilising these peculiarities, a number of ingenious alarms have been devised. A coil forming part of the plate-circuit of a "trigger" valve, can be placed near a safe or window, and the approach of a burglar detunes the circyit sufficiently to set the trigger valve into self-oscil-lation. This in turn causes the plate current to alter suddenly, thereby es-' tablishing a contact which rings an alarm bell. In a German factory the pilfering of valuable metal by workmen has been detected by means of a very similar circuit arranged around a special exit door. The door windings are connected to a superheterodyne receiver, which is sensitive enough to detect the presence of any hidden metal, by giving rise to an alteration of the normal tuning note in the ’phones as the culprit passes through. A similar principle underlies an arrangement used in prospecting for the presence of metals. A part of the detector circuit is slung in the form of a Hertzian oscillator below the fuselage of an aeroplane, which flies over the area to be explored. The presence of mineral deposits below the ground surface, exercises a "damping" effect on the "search" coils, which creates a change in the characteristic beat-note of a super-sonie receiver, and so gives a reliable indication to the prospecting aviator.

-The combination of a selenium cell with a thermionic amplifier has been used to detect the presence of unauthorised persons wheh opening a safe or strong-room door. .The change in the lighting’ conditions alters the resistance of the selenium cell, which in turn sets a trigger valve into oscillation, and so rings an alarm. A very similar combination has been used to give automatic indication of the approach of fog. A ray of light from a distance is focused upon an optical cell, which is: connected to a "trigger" valvé circuit in such a way that when the atmosphere becomes opaque, thus cutting off some of the light, a relay is actuated and automatically brings the harbour fog signal into operation. ’ An identical arrangement is used inside a chimney stack to give a warning when the smoke emitted contains more than the regulation percentage of soot and other impurities, Dixcessive atmospheric pollution is thus_prevented, an automatic alarm being given when the smoke becomes too apoque, and thus too dirty. Amongst other applications, mention may be made of railway signalling devices in which trap or loop circuits laid on the permanent way co-operate with valve amplifiers mounted in the locomotive, and automatically give audible or visible signals of track conditions ahead. One final’ ingenious application may be mentioned. A valve can be used to give audible warning of the accumulation of dangerous gases in a coal-mine. The operation in this case depends on the change of "tuning note" of a pipe resonator, the fundamental frequency of which’ is determined by the density of the contained column of air; The intrusion of noxious gases alters the original density, and so actuates a local alarm. In conélusion, it would not be an exaggeration to state that the valve is probably: the most marvellous invention of all time, and its applications to various widespread uses are bv no means all discovered.