This was a woven heating pad with as a warp optical fibers and conductive thread.
(made during the optical fiber workshop: http://etextile-summercamp.org/?p=524 of Sarah Taylor and Meg Grant.)
It had a double function: illuminating and changing color. I used two types of conductive thread: for the warp low resistance, to arrive at a total resistance of 10 Ohm for the total of the parallel threads in the warp. And secondly for the connection area silver thread with virtually no resistance. (This was the mistake!)
It works, when it was made, see picture. A few weeks later it did only sometimes, then it stopped altogether. The reason was a mystery. Corrosion? The warp thread seemed ok, but the resistance from the left silver thread connection pad to the right one was now huge, giving no current, and no energy to heat up the thermo chromic ink.
Still working! (Picture Meg Grant)
With an usb microscope I tried to discover the reason. In the end I saw that the thread was burnt at the contact area. The hypothesis now is that the actual connection from the silver connection pad tot the warp thread was having a small contact area, thus having a high resistance, thus generating more heat than average, and finally burning this contact area.
between the silver thread (nearly zero resistance) and the warp conductive thread (other conductive thread, low resistance)
showing contact area: because of open weaving this touching area between the threads is very small, like two circles touching in a point.
pointing to a damaged thread. In fact all threads in the warp are now broken, close to the contact area.
One of the broken threads.
As a contrast a close up from the elastic band with conductive thread we made during the Elasta workshop (see post https://myfablab.wordpress.com/2013/10/18/elastic-heating-pad-workshop-at-elasta-belgium/ )
The band with low resistance conductive thread and high resistance conductive thread.
Here you see the difference of machine weaving with the hand weaving on the baby loom: the thread is much more in close contact.
Close up low resistance
Close up high resistance
This is a close up of a silver thread, the silver is around the thread in broad bands, resulting in nearly zero resistance.
warp thread and silver thread difference, and optical fiber and thread side by side.
Conclusion: by inserting the silver thread I destroyed the total woven heating pad. I should have used the warp ends for connecting the power. But then the total resistance of this woven heating pad, the warp being three times longer than the chromic part of the heating pad would have been three times higher, needing 1.7 (square root of 3) times more voltage.