CRYSTAL TO VALVE

Evening Post, Volume CVI, Issue 126, 24 November 1923, Page 22

CRYSTAL TO VALVE

3 THE RELAY'ACTION

FUNCTION OP THE GRID,

' It -was • explained in last Tuesday's notes that in a valve, containing" a hot filament and a positively charged plate, there is a flow of electrons from the filament to the plate, constituting an electric current, and that the force required to maintain this flow is maintained by connecting a special battery to the filament and the plate. This is the so-called B or plats battery; and it is of course the ultimate source of the energy which flows between'the filament and the plate. The filament is heated by another set of. cells, but their energy •is riot utilised in the generation of the flow. Its-function is merely to place the electrons .in .the-filament in such a condition that they can be easily moved by the force of *the plate battery Any other .possible method of heating the fHameniCwould be just as effective. In order ;tp\ apply the plate-battery force to the; plate and ■• filament, wires are of . course -necessary ; and the current which flows ;thiough the,'wires can be detected or measured . by. ..means of suitable': instruments. : Variations in the current, if of -suitable frequency, may be detected* very effectively by including a telephone receiver in the circuit. Any means, of varying the current at an audible frequency; will cause the telephone receiver.-to emit a, sound of the same frequency. .".'

The; grid is the device used to secure variation- of the plate circuit current. It isviiiterestihg to note here that, in usingU'a crystal! receiver, the energy •whiclg-inoves the diaphragms of the phones;-is simply that which is picked up by the aerial out of the waves radiated .by the transmitter. In a valve set, this, energy is not made audible at all. .It; is utilised solely to control the energy;-of the plate battery; it is the resulting fluctuations of this current become, audible. ELECTRICAL OSCILLATIONS. As every wireless beginner knows, or should-know, the energy which reaches his ;instruments is in the form of extremely rapid oscillations of electrical ■ force.y It is not necessary to discuss here how they-get there. The point to be grasped is that in the tuner of the /receiving apparatus, each coil of wire carries electric currents which travel alternately... in: . opposite directions. In this- respect -they resemble the alternating current commonly used for household • lighting..' The chief differences to be .'.noted., are; the relative feebleness of the'-.Jw.ireless-"current3 and their far. higher frequency. Whereas the alternating'Ccurrent makes its change of 'direction'SO or 80.' times a second, the wireless^falternations occur at the rate of half:a million-per second when 600 metre signals are -.'."being received, and over a million per second when Wellington brqadcasting, on 275 metres, is being received. fi no ascertain the frequency in any case, divide the wave-length in metres into 300,000,000.) r ..■..■.;.--.■;-' ■•• :■ '

Tlie. .oscillating , current may: be described ' as" ■ a periodical- migration of some of the."electrons .in jthe;':material of the'^cbil from bne'end of the coil to. the other and' back again, not unlike the "swinging" of water in a long bath. The end to which a surplus of ■ electrons has moved is for that instant negatively charged, and the other end is posilievly charged, and the ends of the coil thus exhibit positive and negative charges alternately with the frequency of the electrical oscillation. It is these alternations of charge which are applied in the simplest wireless receivers to a crystal detector, where they are bo modified that they become converted into a '..form of ■.•current that : can affect a telephone receiver diaphragm. A TARGET FOR ELECTRONS. .

In the valve receiver, the ends of one of the tuner coils (in a loose coupler, the ends of the secondary coil) are connected to the grid and the filament respectively. The filanie-.it, being also connected to the two batteries, which form, a large mass, may ./be regarded as 'practically earthed, in view of the smallvariations of electrical pressure in the coil. The grid, on the other hand, is ot very small mass, and takes the .full electrical pressure due to the oscillations, becoming alternately positive and'negative. .It will be recalled that the grid, a spiral of thin Svire, Nlies between the hot filament and the positively charged plate, and is therefore in the track of the electronic stream passing from the filament-to the plate.. While the grid circuit is .not carrying any oscillations,' the grid will/exert no appreciable effect on the flow of electrons. But when oscillations begin, the grid becomes alternately positive and negative, though usually only in_ a small degreei At the moment when it is positive, it exerts an additional attraction upnn the loose electrons of the filament, and they are discharged more freely. Some of them strike the grid,, and enter its atoms. But the majority miss it, because they travel at an enormous velocity, and their course is not very easily altered; and so the net result of a positive charge on the grid' is that more than the normal number of electrons pass to the plate. AN EFFECTIVE SWITCH _ At the opposite phase of the oscillation, the grid becomes" negative. The result of this is a repulsion of the negative electrons, and' a remarkably small negative charge Yon the grid -will practically stop the whole flow towards the plate. Thus the effect of the alternating charges on the grid is to increase and decrease the current flowing to the plate and through the plate-battery circuit, ■with exactly the same frequency as • the oscillations in the grid circuit. Under certain conditions it is possible to use very high electrical pressures in the plate circuit —100 volts is quite common in receiving sets—so that the tiny currents affecting the grid circuit take con- J trol of very much larger currents in the ! plate circuit, in much the same fashion \ as a small and light-running hand-wheel can steer a huge steamer by controlling a powerful steam Bteering gear. fhe effects of the grid charges -upon the electronic stream depend upon their intensity, and this depends upon the smailness of the capacity in the grid circuit. Too much insistence cannot be ■ laid upon the desirability, of tuning receiving sets by means of inductance rather than capacity; and this rule applies with special force; to trie grid circuit, in which the effect of high capacity is to decrease materially the voltage. applied to the grid. "'. .:."■.' . . ' .REGENERATION; The conditions just referred to constitute the. amplifying properties of the i valve. In describing in general terms the conversion di ■-'• a crystal set- to. a ' valve set, it. was remarked, that the result will be disappointing unless regeneration is applied. This is because a valve, when used as a detector, is not : worked at. high . amplifying efficiency. \B_ut it dfigg- ampljfyj ajjd regene£ation

consists of using the current in the plate circuit to increase that in the grid circuit, ■ and so to increase the variations of the electrical conditions of the grid, and secure a plate current still more increased. The usual way of securing regeneration is to include in the plate circuit .a coil like that of the tuner secondary, and loose-couple it with the secondary. The latter is then acted upon by the plate-battery currents in exactly the same way as by the currents in the primary of the loose-coupler.