How many loops are necessary to create a solenoid with a uniform magnetic field?   Build a solenoid by adding current loops to the simulation.
When you put the first coil down (or any later coils), you can move it around .. and if you click-drag one of the ends, you can change the radius of the coil ... watch how the field strengths change as you change the coil size. (If you drag one end "across the x axis", you will reverse the direction of the current in the coil (remember, for the current directions, OUT=RED and IN=BLUE).
You can measure the magnetic field by click-dragging the mouse and observing the yellow message box.
You can also superimpose field lines by double-clicking inside the animation.