MGM092692, The End of line resistors used in a Fire Panel, has a Known value, say 2200 ohms.
Detectors need 2 wires for power and a pair for supervision, In a alarm condition a detector in the circuit will close making it zero ohms, triggering an alarm, If the wire is cut, resistance will go high and show as line trouble.

Stan:

Beg to differ a tiny bit. See http://ecatalog.squared.com/pubs/Mac...0140CT9201.pdf

pdf page 69, document page 65 figure #1

A part-voltage motor starter CAN use resistors.

Shunts or low value resistors allow one to measure currents by looking at the voltage drop across the resistor. Typical values might be 50 mV drop at 100 AMPS.

Shunt ammeters.

Dynamic braking for motors can also use resistors.

These are the applications of pure resistors in an electrical AC environment I can think of.

Lets go back a ways in the old automobile ignition system consisting of points, condenser and a coil to generate a spark. The ignition switch would provide full voltage to the coil when starting. When running, the current would be limited by a resistor mounted on the firewall. This resistor extended the life of the points. I know, this isn't what you asked.

Now, triggering is replaced by a magnetic sensor and a computer. When I was 16ish, I converted my ignition system in my first car from points to an optical system.

Generally you will be considering resistive losses in a circuit. The wire size you pick, for instance must have less than a particular loss. Measuring the contact resistance when troubleshooting. Wire has resistance.

MGM, A capacitor is what you were describing.
However, the Resistor used as an end of line resistor is needed to show an intact circuit, without it, if you had a short across the pair, it would not be seen without the resistor. It MUST be within a couple of hundred ohms(more or less depending on Panel) or it will show as a fault.

Commercial fire alarm panels that I had to interface with used 4700 ohms for monitoring of a contact closure.

I used that plus a three wire system, so I also had an indicator. Off when tripped or turned off because we routinely disabled the panel during fire alarm testing.