Why outlets spark when unplugging - EMF & Inductors

OK, now we know the reason why: momentum. How do we stop it? Plumbing has special devices/arrangements to stop water hammer. Is there a special device/arrangement to stop the spark? I have an immersion heater that causes the switch to spark when turning off. Some switches spark more than others. Some new switches spark more than old switches. Why? Is the immersion heater broken because it draws more than the switch is designed to handle?