THYRATRON 



situation is reversed and the thyratron conducts. The conduction period is 

 approximately half a cycle. If the grid is made more negative than 



(lyi^ X 350 

 Control ratio 



then the thyratron is prevented from firing at any time, i.e. the switch is 

 'off'. If the grid is made rather less negative than this, conduction will occur 

 over less than half a cycle and an intermediate amount of power is supplied 

 to the load. This is the theory of the grid-controlled rectifier, which is com- 

 plicated and probably of little interest in connection with biology. It is 

 worth mentioning if only to draw attention to the fact that it exists, and to 

 emphasize that in the design of simple on-oflf thyratron switches anomalous 

 results may ensue if care is not taken that the shut-off bias is large enough. 



131 



