Lighting a LED with a 9V battery and 3 choices of resistors connected either to the + side or the - side of the 9V battery. Play around with the switches to connect as many of the resistors as you want and check the result on the LED and on the voltage in the circuit. The experiment in the book starts by connecting a 2.2k resistor on the negative side of the battery. I have added a switch to take the LED out of the circuit as well. I = E / R becomes simpler to calculate then.
