A betatron is a device used to accelerate electronics to high energies by allowing them to be acted on by induced electric fields. Although the betatron is not widely used today, we describe it because it is a perfect example of the reality of these induced fields.
A cross section of a betatron in a plan that contains its vertical axis of symmetry. The magnetic field B (r, t) that has several function.
1)   It guides the electrons in a circular path.
2)   The changing magnetic flux generated an electric field that accelerates the electrons in this path.
3)   It keeps the radius of the electron orbit essentially constant during the acceleration process. 
4)   It injects the electrons into the orbit initially and extracts them from the orbit afer they have reached their full energy. 
5)   It provides a restoring force that resists any tendency for the electrons to stray away from their orbit, either vertically or radically. It isremarkable that all these things can be done through proper shaping and control of the magnetic field. Although the concept of the betatron had been proposed by others, it was don W. Kerst, at the University of Illinois, who first succeeded in 1941 in providing a magnetic field that performed all these functions in a working betatron.