Most everyday electrical and electromechanical objects emit small amounts of electromagnetic (EM) noise during regular operation. Imagine if your iphone were aware of any device you touched and could identify that object, or where you were located in your house based on this technology? This also makes an excellent pipe, stud and hidden household electromagnetic object detector!
There is an example of this technology published by Disney Research, although it requires a small SDR(software defined radio) receiver, also similar is the Touche sensor which has another referencing instructable. This instructable shows a simpler, cheaper way to achieve the same results as the Emsense, using an app for the iPhone, a resister and a single cable. You can also map the electric fields in your house with the app, as they change throughout different rooms, but change most when touching different metallic objects.
The app also has a very basic classifier implemented so that you can train it to recognize the different objects you touch, and then send the recognized objects anywhere on your network via OSC. Check out the example in the youtube videos!
You are going to make a cable as per the paper description. You will need a TRRS cable and a 4.7K resistor, and some exposed metal at the end. I include also a picture of the cable that I made.
Then, you should download the app from the app store. Here is a link: