TM 5-3820-256-10
c. Resistivity Instrument. The resistivity instrument uses direct current and is of the 'null' reading type. The
instrument reads directly in ohm-feet. The instrument's controls, shown in figure 2-41, operate as described In
the following paragraphs.
(1)
Galvanometer. Zero-centered micro-ammeter. All operations involve returning the meter to its zero
position.
(2)
SP shut-off switch. Located in upper left corner of the panel. Spring loaded switch to automatically
disconnect the circuit from the 1-1/2 volt 'C' battery when the instrument lid is closed.
(3)
Self-potential potentiometer. Balances the spontaneous potential that always exists between any pair of
potential electrodes. Do this before making any resistivity reading. Balance is indicated when the
galvanometer needle goes to zero. If the SP curve is desired, merely record potentiometer reading. Each
small division equals one millivolt; full scale is 1 volt.
(4)
SP Polarity-reversing switch. Located directly below the Self-potential potentiometer; it indicates the
polarity of the particular cable electrode being used and enables the operator to change the polarity of the
injected voltage as required by bore-hole conditions.
(5)
Function switch. A three-position selector switch located under the galvanometer. Its three positions are:
(a)
CUR (current), to check the system to be sure whether or not the proper amount of current is being
used;
(b)
CAL (calibrate), to calibrate the instrument, as is required at the start of the logging operation and after
every 50 feet of logging; (c) LOG, to set the instrument for logging operations.
Figure 2-41. Resistivity Instrument
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