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of illumination. To meet this need the Ferranti Valve the baking and processing schedules. The glass envelope
Development Laboratories produced a range of single is silicone coated to render it moisture repellent and
tetrode and double tetrode indirectly heated electro- carries internal and external guard rings R 1 and R 2
meter valves, employing oxide-coated cathodes. These respectively. The internal guard ring, on which part
valves were of normal radio valve dimensions and the of the barium getter film is deposited, is connected to
aim eventually became that of producing similar type the anode via the spring contact S. The external guard
electrometer valves of smaller dimensions and, if ring is connected to the anode pin of the base via the
possible, more rigid construction. nickel gauze ring N.
Directly heated subminiature electrometer valves had In assembling the valve particular attention is paid
already been produced by home and overseas manufac- to ensuring that
turers but, in many cases, they did not incorporate (i) The control grid is mounted (via its insulators)
guard rings and the valves were far more unstable than rigidly relative to the anode,
their predecessors. Such valves, it is true, satisfied the
immediate need for a means of detecting radiation, and (ii) The micas hold the screen and the cathode rigid
for short-term measurement, but many of them were relative to the anode.
relatively useless for quantitative, long-term, or repeti- The cathode is rigidly connected to the bottom mica
tional measurements-due to their excessive sporadic and is able to expand through the central hole in the
drift of characteristics. In consequence our desire top mica. The screen support wires are rigidly con-
became that of miniaturising the indirectly heated valve nected by a nickel bridge and one of them is welded
whilst retaining, unless able to improve upon, our to base pin 6. The bottom mica is picot-edged and
existing methods of obtaining high stability-and the grips the internal surface of the glass envelope.
outcome has been the development, on a B7G base, of
the indirectly heated single tetrode electrometer valve The order of location of the electrodes (starting from
type BM30. the centre) is heater, cathode, screen (or accelerator)
grid, control grid and anode. The guard rings are both
DESIGN connected to the anode.
The BM30 is shown, full size, in Fig. (2). The com- ELECTRICAL CHARACTERISTICS
ponent parts are, as shown in Figures (1) and (3), as OF THE BM30
follows:-
The cathode C, in the form of a nickel tube carrying
the oxide-coating 0, is supported coaxially by the disc
micas M 1, M 2 within the screen grid g 1 and the anode
A of the valve and surrounds the heater h which con-
sists of a v-shaped spiral of tungsten wire coated with

an insulating refractory material. The control grid g 2
is located between, and coaxial with, the anode and the
screen grid and is supported (by the insulators I 1, I 2)
so as to be parallel with them and everywhere out of
contact with the micas. The insulators I 1 and I 2 take
the form of metal/glass tubular seals, as shown more
clearly in the section of I 1 (Fig. (l)), and are rigidly con-
nected to the anode by the metal straps S 1, S 2. During
their construction the glass tube is fused centrally, by
the application of heat, to cause it to collapse upon the
straight grid support wire (whose coefficient of expan-
sion matches that of the glass) with which it forms a
seal and, during the process of fusion, its ends are suit-
ably supported to maintain them coaxial with the
control grid support wire. The tubular ends of the
insulator now enshroud the grid support wire and pre-
vent the deposition on the insulator, either during the
process of flashing the cathode or of firing the getter,
of a conducting film joining the control grid to the
remainder of the assembly. The control grid connection
is also surrounded by a similar type glass-sleeve insula-
tor I, which is fused into the top of the bulb of the
valve. The assembly is suitably mounted on a B7G Fig. (4)
base and the contact spring (S) for the internal guard
ring, and the getter frame (F), are added (see Fig.. 1) Characteristic curves for the BM30, for each of the
before it is sealed into the cylindrical glass bulb (Fig. three sets of typical operating conditions specified, are
3) which is then evacuated and stored in readiness for shown in Figs. (4) and (5).

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