Setting up…

The flow of electricity round a complete loopWhere the loop breaksBattery, switch, bulb and three leadsTwo 1.5 V cells make 3 V, enough for a 2.5 V bulb that takes about 0.3 A. Electricity can only flow round a closed loop, so the bulb lights only when there is an unbroken path from one end of the battery, through both of the bulb's contacts, and back to the other end, with the switch closed. A gap anywhere stops the whole loop; a lead on the wrong terminal leaves no way back; a lead straight across the bulb lets the electricity go round without passing through it. Where the bulb sits on the board makes no difference.