22-2. Electronic Ammeters. The oscilloscope may be used to measure currents, although an indirect method must be adopted. In this measurement, the current to be measured is passed through a calibrated resistor, the resulting voltage across the resistor being given on the oscilloscope screen. The current may then be calculated from the known voltage and resistance. However, owing to the fact that the input impedance of the oscilloscope is only moderately high, the oscilloscope is limited to the measurement of relatively high
Fig. 22-12. triodes.
A d-c voltmeter employing
Fig. 22-12. triodes.
A d-c voltmeter employing currents, of the order of that possible with good-quality d-c instruments, say 0.1 juamp.
Electrometer Tubes. The amplification properties of a vacuum tube may be used to amplify extremely small currents, which are then indicated on a sensitive galvanometer. In order to measure these small currents, it is essential that the grid current of the tube be very small compared with the currents to be measured. Normal tubes, if selected for quality, and if operated at reduced electrode voltages, are satisfactory over rather wide limits. Special tubes with unusually small values of grid current are available. These include, among others,
A circuit utilizing a General Electric FP-54 electrometer tube and operating from a 12-volt storage battery is given in Fig. 22-13. Even though the FP-54 tube is a low-grid-current type, great care must be exercised in keeping leakage and surface currents to a minimum if high sensitivity and dependable operation are desired. This is frequently accomplished by incorporating the entire circuit in a suitable probe type of construction, which is shielded electrostatically and which is kept dry by the use of a drying agent. Also, the grid current is kept small by driving the circuit at low potential from batteries, the circuit being kept in continuous operation for long periods of time in order to reduce tube drift. With the simple circuit shown, the indicating instrument being a sensitive wall galvanometer, currents of the order of 10~14 amp may be reliably measured.
22-3. Phasemeters. Lissajous Patterns. A Lissajous pattern is the figure created on an oscilloscope screen when sine-wave voltages are applied to both the horizontal and vertical deflecting plates. If the frequencies of these two component voltages are the same but differ in phase, the resulting pattern is a measure of the phase difference between the two waves.
General Electric Co. FP-54 Ig ~ 10"15 amp Victoreen Inst. Co. VX-41 I„ ~ 10~15 amp Raytheon Mfg. Co. CX-570A Ig ~ 10~u amp
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