My group members and I are trying to build a snow heating mat. So far, we are using nichrome heating wires sandwiched in silicone rubber connected to an arduino that controls a solid state relay to turn on and off electric current from a wall plug. What I want to know is how to control the temperature. We decided to stick with an on and off control since it's the easiest to work out but I want to know how you implement a P or PID control. Do I need like a variable resistor or something? Or what way is the PID control supposed to vary the power output?