522 



NATURE 



[April 22, 1922 



Applications of the Thermionic Valve.^ 



By J. Joseph. 



T^HE control of energy at distances of thousands 

 of miles without any other medium than the 

 aether has been made possible by the evolution of the 

 thermionic valve. This remarkable invention can be 

 described briefly as a highly exhausted glass bulb, in 

 which is mounted a tungsten or tantalum filament 

 heated by a battery giving about 6 volts. Electrons 

 are emitted by the heated filament. The filament 

 is surrounded by a grid or gauze cylinder, which 

 is insulated and kept at the negative potential of the 

 filament, while a plate of metal mounted inside the 

 bulb is kept at a high potential of from fifty to several 

 hundred volts by means of a battery or some other 

 source of continuous current. The bulb is highly 

 exhausted, and while the grid is kept at a normal 

 negative potential, steady current passes from the 

 filament to the plate or anode, but as soon as the 

 grid is made slightly positive or negative, the current 

 passing between the filament and anode by virtue of 

 the electronic conductivity is increased or decreased. 

 A valve can be used as a rectifier, as it can be made 

 unilateral in conductivity by suitable adjustments of 

 "grid potential." It can also be regarded as an 

 inertialess relay, it being only necessary for the grid to 

 be affected by the most minute change of potential 

 for the valve to become more or less conductive, 

 when it may be used indirectly to close a circuit and 

 control magnetic or electrical operations. 



One of the most important applications of the valve 

 is the amplification of telephone currents in long- 

 distance telephone trunk lines. Here, owing to the 

 length of the cable and to the electrical constants in- 

 volved, speech becomes greatly attenuated, and 

 thermionic relays or repeaters are introduced about 

 every thirty miles which amplify the speech to its 

 original degree of loudness. In addition, cable of 

 much smaller diameter and weight can be employed, as 

 currents producing almost inaudible sounds can be 

 amplified to any degree of strength. The introduction 

 of these valve relays has effected a saving of thousands 

 of pounds in many of our trunk telephone lines. 



Another recent application of the valve is the magni- 

 fication of the sound of the heart-beat. This is effected 

 by means of a special transmitter, which rests by its 

 own weight over the heart of the patient under examina- 

 tion. The heart creates vibrations in an air-chamber 

 which reproduce exactly the complex action of the 

 blood when passing through the valves of the heart. 

 When connected to a thermionic valve amplifier and 

 a special receiver attached to a large horn, the beat 

 of the heart can be made audible to a number of people 

 in a lecture-room. 



The valve has also been used for the simultaneous 

 reproduction of speech with the projection of a film 

 on a screen, both picture and sound vibrations being 

 photographed simultaneously on the same film, thereby 

 ensuring perfect synchronisation. The vibrations of 

 the voice are, by means of microphones, made to 

 agitate a small mirror fitted on the camera adaptor, 



'Substance of a contribution to a discussion at .the 'Institution of 

 Electrical Engineers on March 6. '' 



NO. 2738, VOL. 109] 



and a shaft of light passes from the mirror through 

 a narrow slit. As the mirror vibrates, the band of 

 light is reflected at constantly changing angles, and 

 a wave form is produced which corresponds to the 

 vocal sounds of the person speaking, as in the oscillo- 

 graph. The wave form appears on the side of the 

 film and is reconverted into sounds by means of a 

 selenium cell, which, as is well known, possesses the 

 peculiar property of resisting the passage of electricity 

 in proportion to the intensity of light to which it is 

 subjected. The variations in resistance caused by 

 the passage of the film through the cinematograph 

 are amplified by thermionic valves and made audible 

 through a loud-speaking telephone. There are wide 

 possibilities in this application of the valve. 



An important feature of the valve is its great 

 adaptability to the production of sounds of any 

 frequency from one to many millions per second. A 

 valve can be made to generate oscillations if the 

 grid and anode are coupled to coils so as to form a 

 transformer, the circuit of the coils being completed 

 through a battery of 150 volts or more. By con- 

 necting a condenser across the anode coil, oscillations 

 are set up, the frequency of which depends on the 

 capacity of the condenser. If a third coil is coupled 

 magnetically to the anode circuit, a note will be emitted 

 corresponding to the frequency of the circuit, and by 

 varying the capacity of the condenser, a wide range of 

 frequencies can be generated for various testingpurposes. 

 The note emitted by the receiver is very clear and sharp, 

 and the ease and rapidity with which the frequency 

 can be changed renders the method particularly suitable 

 for aural surgery, where frequencies covering a range of 

 200 to 3000 are often required. It is well known to 

 aural specialists that certain people have what is 

 known as a silent zone at particular frequencies. For 

 instance, a patient's hearing might be normal for fre- 

 quencies 200-500 and although he is deaf to frequencies 

 500-520. The aural appliances at present in use 

 are not suitable for the rapid and accurate production 

 of frequencies of any desired value. With a thermionic 

 generator and a calibration chart, however, the 

 frequency can be varied at will, and if a telephone 

 head-receiver is worn by the patient and connected 

 in series with a variable air condenser and the output 

 or coupHng coil, it can be determined readily what 

 frequencies are inaudible to the patient. Further, 

 by varying the capacity of the condenser the sound 

 can be reduced gradually to inaudibility and, by 

 calibration, a scale obtained which will give positions 

 for normal hearing, imperfect hearing, and so on. By 

 this means the effect of treatment can be determined 

 to a very fine degree. 



The human ear will not easily respond to frequencies 

 greater than 3000 per second, although frequencies of 

 18,000 can be detected and instances have been known 

 where 30,000 to 40,000 have also been heard. The fre- 

 quencies used in wireless telegraphy are governed by the 

 wave-length, and values of 500,000 per second, which 

 correspond to a wave-length of 600 metres, are quite 

 common. In spark telegraphy, the wave trains are 



