4 A voltmeter was used to measure the voltage of a cell with

4 A voltmeter was used to measure the voltage of a cell with an internal resistance of 750 , what must the internal resistance of the meter be if the relative error in the measurement is to be less than (a)-1.0%, (b)-0.10%?

Solution

Relative error = (measured value -true value)/(true value)

So here relative error is -1% or -0.01

So -0.01 = (750/true value) - 1

So 0.99 = 750/ true value

So true value is 757.57

So internal resistance = 757.57-750= 7.57 ohm

For -0.10% = -0.0010 error

So, -0.0010 = (750/true value) -1

So 0.9990 = 750/true value

So true value is 750/0.9990 = 750.75

So internal resistance is 750.75 - 750 = 0.75 ohm