I have a General Radio 1650A Impedance Bridge. It has two positions for

measuring unknown inductance, Lp (parallel inductance) and Ls (series

inductance). The book is not clear as to the differences. I can get

significant differences when measuring the same coil on the two settings.

The book says that Ls equals Lp within 1% when the Q of the coil is

greater than 10. The coils I'm measuring have a Q of less than 10 at the

bridge frequency of 1 Khz (2 or 3). Which scale should I use? What am I

missing here?

Any combination of reactance (inductive or capacitive) and resistance may

be represented as either a perfect reactance and resistance in series, or

a perfect reactance and resistance in parallel. The values of both

reactance and resistance in each case are *not* equal. The values of

reactance in each case become close when Q>10.

With a Q between 2 and 3, the values of series and parallel reactance,

hence inductance will differ significantly.

Which value to use? It all depends... In the case of small, air-cored

coils, it is usual to use the series case. With big, iron-cored coils, the

parallel case is usual.

Google for "hay maxwell bridge" - plenty of information.

Also try googling for "series parallel impedance conversion"

Omit the quotes in both cases.