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National Advisory Committee for Aeronautics, Technical Notes - A Multiple Bridge for Elimination of Contact Resistance Errors in Resistance Strain Gage Measurements

A multiple—bridge circuit is described that eliminates contact—
resistance errors of the first order between the arms of the ordi-
nary Whoatstcne bridge. This circuit is applied to the elimination
of the effects of contact-resistance variations that may Occur in
resistance strain-gage measurements made through switch contacts or”
slip rings. No additional wiring is required at The strain guess,
and the number of slip rings need net necessarily be increased.

The general.theory is derived and correct circuits and constructions
are described The methods allow considerable freedom in choice of
circuit combinations and are appli able to multiple~point strain
measurements, particularly on rotating shafts or propeller blades.
Other applications of the multiple—bridge circuit are noted.

In measuring structural strains by observing the change in
electrical resistance of a wire or a carbon strip attached to the
surface of the test member, it is necessary to detect accurately a
resistance change of a fraction of 1 percent. The fractional change
in resistance of an Advance wire strain gage that is in common use
is approximately twice the numerical value of the elastic strain
(reference 1); for example, a stress oi 10, OCO pounds per square inch
in aluminum inwolves a resistance change of O. 2 pcicent. In order
to achieve an.accuracy of 2 percent in such a measurement, the
resistance must be measured to an accuracy of 0.004 percent.

Although resistance changes of such small magnitudes may be
measured quite readily by ordinary bridge methods in those cases
where direct connection to the strain gage can be made i.hrough.well—
soldered connections, the nro‘blem becomes difiicult when connection
to the strain gage must be made through switch contacts or slip rings.

In such cases, the possible variation in contact-resistance
between switch‘contscts or between'brush'andfslip.ring mnx_be of
the same order of magnitude as the resistance change due to—nctual
strsin variation and, if-this‘contsct resistenCe lies in series
with the strain gage itself, large errors are introduced into the
measurements.