Electronic Principles
The core of an electromagnet is said to be in saturation when:

changing the polarity of the current in the coil changes the polarity of the magnetic field around the core
the core is permitted to move freely within the coil
frequent changes in the direction of current through the coil causes heating effects in the core material
an increase in current through the coil causes no further increase in the mmf
None of these

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Electronic Principles
A low-pass filter:

reduces the level of all frequencies by a significant amount
passes higher-frequency signals and reduces lower-frequency signals
passes lower-frequency signals and reduces higher-frequency signals
None of these
has no effect on ac signals

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