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In its modern form a thermionic valve of the latter type com- prises a highly exhausted glass bulb having in it a filament of tung- sten, or thoriated tungsten or of platinum wire coated with oxides of barium and strontium (see fig. 3). This is rendered incandescent by electric current from a storage battery. Around the filament js a spiral of nickel wire or else a cylinder of nickel wire gauze. This is technically called the grid. Around that again is a cylinder of sheet nickel called the plate. The plate and the grid are carried on separate wire stems sealed through the wall of the bulb. Although the three-electrode valve was originally devised as a detector of electric oscillations as described below yet about 1913, or before, it was found that both the two-electrode valve and also the three- electrode valve can produce electric oscillations as well as rectify or detect them. When the filament is rendered incandescent tor- rents of electrons or particles of negative electricity are emitted from it. If the plate is given a positive potential relatively to the filament by means of a battery called a plate battery, these elec- trons are attracted to it, and this creates a movement of negative electricity called a thermionic current. If the bulb is highly ex- hausted and has a grid in it between the filament and plate, the electrons can only reach the latter by passing through the holes in the grid. If the grid is given a negative potential it reduces or stops the thermionic current. If it is given a positive potential it increases the current. The relation between thermionic current and grid potential can therefore be represented by a characteristic curve as shown in fig. 4.

Potential of Grid with respect to Filament

FIG. 4. Characteristic curve of a three electrode thermionic i valve.

The three variables, viz. plate and grid potential (v p and v a ) and the thermionic current (i p ), may be regarded as three rectangu- lar coordinates which define a characteristic surface, and sections of this, parallel to the i p v a axes or i p v p axes, delineate the principal characteristic curves. The central portion of this surface, corres- ponding to zero grid potential, is nearly a plane and has therefore the equation i p = av a -\-bv p where a and b are certain constants or coefficients.

If we pass a current by means of a high voltage battery from the filament to the plate and send this current also through the primary coil of an oscillation transformer, the secondary circuit of which is connected to the filament and grid, we have an arrangement by which continuous electric oscillations are produced and maintained. For, if properly connected, any variation of the grid potential will increase or decrease the plate current; and this acting through the transformer will in turn create the changes of grid potential which act to sustain the variations of plate current. The action is just like the well-known experiment of the singing telephone. If a magneto-telephone receiver is in series with a carbon transmitter and with a battery, then, when the transmitter is held near the receiver, the latter emits a shrill note. The sound given out by the receiver acts on the transmitter and this in turn, actuates the receiver. The energy is drawn in both cases from the battery.

The three-electrode valve so used is called a transmitting valve, and the sustained electric oscillations it can produce, as above described, can be transferred to an aerial wire and cause it to radiate continuous electric waves.

Very large thermionic valves are now made with glass or silica bulbs, a foot or more in diameter, for use as transmitting valves, and numbers of these can be associated together to form a thermionic generator of large power. In this case the high voltage required to pass the plate current through the valve is obtained by the use of a battery of Fleming two-electrode valves which rectify a high ten- sion alternating current of low frequency. A complete valve panel, as it is called, comprises the battery of rectifying valves, and three- electrode valves and also the necessary transformers, induction coils ami condensers (see fig. 5). Large valve panels are now constructed to transform electric power from I kilowatt to 50 kilowatts or more into high frequency electric oscillations of great energy.

Such valve generators are extensively used by Marconi's Wire- less Telegraph Co. and others for the production of continuous waves, and are employed at Clifden Station in Ireland for the trans- mission of wireless messages across the Atlantic Ocean.

XXXII. 33.

Detectors. We must in the next place notice the improvements which have taken place in means for detecting continuous waves (C.W.) as used in wireless telegraphy. The reader may refer to the earlier article on Wireless Telegraphy (see 26.535) for an account of the principal appliances used in connexion with spark or damped wave telegraphy for the detection of electric oscilla- tions, and especially to the section on the oscillation valve or two-electrode thermionic detector, from which other types of im- proved thermionic detector have been developed. Subsequently to the introduction of the two-electrode, but prior to the advent of the three-electrode thermionic detector, much use was made of crystal or rectifying detectors.

It will be remembered that the electric waves sent out from the transmitting aerial wire, which are identical in nature with light waves except for their much greater wave length, fall upon an aerial wire at the receiving station, and create in this latter extremely feeble, high frequency electric currents or oscillations which are a copy on a very reduced scale of the electric oscillations established in the transmitting aerial. The strength of these feebly received currents reckoned in amperes (l r ) can be approximately computed from the strength of the currents in the sending aerial (I.) reckoned in amperes by means of an empirical formula valid up to about 2,000 m. due to L. W. Austin and L. Cohen which is as follows:

where h, and hr are the heights of the sending and receiving aerial wires in kilometres, d is the distance apart of the stations in kilo- metres, X is the wave length in metres, the base of the Napierian logarithms, and R the total resistance of the receiving circuits in ohms. 1

The received currents may be something of the order of 5-10 microamperes more or less in the case of long distance working. To detect these feeble oscillations special appliances called detec- tors are in use. The so-called rectifying detectors do this by con- verting the received oscillations into feeble unidirectional currents, which in the case of damped waves are equivalent to short gushes of electricity in one direction corresponding in frequency to the condenser discharges in the transmitter. These can then be detected by a telephone, as they create in the latter a musical sound agreeing in pitch with the wave group frequency, and this, by the action of the key in the transmitter is cut up into dot and dash audible Morse signals. One of the first rectifying crystal detectors was carbo- rundum discovered in 1906 by H. H. C. Dunwoody in the United States. This material is a crystalline carbide of silicon produced in electric furnaces, and, in certain specimens, as shown by G. W. Pierce, has an electric conductivity 40 or 50 times greater in arc direction than in the opposite along one crystalline axis.

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FIG. 5. Valve panel for generating high frequency oscillations in the transmitters for wireless telegraphy, as made by Marconi's Wireless Telegraph Co., Ltd. (By permission.)

The same properties are exhibited by hessite and anatase as well as by molybdenite and other native sulphides. Furthermore, it was found by L. W. Austin and G. W. Pickard that contacts between certain pairs of substances such as tellurium and aluminium, or zincite and chalcopyrite, also plumbago and galena, have the same kind of unilateral conductivity and can be employed for " rectify-

1 The problem of predetermining the electric and magnetic force at any point on a conducting sphere due to a Hertzian oscillator at some point on it is a very difficult one. The reader will find references to the work of Macdonald, Nicholson, Love, Rybczyn- ski, and others in The Principles of Electric Wave Telegraphy, Fleming. 4th ed. chap, ix., and also in a paper by Balth van der Pol in Phil. Mag., vol. xxxviii. (Sept. 1919). The final result is that diffraction alone will not account for long distance radiotelegraphy.