Problem 16-3    Source:    Problem elaborated by the author of the site.

Consider a full scale 1 mA galvanometer and have a internal resistance of R_i = 400 ohms, as show in Figure 16-03.1. Calculate the value of R₁ , R₂ and R₃ so that we can measure tensions of 1 V , 10 V and 100 V, respectively, and calculate the sensitivity of the voltmeter.

Solution of the Problem 16-3

Knowing the internal resistance of the milliammeter we can calculate the voltage drop on it when a current of 1 mA pass on it.

Solution to 1 volt

It is clear that there must be a voltage drop in R₁ of V_R1 = 1 - 0,4 = 0.6 volt. How should circulate a current of 1 mA, then the value of R₁ is:

Solution to 10 volts

Now that we know the procedure required for the calculations, let's repeat it for the calculation of R₂. Here V_R2 = 10 - 0.4 = 9.6 volts.

Solution to 100 volts

By repeating the procedure, we can calculate V_R3. Here V_R3 = 100 - 0.4 = 96.4 volts.

Finally, the sensitivity of a voltmeter is given by the inverse of the full scale current, which is equal to 1 mA. So: