472 



Mr. Gr. Stead on the 



ratio of the grid current to the anode current. This is shown 

 by the numbers in the following table, which were obtained 

 by measurements on the original characteristics. 



Distance from gr 

 to filament. 



id 



Maximum slope. 

 (V=40). 



Gr 



d current. 



An 



ide current. 



3 mm. 





0'6 milliamp. per volt. 





3-42 



7 „ 





022 





105 



10 „ 





O20 





089 



The second column gives the maximum slope of the anode 

 current-grid voltage curve for an anode voltage of 40. The 

 third column gives the ratio of the grid and anode currents 

 for the case when the anode voltage is 40 and the grid 

 voltage 15. 



(ii.) Experiments with a grid of variable spacing. — A dia- 

 gram of the special valve used' for these experiments is 

 shown in fig. 3. It was of cylindrical type and the grid 



Fig 1 . 3. 



consisted of a steel spring, of helical form, which could be 

 stretched by means of a thread attached to a small winch 

 W operated from the outside through the ground glassjoint G. 

 The anode was a brass cylinder about 3'5 cm. in length and 

 2 cm. in diameter. Small holes were drilled through the 

 anode cylinder so that the grid and filament were visible 

 from the outside. The filament was a straight tungsten 

 wire about 2 cm. long and *0082 cm. in diameter. The tube 

 connecting the winch with the valve was made long, so that 

 the ground glass joint could be outside the oven during the 

 baking process, and the conduction of heat from the oven to 

 the ground glass joint was prevented by coiling round the 

 olass tube a thin lead pipe through which cold water was 

 allowed to flow. 



