ELECTRON THEORY KLOEFFLER 



251 



travel through space very far. The various low-frequency signals to 

 be transmitted are impressed upon the high frequencies by a process 

 of forming, molding, or modulating. At the receiving end of a radio 

 circuit the impressed signals must be demodulated or detected and 

 thus recovered from the high-frequency carrier. 



The three-electrode tube or triode is used first as an oscillator in a 

 resonant circuit for producing the high-frequency carrier current. 

 Secondly, this type of tube is used to modulate or mold the low- 

 frequency signal into the high-frequency carrier. In the third place, 

 the triode may be used in the receiver as a detector or remover of the 

 low-frequency signal from the transmitted carrier. Lastly, triodes 

 are used in cascade for amplifying the feeble received signal so that 

 it may be suitable for the desired purpose. 



Additional electrodes, usually some type of grid, have been added 

 to the three-electrode tube to increase the range of amplification, to 

 improve the fidelity of amplification, or to 

 prevent oscillations. The heavy loading 

 of a triode by wide fluctuations in the 

 grid potential and the resulting plate cur- 

 rent may introduce distortion of the ampli- 

 fied signal and may produce oscillations 

 because of the feedback of the grid-plate 

 capacitance. To reduce such undesirable 

 results, a fourth electrode known as a 

 screen grid may be introduced between 

 the control grid and the plate, as shown 

 in figure 6. This screen grid is connected 

 to a source giving it a fixed potential 

 somewhat under the normal plate voltage. 

 The screen then serves to establish a fixed 



potential in space giving a constant attraction upon all emitted elec- 

 trons regardless of any variation of plate voltage and it greatly 

 reduces the grid-plate capacitance. 



A curious phenomenon may result in the use of the screen-grid tube 

 if a low plate voltage be encountered. Thus, at all positive values of 

 plate voltage above 10 to 20 volts, the electrons hitting the plate 

 produce secondary emission (splash out electrons from the plate). In 

 the triode these electrons of secondary emission are attracted back to 

 the plate but in the screen-grid tube for a certain range of low values 

 of plate potential, the splashed-out electrons are attracted back to the 

 screen grid (fig. 7). In special cases it is possible for more electrons 

 of secondary emission to go to the screen grid than there are electrons 

 arriving at the plate. Under this condition the plate current will 

 reverse or become negative. Even though the plate current does not 

 reverse, the electrons of secondary emission, which pass to the screen 



Figube 6. — Diagram of four-electrode 

 tube or tetrode. 



