Magnetic Fields

TM 11-6625-524-14-2

local radio transmitters, grid dip meters, and

noise is attenuated at the input by 60 db, it is still

amplifiers with parasitic oscillations.

possible for high level unwanted signals to get

d. Magnetic Fields. Operation of the voltmeter

through and cause errors. The best test for this

in strong 60 Hertz (Hz) magnetic fields, such as

condition is to reduce the test signal to zero level

t h o s e surrounding unshielded power trans-

and note whether the voltmeter continues to read

formers, should be avoided. The magnetic field

some spurious signal level. In some cases, extra

induces small 60 Hz currents in the amplifier

shielding may be necessary around the probe

section of the instrument which, due to the ex-

connections to reduce stray field pickup. Typical

tremely high gain at this frequency, appear as an

sources of spurious radiation are induction or

dielectric heating units, diathermy machines,

indication on the meter.

3-4