This is a wheat stone bridge circuit. Here you can find 3 resistor of 1k ohm but one is not of 1 k ohm . On adjusting it to 1 k ohm -you found that the lamp stop glowing. ...as p.d. on lamp became zero. And this is wheat stone.
Required conditions for this is
R1/R2=R3/R4
This also happens in case of capacitors.
In this case the equipment at middle (as lamp here) does not work.
