Range Selection

1-23 Q21. How is an ammeter connected to the circuit under test? Q22. How does an ammeter affect the circuit being measured? Q23. How is the ammeter’s effect on the circuit being measured kept to a minimum? Q24. What is ammeter sensitivity? Q25. What is used to allow an ammeter to measure different ranges? Range Selection Part of the correct use of an ammeter is the proper use of the range selection switch. If the current to be measured is larger than the scale of the meter selected, the meter movement will have excessive current and will be damaged. Therefore, it is important to always start with the highest range when you use an ammeter. If the current can be measured on several ranges, use the range that results in a reading near the middle of the scale. Figure 1-24 illustrates these points. Figure 1-24.—Reading an ammeter at various ranges. Figure 1-24(A) shows the initial reading of a circuit. The highest range (250 milliamperes) has been selected and the meter indication is very small. It would be difficult to properly interpret this reading with any degree of accuracy. Figure 1-24(B) shows the second reading, with the next largest range (50 milliamperes). The meter deflection is a little greater. It is possible to interpret this reading as 5 milliamperes. Since this approximation of the current is less than the next range, the meter is switched as