Page 35 - IOE_Winter 24

FROM THE ARCHIVES

The High Stability Electrometer Valve Type BM30
and its Performance in a Balanced Bridge Circuit

by W. BIRTWISTLE*, M.Inst.E.
The characteristics of the Ferranti Single Tetrode Indirectly Heated Electrometer Valve Type BM30 are outlined-
with particular reference to the means of securing high stability performance. The effects of operating the tube at
various electrode potentials are indicated and details are given of the operation of the valve in a single-battery circuit
which is stabilised against battery voltage variations by adjusting the resistance network, which supplies the requisite
electrode voltages, until the effect upon the output current of small changes in battery voltage is negligible.

The BM30. The BM30. The BM30.
Front view of (Actual size) Side Elevation
Assembly Fig. (2) Fig. (3)
Fig. (1)
INTRODUCTION ment normally involves supporting the control grid by
An electrometer valve is, ideally, a device for measur- suitably sheathed beads, or columns, constructed either
ing an electromotive force without extracting energy of glass or of highly polished amber.
from the source producing the electromotive force- The need to measure currents smaller than those
and such a valve, employing a perfectly insulated con- formerly considered infinitesimal became evident as the
trol grid maintained negative with respect to the cath- study of photoelectric effects and of radioactivity con-
ode, would be able to control the anode current (used tinued, with the result that the relatively stable directly
as a means of measuring the control grid potential) heated electrometer valve, whether of the form of the
without loss of charge. In practice, however, such ideal proximity-grid triode or of the more conventional triode
insulation cannot be approached by using standard and tetrode assemblies, became an object for improve-
valve design techniques. The control grid of the valve, ment. The existence of electrometer valve devices
for example, cannot be mounted on mica, as is the sufficiently sensitive to measure, under ideal conditions,
conventional radio valve technique, for although current of the order of 10 -16 ampere-and to likewise
selected specimens of commercial radio valves may detect currents of the order of 5 X 10 -18 ampere-was
initially have grid currents of less than 10 -9 ampere much appreciated but amplifiers free from long term
such currents would increase appreciably during the life drift were now desired for such purposes as the measure-
of the valve. By special construction and operation it ment of ion current in mass spectrometer and ionisation
is possible to reduce the control grid current of an chamber work and of vacuum photocell currents pro-
electrometer valve to 10 -16 ampere and such achieve- duced during the measurement of very small intensities
*Ferranti Ltd.
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