Therefore, a power measurement in a dc circuit can be generally carried out using a voltmeter (V) and an ammeter (A) according to one of the arrangements shown in Figure 3.1.
In the arrangement of Figure 3.1(a), the ammeter measures the current flowing into the voltmeter, as well as that into the load; whereas in the arrangement of Figure 3.1(b), this error is avoided, but the voltmeter measures the voltage drop across the ammeter in addition to that dropping across the load.
Thus, both arrangements give a surplus of power measurement absorbed by the instruments. The corresponding measurement errors are generally referred to as insertion errors.