algebraic3d
solid coil = cylinder (0, 0, -1; 0, 0, 1; 0.4)
	and not cylinder  (0, 0, -1; 0, 0, 1; 0.2)
	and plane (0, 0, 0.2; 0, 0, 1)
	and plane (0, 0, -0.2; 0, 0, -1);

solid ferrit = 
	cylinder  (0, 0, -1; 0, 0, 1; 0.2)
	and plane (0, 0, 0.6; 0, 0, 1)
	and plane (0, 0, -0.6; 0, 0, -1);
	
solid box = orthobrick (-2, -2, -2; 3, 2, 2) -bc=1;
solid air =  box and not coil and not ferrit -bc=3;

tlo coil -col=[0, 1, 0]; 
tlo air  -col=[0, 0, 1] -transparent;
tlo ferrit -col=[1, 1, 0];