﻿164 Messrs. E. W. B. Gill and J. H. Morrell on Short 



much more closely to the calculable field between two co- 

 axial cylinders than does the field in the French type, wher^ 

 the grid is a spiral coil of fine wire. The diameter o£ grid 

 used was 1 cm. and that of the plate was 2*5 cm. to an 

 accuracy of about 5 per cent. 



4. The preliminary experiments were made with the 

 apparatus arranged as in fig. 2. The valve is shown dia- 

 grammatical ly : F is the filament, Gr the grid, P the plate, 

 LL the Lecher wires, which were of copper wire each about 

 850 cm. long and spaced 5 cm. apart. They were suspended 

 about 200 cm. above the floor from insulators secured to the 

 walls at each end, and from one end were leads about 70 cm. 

 long to the grid and plate respectively. The bridge consisted 

 of two equal condensers, C, 0, joined through the heater-coil 

 of a Paul thenno-j unction, T. The outer plates were fitted 



Fiff. 2. 



with contacts to slide along the Lecher wires. The capacity 

 of these condensers is unimportant, provided it is large com- 

 pared with the capacity of the valve. In practice, the 

 capacities were of the order of 1 milli-microfarad. The 

 terminals of the thermo-junction were connected to a gal- 

 vanometer by two long leads, which are not shown. The 

 sliding contacts were also connected to the negative side of 

 the filament-heating battery B, that on the grid-wire through 

 a high-tension battery V, and that on the plate-wire through 

 a potentiometer S. which could raise the potential of the 

 plate +6 volts above the negative end of the filament. 



Two sensitive milliammeters, A, A, gave the steady currents 

 through the valve to the grid and filament respectively. A. 

 rheostat, R, controlled the filament-heating. In all cases 

 potentials are measured with regard to the negative end of 

 the filament. 



electrons set free at the 



With this arrangement th< 



