The locus of points  which have the same power with respect to the two given spheres s1, s2 is a plane called radical plane.

Let us introduce a third sphere s3. Now we have three radical planes that form a pencil whose axis is the straight line. This line is called the radical line of the three sphere.

Now add a fourth sphere s4, and we have four radical lines. These four lines are clearly concurrent at the radical center.

Warning, assume that the parameter in the parametric equations is _t