EMF Detector Sensitivity
I watched the video - the resistor will change the sensitivity of the device.  

It's basically a charge detector. The antenna picks up a charge from a nearby object which turns the input FET on/off.
The resistor controls how quickly the charge will dissipate to ground, and therefore how sensitive the circuit is to external e-fields.  

You can build the same thing using one FET, 9V battery, resistor and LED.  

Here is an example circuit with an excellent description of what is happening.  

In the second version of the Arduino version an LCD + averaging algorithm is used to low pass filter the readings and provide a numeric display of the field strength.