When a progressive tense is used in the independent clause, the dependent clause typically uses the tense.
simple past
simple present
progressive past
progressive present

simple past

Explanation: A progressive (also called continuous) tense indicates an ongoing action. If we have a past progressive verb in the independent clause, the typical use in a dependent clause is that of a simple past tense verb to indicate that an action began and finished while the action in the independent clause was in progress. Example: "I was taking a shower when the phone rang."

the direction of the magnetic field produced by an electric current will change when the direction of the electric current changes. for example : if a current is flowing in a straight length of wire, the magnetic field is described by a concentric circle (using right-hand rule).