also see: conductive yarn
conductive threads are usually manufactured for anti-static, electromagnetic shielding, intelligent textiles, wearable technology, data transfer and heating purposes. Most threads are metalized with an alloy of various metals, which can include silver, copper, tin and nickel. The core is normally cotton or polyester.
Conductive threads are uninsulated and sewing them tightly to metal usually makes for a good connection, though this connection tends to loosen over time where movement occurs. One way of avoiding this is to include a squishy material, such as stretch conductive fabric underneath the stitches, or a non-conductive material, so long as it does not obstruct the electrical connection.

see also: Syuzi Pakhchyan’s summary of conductive threads on Fashioning Technology
>> http://www.fashioningtech.com/page/conductive-thread

About

Thread is a type of yarn intended for sewing by hand or machine.
Example: 117/17 2ply thread
- 117 is the Denier weight (grams per 9000 meters)
- 17 is the number of filaments
- 2ply is the number of strands

Silver Plated Nylon 117/17 2ply

100 Ohm / 20 cm
Manufactured by Shieldex, distributed by LessEMF and Sparkfun.

Silver Plated Nylon 234/34 4ply

17 Ohm / 20 cm
Manufactured by Shieldex, distributed by Sparkfun.
>> http://www.shieldextrading.net/

Sparkfun writes that they have worked with their supplier to reduce fraying.
Right photo from Leah Buechley’s Flickr >> http://www.flickr.com/photos/leahbuechley

Shieldex Conductive Threads

Manufactures both Silver Plated Nylon 117/17 2ply and 234/34 4ply threads and many more variations thereof, that I have yet to test…

Resistive thread 66 Yarn 22+3ply 110 PET

4 K Ohm / 20cm
Distributed by LessEMF, great for sewing resistors and nice to sew with.

Lame Life Saver

20 Ohm / 20 cm
High conductivity, a bit thick, but okay to sew with by hand and machine.

Close-ups of thread taken by Lame Life Saver:

Stretch conductive thread

800 - 1.8K Ohm / 20 cm
Manufactured by Fine Silver Products. The conductive part of the thread is not stretchy itself, but it is wound around a stretchy fiber. It is nice, but annoying to work with.
>> http://www.fine-silver-productsnet.com/elcoya1.html

Gunze & Mitsufuji (ETC SI30)

6 Ohm / 20 cm
The highest conductive thread i’ve tested so far. Wonderful to work with, hard to obtain and expensive.

Karl Grimm

These threads have thin flattened wires wrapped around them to make them conductive. this makes them stiffer and less sewable than metallized yarns, but you can solder to them!

Baekert bekinox

Some more Bekinox samples provided by The Swedish School of Textile

Bekinox VN12/1*275/100Z
When current is applied, it emits heat. (we tested with 5V 400-500mA to change the thermochromic textile paint. This will differ with kinds of thermochromic paints and environmental temperature)

Bekinox Yarn 20% stainless steel fiber 80% polyester (the tag was missing and not for sure)
very similar to Schoeller Nm 50/2 conductive yarn.

Micro-Coax

X-Static

>> http://www.x-staticfiber.com/

Offray

Offray is a company that makes narrow fabrics and some of these include conductive traces.
Offray >> http://www.osnf.com/ (USA)

Jumbo

This German company makes stretchy conductive traces inside narrow fabrics, but I have not been able to obtain any samples.

Make your own conductive thread

>> http://www.instructables.com/id/Conductive_Thread_Wind_up/

Above photo by Lynne Bruning >> http://lynnebruning.com/

Suppliers

Less EMF >> http://www.lessemf.com/fabric.html (New Jersey, USA)
Gunze >> http://www.gunze.co.jp/e/english/index.html (Japan)
Mitsufuji >> http://www.mitsufuji.co.jp/e/index.html (Japan)
Sparkfun >> http://www.sparkfun.com/ (Colorado, USA)
Karl Grimm >> http://karl-grimm.com (DE)
Bekaert Bekinox >> http://www.bekaert.com (Belgium)
Shieldex >> http://www.shieldextrading.net (USA)
Lame Lifesaver >> http://members.shaw.ca/ubik/thread/order.html (Canada)
Fine Silver Products >> http://www.fine-silver-productsnet.com/pycoya.html
http://www.shieldextrading.net/ (USA)
X-Static >> http://www.x-staticfiber.com
http://www.noblebiomaterials.com/category.asp?itemid=51
Statex >> http://www.statex.biz/ger/index.php (DE)
Micro-Coax Aracon >> http://www.micro-coax.com/pages/technicalinfo/aracon/aracon_about.asp (USA)
Ajin Electron >> http://ajinelectron.co.kr/eng/index.php (Korea)
Syscom Advanced Materials - Amberstrand >> http://www.amberstrand.com/ (USA)
StaticFaction Inc. >> http://www.staticfaction.com/products-yarns.html (MA, USA)
Offray - Specialty Narrow Fabrics >> http://osnf.com/ (USA)
>> http://www.mutr.co.uk/index.php?cPath=6_572 (UK)
>> http://www.3ltex.com/ (China)