Chapter 23, Problem 70GP

A circuit you’re building needs an ammeter that goes from 0 mA to a full-scale reading of 50.0 mA. Unfortunately, the only ammeter in the storeroom goes from 0 μA to a full-scale reading of only 500 μA. Fortunately, you can make this ammeter work by putting it in a measuring circuit, as shown in Figure P23.66. This lets a certain fraction of the current pass through the meter; knowing this value, you can deduce the total current. Assume that the ammeter is ideal.

a. What value of R must you use so that the meter will go to full scale when the current I is 50.0 mA?

Hint: When I = 50.0 mA, the ammeter should be reading its maximum value.

b. What is the equivalent resistance of your measuring circuit?

Figure P23.66