Helmholtz's theorems are two ideas about electricity and magnetism. They were figured out by scientist Hermann von Helmholtz.
The first theorem is that a changing magnetic field will create an electric field. This happens because when the magnetic field changes, it pushes on the electricity particles, which then create an electric field.
The second theorem is that a changing electric field will create a magnetic field. This is because when the electric field changes, it pushes on the magnetism particles, which then create a magnetic field.