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Ing " high frequency oscillations. These crystal and rectifying detectors came at one time into great use in wireless telegraphy. 1

The limitation in the power of these crystal or rectifying detectors lay in the fact that the energy used in making the signal is only a portion of that captured by the receiving aerial from the incident waves. An immense improvement was therefore effected by the introduction of the three-electrode thermionic valve, which can act in the manner of a telegraphic relay and employs the received power merely to release a much larger amount of electric power from a local battery, which latter creates the signal in the telephone or other instrument: More- over, this type of detector is capable of being used in series so as to amplify or magnify enormously the signal-making power.

Pen terminals

FIG. 6. Thermionic amplifying and detecting valve of the type usually called the " French " valve with cylinder anode and spiral wire grid surrounding a straight filament.

FIG. 7. Views of various types of three-electrode thermionic valves; (a) detecting and amplifying valve; (6) transmitting or gen- erating valve; (c) amplifying valve of a type made by Marconi's Wireless Telegraph Co.; (d) small transmitting valve with gauze grid.

The modern hard or high vacuum thermionic valve as used for reception and amplification is now generally constructed as fol- lows: a small glass bulb or tube, a few inches in diameter, has sealed into it a filament which can be rendered brightly incandescent by current from a 2-to 3-cell storage battery (4-6 volts). This fila- ment is of drawn tungsten wire, or else platinum coated with oxides of barium and strontium. The bulb is highly exhausted. Around the filament and close to it is coiled a spiral of nickel wire called the grid, and outside that a cylinder of nickel called the plate. The plate and grid are carried on wires sealed through the bulb, and connexions to the grid, plate and filament are brought to four ter- minal pins at the base fixed to a brass collar (see figs. 6 and 7). These pins fit into a suitable socket. A battery of 30 to 200 volts E.M.F. has its negative terminal connected to the filament and positive to the plate, and when the filament is incandescent a stream

1 A fuller description of these rectifying detectors is given in The Principles of Electric Wave Telegraphy and Telephony by J. A. Fleming, 4th ed. chapter vi. (1919).

of electrons (atoms of negative electricity), called the plate cur- rent, flows from the filament to the plate through the apertures in the grid wires. This current completes its circuit through a coil of wire in the external plate filament circuit which may be one coil of a transformer. If the grid has a small negative charge given to it the plate current decreases, and, if a positive one, the plate cur- rent increases. The relation of plate current to grid potential can be delineated by a characteristic curve (see fig. 4). For a certain positive grid potential the plate current becomes saturated and ceases to increase. If the grid and filament are connected to the terminals of the receiving condenser in a wireless telegraph aerial, the incidence of electric waves on the aerial will create alternations of potential in the grid and alternations of plate current, and the amplitude of the plate potential may be five to ten times greater than that of the grid. The thermionic tube is then said to have an amplifying power of five to ten.

If the coil in the plate circuit forms the primary coil of a two- coil transformer the secondary circuit of the latter may be con- nected to the grid and filament of a second valve, and a second amplification of potential may take place. We can thus employ a series of valves in cascade and the total amplification increases as the nth power of the number of valves (n) in cascade. Thus if one valve amplifies potential ten times, three valves will amplify 1,000 times and so on.

This use of thermionic valves in cascade has given us detectors of enormous sensitivity. In order to detect damped oscillations we can adopt one of two methods. If we place a small condenser with a leak across its terminals in the grid circuit then the side of this condenser next the grid becomes negatively charged, and the plate circuit of the valve is reduced/ This charge leaks away almost instantly and the plate current'of the valve rises again. Hence if the incident waves are in " damped trains " a telephone receiver inserted in the plate circuit of the valve will give a sound of the pitch of the train frequency, and this can be cut up into signals. In this case the valve is used as a rectifier as in the case of the Fleming valve. The second mode of use depends upon the form of the characteristic curve. If we employ a small battery of cells to give the grid a certain positive potential we can operate the valve at a point on the curve near to the saturation point so that a small reduction in the potential of the grid lowers the plate current, but a small increment of potential does not increase it. Hence if the grid is connected to one terminal of the tuning condenser of a receiving aerial it will oscillate in potential when a train of electric waves falls on the aerial. This, however, will cause a drop in the plate current and hence a sound in a telephone receiver included in that circuit. If the incident waves are damped or intermittent trains the result is to make a steady musical sound in the telephone of the pitch of the train frequency. Accordingly, by interrupting the groups by the sending key, audible Morse signals can be received.

The above described methods of reception are however only applicable in the case of damped or intermittent trains of waves. If the electric waves are continuous as sent out by an alternator, arc, or valve transmitter, then we can only detect signals made with them by converting the continuous waves into the equivalent of a | series of damped trains. This is done by generating in the receiving aerial electric oscillations by a local valve generator which have a i frequency differing from that of the incident waves by about 300 to 1,000. The result is to create in the receiving aerial resultant electrical oscillations which fluctuate periodically in amplitude just as " beats " in musical sounds are produced when two organ pipes slightly out of tune are sounded together. The number of ' beats per second is equal to the difference in the frequency of the two separate oscillations. In the aerial wire these electrical beats can then be detected by any of the types of detector and receiver used in spark wireless telegraphy. This method is therefore called " beat reception." The beats disappear when the signal bringing waves are interrupted at the sending station in making the spaces between the Morse code signals.

One great use of the three-electrode valve, or triode, as it is some- times called, is in amplifying feeble signals. It has been explained already that when the grid of the valve is electrified positively or negatively it increases or decreases the plate current and, therefore, the plate potential. The amplitude of the plate potential varia- tions may however be five or ten 'times or more that of the grid potential variations. Hence the valve acts as a relay or magnifier of potential. Again we can interconnect a number of such triodes in series by induction coils so that the variations in plate current of one valve are made to vary the grid potential of the next. Hence by using a series of valves in cascade we can multiply potential variations of the grid of the first valve in a geometric progression and enormously magnify them. The remarkable achievements of modern long distance wireless telegraphy are chiefly due to the use of such cascade amplifiers. In fig. 8 is shown a view of such a detec- tor made by Marconi's Wireless Telegraph Co. in which six valves are used as amplifiers and a seventh valve as a detector. So sensi- tive are these cascade receivers that it is not necessary to employ any long aerial wire to receive wireless signals from distant stations. It suffices to construct a large rectangle of a few dozen turns of insulated wire called a frame aerial and connect this in series with a condenser of suitable variable capacity and tune the arrangement