I think your answer is volume
The nucleons(protons and neutrons) are held together by means of this strong force. If this strong never existed, all the nucleus will blow themselves due to strong repulsive force between protons(neutron has no charge).
Thats it!
If I explain beyond, it will surely bounce off your head. Anyways, if you wanna know more bout it, ping me. (:
electromagnet
When a electric current is passed through an insulated wire that is coiled around an iron core, like a nail, an electromagnet is created.
1) First of all, let's find the resistance of the wire by using Ohm's law:

where V is the potential difference applied on the wire, I the current and R the resistance. For the resistor in the problem we have:

2) Now that we have the value of the resistance, we can find the resistivity of the wire

by using the following relationship:

Where A is the cross-sectional area of the wire and L its length.
We already have its length

, while we need to calculate the area A starting from the radius:

And now we can find the resistivity: