What is a Circuit?
Sparkfun
https://learn.sparkfun.com/tutorials/what-is-a-circuit
Voltage, Current, Resistance and Ohm's Law
Sparkfun
https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law
Series and Parallel Circuits
Sparkfun
https://learn.sparkfun.com/tutorials/series-and-parallel-circuits
Resistors
Sparkfun
https://learn.sparkfun.com/tutorials/resistors
Voltage Dividers
Sparkfun
https://learn.sparkfun.com/tutorials/voltage-dividers
Ohm's Law
the Physics Classroom
http://www.physicsclassroom.com/class/circuits/Lesson-3/Ohm-s-Law
Demo: Using a Thermistor
Adafruit
https://learn.adafruit.com/thermistor/using-a-thermistor