P28_066.PDF

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Chapter 28 - 28.66
66. (a) They are in parallel and the portions of A and B between the load and their respective sliding
contacts have the same potential difference. It is clearly important not to “short” the system
(particularly if the load turns out to have very little resistance) by having the sliding contacts too
close to the load-ends of A and B to start with. Thus,we suggest putting the contacts roughly
in the middle of each. Since R 1 >R 2 ,larger currents generally go through B (depending on the
position of the sliding contact) than through A . Therefore, B is analogous to a “coarse” control,
as A is to a “fine control.” Hence,we recommend adjusting the current roughly with B ,and then
making fine adjustments with A .
(b) Relatively large percentage changes in A cause only small percentage charges in the resistance of
the parallel combination,thus permitting fine adjustment; any change in A causes half as much
change in this combination.

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