Conductive thread is similar in properties to ordinary sewing thread but, it also has the ability to conduct a small amount of voltage through it. It can do this as it has metal incorporated into it (usually silver, nickel, tin or copper) with a core of normally cotton or polyester. The thread is not insulated and therefore attaching it to a metal component within a circuit in place of the usual wires means the circuit is much more flexible allowing you to maintain many of the original properties of the material such as drape and feel. As it is a thread it also allows you to sew by hand or machine and even embroider designs into textiles. Its resistance properties are 4Ω per 100mm. When using by machine it is not necessary for the second thread to be conductive thread too just the spool for the side of the design you wish to have the circuit on.
The mixed properties of electronics and textiles was unheard of a few years ago but times have move on. With technology moving as fast as it has in recent years, the possibilities of clothing and accessories with visual and audio effects by the use of flashing lights, sensors and piezo-electronics has now been made much easier in a domestic situation with the availability of conductive thread.
The conductive thread used by Light Stitches is a medium weight and comes on a bobbin of approximately 6M or 150M or 250M reel. The HC12 150M reel is thicker than the HC40 250M reel this could be used for finer textile projects.. The thread is much stronger than domestic poly/cotton thread, and somewhat thicker. If using on a machine you may wish to try a larger needle to help with threading up and less chance of fraying by being caught on the point of the needle.
This thread also has medical uses (silver has antiseptic qualities) and is used to create ‘soft’ circuits. An example of one of its uses is a fencing jacket. The jacket is made with conductive material scoring areas which can become extremely worn with time. The jackets are expensive, and fencers usually try to get them repaired by darning the worn areas. Conductive thread can be used for this quite successfully and also sewn into the fabric of a jacket where the conductivity of the material has been lost over time.
HC12 – 150M reel 
6M bobbin