Demonstrating back emf in a d.c. circuit

 I set up the circuit above with two bulbs in parallel. The second bulb is wired through a solenoid coil which sits on a laminated soft iron core. With the circuit connected to d.c. the switch is pressed. Needless to say, the bulb wired on its own comes on straight away. But you have to wait a second or two for the bulb with the solenoid to come on. This is because the solenoid turns into an electromagnetic. It goes from having no magnetic flux inside it to having a high magnetic flux inside it. There is flux linkage in the coil. By Faraday's Law, an emf voltage is induced in the coil if there is a rate of change of flux linkage. By Lenz's Law it has to fight the voltage from the battery pack, so it is a back emf. Whilst it is fighting the battery pack voltage, there is a momentary dimming of the bulb. Once the circuit is switched on, the electromagnet's flux remains fixed at a high level and there is no rate of change of flux to cause a back emf. The top picture shows the second bulb just beginning to come on. Whereas if you connect to a.c, there will always be changing flux in the electromagnet and always a back emf, so the bulb never comes on. See below!