How can you change the strength of an Electromagnet?

I am going to do an experiment to measure how the number of coils of insulated wire around the iron core effects the amount of paper clips attracted to the electromagnet.
These are the things that could affect my results - weight of the paper clip, number of coils, shape of the core and the material it is made out of, the size and age of paper clip and how old the nail is. If I didn’t keep these variables in mind they could affect the pick up number and therefore not made the results fair.

For this experiment I will use: a clamp, a nail, an ammeter, a power pack, four wires and some paperclips.

Firstly I will set up the apparatus (mentioned above) as shown in the diagram below. I will then increase the number of coils on the nail each time by five and see if this makes a difference as to how many paper clips the nail will pick up.

In my experiment I will measure the number of paper clips picked up by the nail and the current (number of amps) the ammeter is on. To che...