Moving-coil meter has resistance of 1kohm and f.s.d. of 10mA. what is the value of resistor placed in series?

Question

to enable it to read up to 100v?

Answer 1

Current flow in a series circuit is always the same in all parts of the circuit.

Ask yourself, what is the voltage drop across the meter when it reads full scale at 10 mA?  The internal resistance of the meter is given at 1.0 K ohms.

E = I x R, thus E = 0.010 A x 1000 ohm = 10 volts.

Which means that, if 100 volts is applied across the meter leads, 90 volts has got to be dropped across the series limiting resistor.  So, what is the necessary limiting resistor value?

R = E / I, limiting series resistor = (100 v - 10 v) / 0.010 A

R = 90 volts / 0.010 A = 9,000 ohms, or 9.0 K ohms.

Check this: (9.0 K + 1.0 K meter resistance) x 0.010 A = 100 volts.  Which is the applied voltage across the meter leads.