The electric clocks in my house vary (relative to the correct time)
within a range of only about 18 seconds.  Is this always true, for all
electric companies?  Clearly, some mechanism ensures that the electric
current is in sync -- plus or minus 9 seconds -- with the master clock
(National Institute for Standards and Technology, or NIST).  But why
does it need to vary at all: why can't it be EXACTLY in sync with the
master clock?

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Request for Question Clarification by mathtalk-ga on 14 Jul 2004 09:17 PDT

Hi, colonoscopy-ga:

It is possible for clocks to be synchronized (through radio
broadcasts) with NIST "master clock" timings.  However this is not
provisioned by electric power networks at this time.

Are you more interested in finding a clock that has this
synchronization, or in understanding what sources of frequency
variation there are in electric power feeds and how these affect
simple AC electric motors?

regards, mathtalk-ga

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Clarification of Question by colonoscopy-ga on 14 Jul 2004 11:26 PDT

I am interested in "understanding what sources of frequency variation
there are in electric power feeds".  Thanks for helping me make the
question more precise.