190 Mr. Appleton and Dr. B. van der Pol on a Type of 



In the theoretical discussion given above, it was found 

 that for conditions represented by Region 2 (see fig. 4) it 

 should be possible to start a stable oscillation by means of 

 an electrical impulse greater than a certain amount. This 

 theoretical prediction was experimentally verified in a 

 qualitative way by using transient induced electromotive 

 forces produced with the aid of a coil and a bar magnet. 



From the point of view of certain practical applications of 

 triodes (such as triode relays and quiescent aerial telephony), 

 it is of interest to consider how the difference between the 

 limiting parameters necessary for starting an oscillation and 

 those necessary for stopping one vary with the values of the 

 maintained electrode potentials of the triode. Experiments 

 were therefore carried out to test this point. 



A triode that had been previously carefully tested for 

 steadiness was used in the circuit of fig. 1, care being taken 

 to allow the thermal conditions of the triode and its supports 

 to become steady before measurements were made. The 

 resistance R of the oscillatory circuit was non-inductive 

 (CuS0 4 solution) and continuously variable. The values of 

 the limiting resistances for starting and stopping an oscilla- 

 tion were found for various values of the applied steady grid 

 potential v g0 . The existence of a sustained oscillation was 

 indicated both by means of an autoheterodyne circuit some 

 metres away and also by means of a loosely-coupled sensitive 

 detector-galvanometer circuit. The experimental results 

 are indicated in fig. 7, where the ordinary grid potential- 

 anode current characteristic is also shown. 



It will be seen that there is a region of grid potentials 

 (<?. g., from — 5 to +1 volts in fig. 7) in which the limiting- 

 resistances for starting and stopping the oscillations are 

 the same. For these values of grid potential the relation 

 between the oscillation amplitude and the resistance was 

 quite reversible, no hysteresis of the type illustrated in fig. 5 

 being apparent. 



For other values of grid potential the limiting conditions 

 for starting and stopping an oscillation were not the same. 

 For such cases, evidence of oscillation hysteresis could always 

 be obtained. It was also noticed that in every case the 

 oscillations started suddenly with a large amplitude, when 

 the value of R was continuously reduced to the critical value 

 (R starting) shown in diagram, and that also on increasing 

 R again the amplitude fell suddenly to zero when a certain 

 value (R stopping) was reached. 



More careful experiments were afterwards carried out to 

 test whether the limiting conditions for starting and stopping 



