Of course it may be hard to get a hold of, but ECG elctrodes are the best
for this sort of thing.  And a low current, high frequency source to pass
through the leads.  Changes in resistance of less than an ohm indicate
expansion and contraction.

Rob

At 01:39 PM 1/24/98 +0100, you wrote:
>Maybe a little ot ...
>I'm trying to measure the expansion of chest during
>breathing using a pic (there You are ;-) )
>How would You measure a span of about 10cm ?
>Usually flexible rubber tubes with quick silver are used,
>the change of lenght is measured by taking the resistance
>with a wheatstone bridge. only, Hg on the floor can be hard
>to hunt ;-) (and sooner or later it gets there ...)
>
>The device should be rather small and to be attached
>to a person easily. And it should be as "electronical"
>as can be, that is: preferably no mechanical parts ...
>
>Any ideas?
>
>Christoph
>
>