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. The figure below illustrates these points.
Reading an ammeter at various ranges.
View(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. View(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 shown in view(C). The range of
the meter is now 10 milliamperes and it is possible to read the meter
indication of 5 milliamperes with the greatest degree of accuracy. Since
the current indicated is equal to (or greater than) the next range of
the ammeter (5 milliamperes), the meter should NOT be switched to the
AMMETER SAFETY PRECAUTIONS
When you use an ammeter, certain precautions
must be observed to prevent injury to yourself or others and to prevent
damage to the ammeter or the equipment on which you are working. The
following list contains the MINIMUM precautions to observe when using an
All the meter movements discussed so far react
to current, and you have been shown how ammeters are constructed from
those meter movements. It is often necessary to measure circuit
properties other than current. Voltage measurement, for example, is
accomplished with a VOLTMETER.
VOLTMETERS CONNECTED IN PARALLEL
While ammeters are always connected in series, voltmeters are always connected in parallel. The figure below (and the following figures) use resistors to represent the voltmeter movement. Since a meter movement can be considered as a resistor, the concepts illustrated are true for voltmeters as well as resistors. For simplicity, dc circuits are shown, but the principles apply to both ac and dc voltmeters.
Current and voltage in series and parallel circuits.
In the figure above view (A) shows two resistors connected in parallel. Notice that the voltage across both resistors is equal. In figure above view(B) the same resistors are connected in series. In this case, the voltage across the resistors is not equal. If R1 represents a voltmeter, the only way in which it can be connected to measure the voltage of R2 is in parallel with R2, as in the figure above view(A).