What causes the magnetic field inside a coaxial cable?

[Prinsen]

Hi. In this task we're asked to calculate the magnetic field inside the small cylinder (0>r>r1). The answer is zero, which I don't quite understand...Here is the picture:
http://bildr.no/image/WTJ1VG5F.jpeg

As the picture shows there's a current I going through the inner cylinder. Wouldn't this current generate a magnetic field at a distance 0 >r>r1?

 

[mikeph]

From the shading it looks like the inner conductor is a cylindrical shell rather than a solid, in which case there will be no inside magnetic field.

If it's a solid conductor the magnetic field will increase linearly with r.

 

[Prinsen]

Ok, thanks. So the current "flows" along the cylindrical shell? I.e inside the shell we don't enclose any current.