We conclude our discussion of practical sources and source transformations with a few observations. First, when we transform a voltage source, we must be sure that the source is in fact in series with the resistor under consideration. For example, in the circuit of Fig. 5.21, it is perfectly valid to perform a source transformation on the voltage source using the 10 resistor, as they are in series. However, it would be incorrect to attempt a source transformation using the 60Vsource and the 30resistor—a very common type of error