A moving coil galvanometer has a resistance of 900 Ω. In order to send only 10% of the main current through the galvanometer, what shunt resistance is required?

Let G be the resistance of the galvanometer and S be the shunt resistance. Let I be the main current and Ig be the current through the galvanometer. We are given that G = 900 Ω and Ig = 0.1I (10% of the main current). The potential difference across the galvanometer and the shunt resistance is the same. Therefore, we have:

Ig * G = (I - Ig) * S

Substituting the given values:

0.1I * 900 = (I - 0.1I) * S

90I = 0.9I * S

Dividing both sides by 0.9I (assuming I is not zero):

S = 90I / 0.9I

S = 100 Ω

Therefore, a shunt resistance of 100 Ω is required.