NATURE 



[August 5, 1920 



method of employing it as a detector was as 

 follows : 



If we connect the plate of the valve with the nega- 

 tive terminal of the filament-heating battery, and 

 insert in that circuit a battery for creating a 

 thermionic current, we can delineate a characteristic 

 curve, as already described, by varying the E.M.F. 

 of the plate circuit battery. That curve has generally 

 some places in it at which the slope changes rather 

 quickly. If we adjust the E.M.F. of the plate battery 

 to work at that point, and then by means of a trans- 

 former superimpose a feeble oscillatory E.M.F. 

 derived from a wireless receiving aerial, the thermionic 

 current will oscillate from one value to another, and 

 it is easy to see from the concave form of the charac- 

 teristic curve that the mean value of this varying 

 thermionic current is greater than the value of the 

 steady thermionic current when the oscillations are 

 not superimposed on the steady or battery voltage. 

 This mode of usage in the case of valves with a 

 certain degree of exhaustion in the bulb gives very 

 great sensitiveness in the detection of radio-signals. 

 It is commonly called the potentiometer method 

 because the extra steady voltage required in the plate 

 circuit is derived by employing a fraction of the 

 voltage of the battery used for incandescing the fila- 

 ment by means of a potentiometer resistance. 



This is, perhaps, the place to refer to another view 

 of the mode in which my valve acts even when no 

 additional E.M.F. is placed in the plate circuit. The 

 characteristic curve of a valve is found not to start 

 exactly from the point of zero voltage, but from a 

 point on the negative side about | to i volt. This 

 means that if the plate is connected to the negative 

 terminal of a filament battery by a wire, there is 

 found to be in it a small negative electric current 

 flowing from the plate through the external circuit 

 to the negative terminal. The reason probably is that 

 the electrons are shot out of the filament with a 

 certain velocity and accumulate round the plate. 

 The result is a tendency for them to diffuse back 

 through the external circuit, creating a feeble electron 

 current which can be stopped only bv introducing a 

 small counter E.M.F. into* that circuit. Hence the 

 characteristic curve starts from a negative point on 

 the voltage axis. At the place where it crosses the 

 zero voltage point that curve is concave upwards, 

 and hence, for the reason just explained, the intro- 

 duction into the external thermionic circuit of a feeblf^ 

 alternating high frequency electromotive force will result 

 in an increase in the mean or average thermionic 

 current. Hence the valve is sensitive to feeble elec- 

 tric oscillations and rectifies them, not by quite sup- 

 pressing all current irt one direction, but because the 

 thermionic current is greater for a given E.M.F. 

 applied in one direction in the thermionic current than 

 when that E.M.F. is applied in the opposite direction, 

 whilst the mean value of the thermionic current 

 throughout the complete cycle is greater than its 

 value when the alternating E.M.F. is not applied. 



We must now turn to consider an improvement 

 which was introduced in 1907 into the thermionic 

 valve, for which credit must be given to Dr. Lee de 

 Forest. He placed a crid or zigzag of wire carried 

 on a separate leading-in wire between the plate and 

 the filament of my valve, and therebv made what is 

 now called a three-electrode valve (Fig. i). 



In modern thermionic devices the grid takes the 

 form of either a spiral wire or else a metallic gauze 

 cylinder, which surrounds the filament without touch- 

 in£f it, and is in turn surrounded bv thf" plate or 

 cylinder which does not touch the grid. This addi- 

 tion enables the valve to act as an amplifier of electric 

 oscillations as follows : 



Suppo«;e we insert in the external plate circuit a 



NO. 2649, VOL. IO5I- 



battery Bj (see Fig. i) giving an E.M.F., say, of 

 100 volts, and also a current-measuring instrument A. 

 If the battery has its positive terminal connected ta 

 the plate, the stream of electrons emitted by the fila- 

 ment will be drawn to the plate and give a thermionic 

 current of three or four milliamperes if the valve is 

 highly exhausted. This stream of electrons will reach 

 the plate by shooting through the holes or inter- 

 spaces in the mesh or spiral grid G. 



Let us now suppose that we give the grid a small 

 negative charge by a battery B3. This will cause the 

 electrons coming out of the filament to be partly 

 repelled, and therefore the thermionic current in the 

 plate circuit will be reduced perhaps even to zero. 

 Again, let us give the grid G a small positive charge. 

 This will attract the emitted electrons", and they wilt 

 shoot through the grid with increased velocity. 

 Therefore the thermionic current will be increaseci. 

 The important point to notice is that, owing to the 

 small electrical capacity of thf^ grid, and also owing 

 to the high voltage acting in the plate circuit, a very 

 small expenditure of power on the grid circuit will 

 vary or modulate a much larger amount of power in- 

 the plate circuit. Just as the pressure of a child's 

 finger on the switch may start or stop an electric 



z£ 



/±^ 



3+ 



1 



\j 



j- 



C^I.hZI 



I 



Fig. I. — Conventional diagram of a three-electrode valve. 

 P, a metal plate or cylinder in a highly extiausted 

 glass bulb. G, a grid or perforated plate or spiral wire. 

 F, the lamp filament. Bj, the filament-heating battery. 



motor of several horse-power, or a feeble current 

 passing through a telegraph relay start or stop a large 

 current, so the three-electrode valve acts as a relay. 



If we plot a curve delineating the variation of 

 thermionic current with varying grid voltage or 

 potential for such a three-electrode valve, we find that 

 curve over wide limits to be nearly a straight line. 

 This means that the change in plate current is pro- 

 portional to the change in grid voltage. However 

 rapidly the grid voltage rnay change, so nimble are 

 these little electrons that the thermionic current 

 copies on a magnified scale the changes of grid 

 potential. Hence the arrangement is called a 

 thermionic amplifier. 



We can, however, advance further. If we cause 

 the plate current of one valve to pass through the 

 primary coil of a transformer, and then connect the 

 terminals of the secondary coil of the latter respec- 

 tively to the grid and filament of a second valve, we 

 find that the fluctuations in the plate current of the 

 first valve can be made to generate exalted potential 

 variations of the second valve, and this again to 

 create magnified variations of the plate current of the 

 second valve. This mode of connection is not limited 



