This is the Wheatstone bridge.
it allows you to measure the resistance of the light bulb with a comparison method.
by varying Rc it goes up to the value of the bulb resistance. to understand when you get the result you must observe the amperometer. when the voltage at its terminals is zero, ie when no current flows means that we are at equilibrium:
Ra * Ia = x * Ix; Rb = Rc * Ia * Ix.
calculations show that
Rbulb * Rc = Ra / Rb
749nA are acceptable and approximated to 0A. due to the lack of resolution of the change in resistance you can not arrive at 0A.
Rbulb=(3,34 +- 0,01) kohm
confirmation of a result you can look in
P = I ^ 2 * Rbulb
the bulb is 30mW
from the circuit you can see that it is crossed by 1.5mA
with the inverse formula
Rbulb = P / I ^ 2 = 13,33 kohm
so I wonder: where is the mistake? it is a bug?
what is certain is that the Wheatstone bridge can not go wrong.
Bye.
