Electronic – Is a C2328 Transistor OK to use instead of a 2N2222

This may be a common emitter config but it seems that he's trying to test whether the npn is good or not, sorta like a go - no-go test, which is more like a comparator (saturates the npn). May I suggest a simpler circuit? You dont need two supplies, just a +9V battery. Assuming the LED lights up with ~5mA, you'll get 1.8V to 2V drop across the LED. So the emitter resistor should be 1K, and tie the collector directly to +9V. When the button is pushed, the 100uA or so of base current will be sufficient to make 5mA or so of emitter current. You'll get 5V drop across the 1k, 2v across the LED, and the npn will be in sat. You might want to add a 1M resistor on the base to gnd (to ensure npn is off when button is not pushed).

Windings that are side by side that carry current in opposite directions cancel each others magnetic field - you need to wind the electromagnet so that current travels in the same direction in all turns of the coil. There is no sensible argument for doing differently.

The strength of the magnetic field is related to number of turns and ampere's flowing. For the same number of turns wound on a longer solenoid the field reduces with solenoid length.

These are your only options other than to use superconductors! Fewer turns can be compensated by greater current or reduced solenoid length (stacking turns).

Here's a calculator you can use to estimate field strength and it uses a field value called permeability and this means you can check on various materials and choose one that gives the highest strength BUT for household coil making a piece of iron is fine.

You can also enter the diameter of the coil.