392 
DR. W. S. TUCKER AND MR, E. T. PARTS ON 
this circular opening, and through these the wire is threaded to form a zig-zag grid 
as shown in the figure. To each side of the mica plate is attached an annular plate 
of silver foil (shown by shaded portion in the figure), and to each of these plates is 
soldered one end of the platinum wire. The two plates of silver foil thus constitute 
the electrodes of the wire grid. 
Fig. 1. B, shows another form of grid, for the design and manufacture of which we are 
indebted to the Besearch Department of the General Post Office. The wire is bent 
into three loops, and is supported by a small 
rod of glass-enamel placed diametrically across 
the opening in the mica plate. 
The grids are made in the first place with 
Wollaston wire, the silver sheath being removed 
by means of nitric acid after the wire has been 
mounted in position. During this part of the 
process of manufacture the silver foil electrodes 
are protected by a coating of paraffin wax. 
The grids used in the experiments described in 
this paper were, unless otherwise stated, of the 
type shown in fig. 1, B. The wire, the diameter 
of which was about 0 -0006 cm., carried a maxi¬ 
mum current of about 30 milliamperes, the 
exact amount varying in individual grids ac¬ 
cording to the sample of wire used in their 
manufacture. The average resistance at 10° C. 
was about 140 ohms, and about 350 ohms when 
carrying a safe working current of 25 to 28 
milliamperes. In the case of a particular grid, 
it was found that its resistance was 125-5 ohms 
at 0° C., 133 ohms when carrying a current of 
0 -5 milliampere, 156 -2 ohms at 100" C., and 332 
ohms when carrying its working current of 32 
milliamperes. The working current heats the 
grid to just below red heat. 
For certain purposes grids were made from 
wire 0-0015 cm. in diameter and carrying a 
maximum current of about 55 milliamperes. 
The number of loops was increased in these 
grids to eight, four on each side of the glass- 
enamel support. 
(ii) The “ Holder .”—The manner in which the holder is made up is shown in fig. 2. 
The cylindrical neck A, made of brass, is soldered into the centre of the circular plate 
