Why do I feel 'electricity' when bridging my macbook and a double insulated charger? Let me set the scene:


Macbook with metal case plugged into the wall with 3 pronged plug
iPhone is plugged into an iPad charger with 2 prongs


When I touch both my iPhone and the case of my macbook I perceive a mild 'electric shock' which continues as long as I am touching both devices.

I assume that there is an air gap in the iPad charger so there should be no current flowing between the iPhone and the macbook.

I can't make any sense of what's going on and if someone could school me as to what's happening I'd be very grateful.
Most switching power supplies use an EMI filter on the front, that looks like this:







The "ground" is connected to the "ground" on the secondary side as well as the earth pin on the plug. If the plug has no earth pin (or the socket does not have it), then when you touch it and an earthed object, a small current flows trough caps C2 and C3.

In most power supplies there is also a small capacitor bridging the primary and secondary windings of the transformer (again, for EMI reasons).