Strength of an electromagnet (Larger battery)
The strength of an electromagnet changes depending on the size of the attached battery. A higher voltage battery will produce a larger current through the same wire. This, in turn, results in a stronger magnet.
To prove this is true connect two different sized batteries to an electromagnet and see how many paperclips each pick up. See below:
Here the electromagnet picks up only 3 paperclips if you add a fourth paperclip they all fall off.
With a larger battery we can see that the electromagnet picks up more paperclips.
LARGER BATTERY = STRONGER ELECTROMAGNET
NOTE: The strength of an electromagnet is directly proportional to the current and the number of windings around the solenoid.
That is if you double the current you double the strength of the magnet.