A device that measures the current in a circuit element is called an ammeter. Because charge flows through circuit elements, an ammeter must be placed in series with the circuit element whose current is to be measured.

FIGURE 28.16a shows a simple one-resistor circuit with an unknown emf $\epsilon$. We can measure the current in the circuit by inserting the ammeter as shown in FIGURE 28.16b. Notice that we have to break the connection between the battery and the resistor in order to insert the ammeter. Now the current in the resistor has to first pass through the ammeter.

FIGURE 28.16 An ammeter measures the current flowing through a circuit element.

Because the ammeter is now in series with the resistor, the total resistance seen by the battery is $R_{\text{eq}}=6\text{Ω}+R_{\text{ammeter}}$. In order that the ammeter measure the current without changing the current, the ammeter’s resistance must, in this case, be $\ll 6\text{Ω}$. Indeed, an ideal ammeter has $R_{\text{ammeter}}=0\text{Ω}$ and thus has no effect on the current. Real ammeters come very close to this ideal.

The ammeter in Figure 28.16b reads 0.50 A, meaning that the current through the $6\text{Ω}$ resistor is $I=0.50\text{A}$. Thus the resistor’s potential difference is $\text{Δ}{V}_{\text{R}}=IR=3.0\text{V}$. If the ammeter is ideal, with no resistance and thus no potential difference across it, then, from Kirchhoff’s loop law, the battery’s emf is $\epsilon =\text{Δ}{V}_{\text{R}}=3.0\text{V}$.