Rachel learned about how to make electromagnets using wires, batteries, and nails. She wanted to see if the number of batteries used would increase the strength of the magnet. So, Rachel created different electromagnets. First using 1 battery, Rachel tried to see how many paper clips the magnet could pick up. Then, she repeated the process with 2 and 3 batteries. For each electromagnet, she counted how many paper clips the magnet could hold. What is the independent variable?