Ohms Per Square
Conductive fabrics are made up of different fibers (e.g. nylon, cotton) and conductive metals (e.g. stainless steel, silver, copper). The resistance of a particular fabric depends on what conductor is used and how it is made. When purchasing conductive fabric the unit of resistance will be listed as Ohm/Sq or Ω /▢, meaning Ohms per Square.This unit of measurement calculates the sheet resistance of a material.
What does it mean?
If a fabric is labeled as 2 Ω per ▢ it means that when the material is cut in a square, no matter how large or small that square is, it should be 2 Ohms. If cut in another dimension, such as a rectangle, the Ohms per inch are multiplied by the aspect ratio. For example:
If we define a 1" square an one unit and cut a rectangle that is 1" x 3", the aspect ratio if that rectangle is 3.
2 Ω (per ▢) x 3 (aspect ratio) = 6 Ohms
The thickness is also taken into account when coming up with this unit. If you wanted to calculate your own sheet resistance, two multimeters and 4 probes would be needed. We won't go over how to do this, the resistance you measure using your one multimeter will be more useful to you and your projects. If you would like to learn more, check out this explanation of Four Point Probe Resistivity Measurements.
Figuring Out Resistance
So, now we know what Ohm/Sq. means. This measurement is different than the resistance you will need when figuring out what voltage and current you will need and so on. It is helpful when buying material, look at this measurement as a guide to get a sense of it's conductivity.
The circuits you build will be made of specific sizes and shapes created by you. To get the resistance, cut you basic shapes and keep a multimeter by to test each trace or shape that you make. You may be able to come up with a unit of measurement yourself. If you are cnc cutting swirls and know that one swirl is 6 Ω, you then know that when you make your larger circuit, comprised of 10 swirls, the resistance will roughly be 60 Ω.
Voltage/Current Ratings
Most fabrics I have found and worked with do not state the current or voltage that the material can handle. If it is not available, be cautious when working and email the manufacturer for advisement. Remember that it's uninsulated, so you if you are pumping a good amount of power through an exposed circuit, it can be dangerous. Be extremely careful not to create a short, you could get electrocuted! Jump to the step on insulation to learn how to protect and insulate yourself and the circuit from contact and weather conditions.
Conductive Vs. Resistive
Electrical conductivity measures a material's ability to conduct electrical current. If a material has high conductivity and low resistance, current moves freely through it.
Electrical resistivity is the measurement of how strongly a material opposes the flow of electrical current. If something has high resistance, it therefore has low conductivity.
Fabrics with electrical properties can be put into either category. I can't put a particular cut off point where one material becomes resistive and not conductive, because it will always be conductive and have resistivity. From my experience, when a fabric is called resistive, it usually means that it will measure to be 1K Ω/▢ or more.
When thinking of what makes a fabric conductive, I remind myself that wire typically used for traces and connections can be anywhere from .02 - 10Ω. This is dependent of length too. Always grab the multimeter to test for yourself!
How to Choose for a Specific Purpose
When used to replace traces in an electrical circuit, the fabric you want to use is the one with the lowest resistance.
For contactswitches, the same is true, choose a fabric that has low resistance. You can get away with high resistive fabrics sometimes, but it's easier to stick to one rule.
Capacitive touchswitches can be made using material that has a fairly high resistance, the change in voltage is all that is being detected.
Resistors can be replaced using resistive materials by cutting the right dimension of a resistive material to equal the value you are looking to replace.
When pressure or force is applied to piezoresistive materials the electrical resistance changes. This makes them ideal for creating sensors, especially force sensing resistors (FSRs), bend sensors and stretch sensors.
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