208 



DISCOVERY 



phone receiver, which was joined up to the ends of the 

 coil. 



In the electrolytic type of detectors, as in coherers, 

 the oscillatory currents reduced the resistance of the 

 circuit in such a way as to allow the passage of a current 

 suitable for working a telephone receiver. The detector 

 consisted of two dissimilar metals, such as platinum 

 and lead, immersed in an acid solution, the platinum 

 having only a small point, the lead a large surface, in 

 contact with the liquid. Wlien at rest, a non-con- 

 ducting film formed on the point, causing the circuit 

 through the liquid from one metal to the other to have 

 a high resistance. Oscillatory currents disturbed this 

 film, and the resistance decreased so that the current for 

 working the telephone could flow through the circuit. 



In 1904 Professor J. A. Fleming in this country 

 invented a detector which he called the Oscillation 

 Valve. This consisted of an incandescent lamp with 

 the addition of a metal plate, or cylinder, inside the 

 bulb, connected to a wire sealed through the glass. 

 When the lamp is glowing, the space between the 

 filament and the plate is electrically conductive, but 

 in one direction only — that is to say, it offers a very 

 great resistance to a current in one direction, but very 

 little resistance to a current in the opposite direction. 

 If, therefore, an oscillatory current is applied, it 

 becomes, for all practical purposes, a unidirectional 

 pulsating current which can be used for working a tele- 

 phone receiver. The device acts, in fact, as a valve for 

 the oscillations, allowing current to flow in one direction 

 but not in the other, and was therefore called an 

 oscillation-valve, the resulting current being called a 

 rectified current. 



Soon after the introduction of the Fleming Valve, 

 another form of receiver, called the Crystal Detector, 

 was introduced. The first practical form was used by 

 General Dunwoody in the United States, and consisted 

 of a crystal of Carborundum in contact with a piece of 

 metal. Various combinations of crystals and metals 

 give a rectifying effect— that is, they convert an oscil- 

 latory current, apphed across the contact, into a form 

 suitable for operating a telephone receiver. 



These Crystal Detectors are still very extensively 

 used in ships and small stations, but the Three Electrode 

 Valve, which was developed during the war, is now used 

 for all long-range work. 



This form of oscillation-valve may be used as a 

 generator of electric oscillations as well as a receiver, 

 and is such an excellent receiving and magnifying 

 device that its introduction has undoubtedly opened 

 up new possibilities for wireless communication. 



In tliis valve, as its name implies, there is another 

 electrode in addition to the filament and the plate as 

 shown in Fig. 3. This electrode, called the grid, is 

 usually in the form of a cyhnder of metal gauze 



between the filament and the plate, which may take the 

 form of an outer metal cyhnder. If the telephone 

 receiver is connected up in series with a battery 

 across the filament and plate, there will be no sound 

 in the telephone ; but if the grid is connected to the 

 receiving circuit so that the oscillations produced in the 

 latter by the incoming signals cause variations of 

 electrical pressure on the grid, then the space between 

 the filament and plate becomes conductive, and currents 

 from the battery flow across this space and produce 

 sound in the telephone. 



The valve may thus be looked on as a relay,' the 

 minute oscillatory currents received being used to 

 release ciurents from a battery which can work a 



P = Plate. G=Gricd. F=Filainent. 



telephone or other suitable receiver. This relay action 

 may be further utilised by connecting up a number of 

 valves in such a way that the released battery current 

 from one valve is applied to the grid of the ne.xt one 

 so as to release a larger current, and so on. 



When used in this manner to increase the effects of 

 the received currents, the valves are said to be used 

 as magnifiers ; but there is a practical limit to the 

 amount of magnification that can be produced by 

 increasing the number of valves. It is this magnifi- 

 cation effect that makes the Three Electrode Valve 

 such an enormous advance over all other receiving 

 devices, and it is also one of the properties which make 

 it so valuable as a wireless transmitter. 



' .\ relay is an instrument which enables a small current to 

 start the operations of a larger one. 



