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An ammeter is a device that measures current. Since all meter movements have resistance, a resistor will be used to represent a meter in the following explanations. Direct current circuits will be used for simplicity of explanation. AMMETER CONNECTED IN SERIES In figure 119(A), R_{1} and R_{2} are in series. The total circuit resistance is R_{2} + R_{2} and total circuit current flows through both resistors. In figure 119(B), R_{1} and R_{2} are in parallel. The total circuit resistance is and total circuit current does not flow through either resistor. Figure 119.  A series and a parallel circuit. If R_{1} represents an ammeter, the only way in which total circuit current will flow through the meter (and thus be measured) is to have the meter (R_{1}) in series with the circuit load (R_{2}), as shown in figure 119(A). In complex electrical circuits, you are not always concerned with total circuit current. You may be interested in the current through a particular component or group of components. In any case, an ammeter is always connected in series with the circuit you wish to test. Figure 120 shows various circuit arrangements with the ammeter(s) properly connected for measuring current in various portions of the circuit.
Figure 120.  Proper ammeter connections. Connecting an ammeter in parallel would give you not only an incorrect measurement, it would also damage the ammeter, because too much current would pass through the meter. EFFECT ON CIRCUIT BEING MEASURED The meter affects the circuit resistance and the circuit current. If R_{1} is removed from the circuit in figure 119(A), the total circuit resistance is R_{2}. Circuit current With the meter (R_{1} ) in the circuit, circuit resistance is R_{1} + R_{2} and circuit current The smaller the resistance of the meter (R_{1}), the less it will affect the circuit being measured. (R_{1} represents the total resistance of the meter; not just the resistance of the meter movement.) 
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