Series circuits are useful if you want a warning that one of the components in the circuit has failed. For example, a circuit breaker or a fuse must be connected in series in order for it to work.

Fig. 3: Capacitors in series. You can also replace one capacitor by putting two capacitors in parallel as shown in Fig. 4. In this case, a .0015 mfd capacitor can be replaced by a .001 mfd and a .0005 ...

The current flowing in a series circuit is the same everywhere. Figure caption, A series circuit with two bulbs and one cell. The ammeter will read the same at points 1, 2 or 3. In series circuits ...