What things use series cicuits?

Credendovidis

Dear Mari :
Mostly circuitry that responds to INTERRUPTING the existing wiring loop.
Think for instance alarm systems and fire detection systems, where the continuity of the wiring is important, and checked by a constant current through the series circuitry involving all detectors. That way it is made sure that the circuitry is operational, and any real alarm is detectable.

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KeepItSimpleStupid

Credendovidis:

Fire detection systems are not series circuits. They are most definately parallel because any detector causes a fire alarm. They have "monitored contacts" so that there are really three states: shorted, open, and connected. The last detector in a loop typically has a resistor. This is how a break in the two wire circuit is detected.

Alarm systems operate in much the same way.

I suspect that this is a homework problem.