An extensometer used to measure mechanical strain. Strain gauges are often made from a very thin wire looped back and forth to form many parallel rows. This small package (about 2-5mm on a side) , shown below, is then bonded to a surface along the axis of interest.

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I_g

In this case, the axis of measurement is vertical, since the loops are formed in that direction. The wire has a certain small resistance at rest, and this is measured by applying a small voltage (V_g) and observing the resulting gauge current (I_g). When a load (either tensile or compressionary) is applied to the member (and hence the gauge, if it's properly bonded to the surface of the member), the wire stretches (or contracts) in length. The multiple loops serve to increase the perceived wire elongation. Since the total resistance of a wire is proportional to its length (see this link for details), the change in the wire's length can be measured. Since strain is the ratio of the change in length to the wire's original (unloaded) length, you can compute strain from the change in length of one of the loops.

The small changes in resistance the strain gauge undergoes when it is loaded can be observed with a Wheatstone bridge circuit and an amplifier. This technique is used for many, many applications, such as industrial scales.

Strain gauges measure how much a material stretches or elongates when stretched or compressed. Your standard, off the shelf strain gauge is a zigzagged piece of very thin metal wire or foil arranged to deform along with the underlying material. Imagine the parallel wires of a guitar or harp. When the material underneath stretches, so do each of the parallel wires. All the wires form a single electrical path, allowing the electrical resistance to be measured.

The strain and electrical resistance are related as:

GF= (ΔR /RG)/ε
or
ΔR = ε*GF*RG

where:

• ΔR = change in resistance caused by strain, about 2 for most metal foil gauges
• RG = undeformed gauge resistance
• ε = strain

This type of strain gauge is great for measuring deformation in nondestructive testing, and for aircraft parts while in flight. Plane wings deform when the plane gains or loses altitude, or when it accelerates or decelerates. When the altitude changes, the fuselage also deforms based on the pressure difference between the rarefied air outside and the pressurized cabin.

Experimental aircraft balance safety and performance with the minimum of materials needed to keep them aloft. It's important to know whether a wing is approaching its elastic limit while in flight. Sure, the wings might look fine out there while zooming at supersonic speeds, but how much strain are they under?

Sure, that's nice, you think. But where am I going to need a strain gauge between the times I travel in a plane? Your grocery store, post office and bathroom scale each have them. Digital weight scales measure the minute deformation of a spring underneath the holding plate. Even the minute compression from fractions of a gram are enough to change the resistance of the sensor, so your deli guy can charge a little more for those extra bits of fresh truffle.

The sensors are small, cheap and sensitive enough to measure the minute changes in deformed materials. However, they are not sensitive enough to measure smaller deformations. I mean really small, changes on the order of one part in 10¹⁸ to 10²² from hypothesized gravity waves passing through the earth.

An approach for strain gauges to measure tiny changes is based on an interferometer. When a thin grating interferometer deforms, the wavelengths of diffracted light change. This way, a strain gauge can measure even smaller deformations than before. Are we confirming any theories yet? Sorry, no. Even interferometer-based strain gauges aren't that sensitive.

One approach that hasn't been tried yet could use DNA strands. DNA is a double helix, with base pairs extending between deoxyribose sides. When stretched, the strand will uncoil slightly. Imagine pulling on the ends of a twisted yarn or rope. The strand converts axial tension into torque, and this relationship has already been investigated. A new type of strain gauge might tack down one end of DNA strand and monitor the rotation of the other end with a ligand bonded there.

Microscopic strain gauges

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=121397
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=260613
http://www.opticsinfobase.org/abstract.cfm?URI=ol-18-1-78
http://www.sciencemag.org/content/330/6007/1019.6.citation

DNA mechanical properties

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TBN-41TN4D3-6&_user=10&_coverDate=08%2F31%2F2000&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1557941280&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=de8790ec7cdf347b930367c389036956&searchtype=a
http://www.nature.com/nature/journal/v421/n6921/full/nature01405.html?lang=en
http://www.nature.com/nature/journal/v442/n7104/abs/nature04974.html
http://findarticles.com/p/articles/mi_m1200/is_n17_v151/ai_19377790/

Gauge factor

http://en.wikipedia.org/wiki/Strain_gauge
http://zone.ni.com/devzone/cda/tut/p/id/3092