Magnetic Fields; Operating Practices; Mechanical Operation Of Probe; Degaussing Of Probe Head - HP 428B Operating And Service Manual

Model 428B

3-19. Magnetic Fields.

3-20. If the jaws of the probe are incompletely closed,
the magnetic shielding and the magnetic circuit will have
an air gap. The result is that dc fields, not associated with
the dc current being measured, will cause a shift in the
meter reading.
3-21. However, there will be an indication of a strong
external dc field even with the jaws perfectly closed.
Usually zero setting with the ZERO control compensates
such residual readings for a particular probe location.
3-22. EARTH'S MAGNETIC FIELD. The earth's magnetic
field will affect the reading if the jaws of the probe are not
completely shielded (jaws partially open). The effect of this
field is relatively strong - comparable to deflection due to
about 500 mA of current. Complete closure of the jaws
can be checked by switching to the 1 mA range with no dc
current input. If the jaws mate properly, the zero set
should stay within 0.1 mA while rotating the probe head
with respect to the earth's magnetic field.
3-23. If the zero shift is greater, the mating surfaces of the
jaws need to be cleaned or the probe wiring may be open
(see Section V).
3-24. FIELDS OF PERMANENT MAGNETS. Meter
magnets have strong stray fields, which can cause shift in
the current indication. Such fields are detected by bringing
the closed probe in the area where the measurement is to
be made and observing the zero shift (1 mA range).
3-25. FERROUS WIRE. Wires made out of magnetic
materials can cause a current reading of 2-3 mA without
any connection to the wire. This fact is important as leads
of most transistors are made out of magnetic material.

3-26. OPERATING PRACTICES.

3-27. MECHANICAL OPERATION OF PROBE.

3-28. The probe jaws are opened by simply squeezing
together the two flanges on the probe body. An internal
spring returns the jaws to their proper position when the
flanges are released. (See Paragraph 3-3e.).

3-29. DEGAUSSING OF PROBE HEAD.

3-30. To demagnetize the probe, proceed as follows:
a. Insert probe into degausser at the rear of the
instrument (located on front panel of rackmount
models) with arrow on probe in same position as
arrow marked on chassis.
b. Depress degausser switch S3 to energize degausser.
c. Withdraw probe very slowly for the first few inches
while depressing the degausser switch until probe is
removed approximately one foot.
d. Zero instrument on 1 mA range with ZERO control
3-31. Under normal operating conditions, degaussing
6
may be necessary after measuring current on the
1 thru 10 AMP RANGE.
3-32. Normally, it takes about 10 seconds to degauss
the probe when using the above method (see
Caution, Paragraph 3-3f).

3-33. ELECTRICAL ZERO SET.

3-34. If the instrument cannot be zero set electrically
(with ZERO control) there are two probable causes:
1) Incomplete closure
Magnetization of probe head.
3-35. Dust deposits on the lapped surfaces of the
probe jaws create an air gap. If the jaws are not
completely closed, the earth's magnetic field will
affect the reading. With the RANGE switch at 1 mA,
rotation of the closed probe should not vary the zero
set more than 0.1 mA. Cleaning of the jaws will
restore proper operation conditions (see Section V,
Cleaning of Probe Jaws).
3-36. Magnetic shields protect the probe head from
stray magnetic fields. However, excessive dc
currents (such as short circuit discharge currents
from electrolytic capacitors, etc.) will magnetize the
probe. For demagnetization of probe head, see
Paragraph 3-29, Degaussing of Probe Head.

3-37. POLARITY OF CURRENT.

3-38. The arrow on the probe head indicates the
direction of the conventional current flow for upscale
reading. Reversal of the current flow direction will
reverse the indication on the meter (see Figure 3-2).

Figure 3-2. Polarity of Current.

of probe jaws, 2)