Problem 16-1    Source:    Problem elaborated by the author of the site.

Consider a full scale 1 mA galvanometer and have a internal resistance of R_i = 200 ohms, as show in Figure 16-01.1. Calculate the value of R₁ , R₂ and R₃ to be able to measure currents of 0.1 A , 1 A and 10 A, respectively, with the instrument.

Solution of the Problem 16-1

Initially, it is necessary to calculate the value of the voltage to be applied at the terminals a-b in order to achieve the maximum deflection, that is, to have a current of 1 mA by the instrument. Applying the Ohm's law:

Solution to 0.1 A

We must calculate the R₁ value for the instrument to measure 0.1A. Since the instrument has 1 mA full scale, then it is clear that the current that must flow by R₁ is:

With these data and remembering that the resistor that must be in parallel with the instrument is given by:

Do not forget that for this case, R₁ = R_sh. Then:

Solution to 1.0 A

Knowing the procedure required for the calculations, let's repeat it to calculate the value of R₂. Logo:

Substituting this value into the equation, the value of R₂ is:

Solution to 10 A

By repeating the procedure, we can calculate the value of R₃. Then:

Substituting this value into the equation, the value of R₃ is:

It is found that it is very easy to use a basic instrument and transform it into a multiple-scale instrument.