How can electromagnetic induction be increased?
More turns of wire around the core, more current through the wire, or insert a ferromagnetic core.
Each turn of conductor around a core creates a certain amount of electromagnetism. More turns = more electromagnetism.
The greater the current through a wire, the greater the electromagnetic field around the wire.
Certain materials (primarily, iron, nickel, and cobalt) are ferromagnetic; that is, carry magnetic flux easily. Adding a core inside the turns of conductor redirects the magnetic flux lines through the core.