I know that when a battery is connected to a lamp and the switch is closed the lamp comes on 'immediately'. I also know that the current consists of electrons moving very very slowly.... a few mm per sec. My problem is : what is the speed of the signal/current from the switch to the lamp. Is it immediate, can it be measured and explained. To explain this we compared it to opening a tap to let water flow, it is immediate even though the watr moves slowly. I know that the wire is full of electrons and the pipe is full of water but I cannot see how to get a speed for what the switch or tap does.