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Figure (4) shows the anode- current/control grid Screen
voltage curves (shown as continuous lines) together with Current (I g1) 316 185 115 µA.
the corresponding screen current/control grid voltage Mutual Con-
curves (broken lines). The mutual conductance of the ductance (gm) 65 54 32 µA/V.
valve, given by the gradient of the Ia/Vg 2 curve at the Amplification
working point, is seen to be of the order 60µA/V for Factor (µ) 1.5 1.7 1.7
operating conditions (1) and (2). Control Grid
Current 13.5X10 -15 5X10 -15 2X10 -15 amp.
Control Grid
Insulation (rg 2) 2X l0 14 4 X lO 14 10 15 ohms.

GENERAL DATA

Physical Details:

Base ... ... ... B7G
Top Cap ... ... Type CT1

Max. Overall Length.. . 60mm. (2 / 8in.)
3
Max. Seated Height ... 53mm. (2 / 16in.)
1
7
Fig. (5) Max. Diameter ... 22mm. ( / 8in.)
Inspection of the corresponding control grid current/ Mounting Position ... Any
control grid voltage curves, shown in Fig. (5), reveals: All dimensions
shown are in
(i) The low magnitude of the control grid current Envelope: Silicone coated clear glass millimetres
(max.).
at the recommended working points.
(ii) The relatively low value of the free grid potential
of the control grid. Base Connections:
(iii) The relatively extensive range of control grid
voltage over which the control grid current is Pin 1 Cathode (k).
within the specified range.
Pin 2 Internal Connection.
Heater: Pin 3 Heater (h).
Heater Voltage ... ... 4.0 volts Pin 4 Heater (h).
Heater Current ... ... 0.125 amp.
Pin 5 Internal Connection.
Ratings (absolute values):
Max. Anode Voltage ... 8.0 volts Pin 6 Screen (g l). Base
Max. Screen Voltage ... 6.0 volts Pin 7 Anode (a) and Guard Connections
Rings.
Interelectrode Capacitances: TopCap Control Grid (g 2). Underside View
of Base
... ... ... ... 0.8 pF.
C in
C out . .. ... ... ... 1.9 pF. RECOMMENDATIONS FOR OBTAINING
C a-g2 ... ... ... ... 5.8 pF.
C a-g1 ... ... ... ... 0.8 pF. OPTIMUM PERFORMANCE FROM THE BM30
The electrometer valve, as a means of measuring
Typical Operating Conditions: potential difference, can be used for measuring very
Anode small currents either by (i) measuring the potential
Voltage (V a) 6.0 6.0 5.0 volts. difference when the current passes through a high re-
Screen sistance of l0 ohms or higher, or (ii) measuring the
11
Voltage (V g1) 6.0 4.0 3.0 volts. rate of change of potential of an insulated conductor
Control Grid charged by these currents. To achieve maximum
Voltage (V g2) -2.5 -2.0 -2.0 volts. stability and sensitivity for measurement of such cur-
Anode rents of less than 10 -9 ampere it is advisable to apply
Current (I a) 120 85 35 µA. as many as possible of the following precautions:

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