John (joncoy) asked me this question:
"I am confused about a short circuit. If I put an ohm meter red end (+) on the pick up roller and the black end (-) on a wheel (train car on work bench) it pegs the meter as if I touched the leads together. I always thought that was a short in the circuit. I now (because the lights light) assume that it is the return path for the circuit. When is it a short?
“If I put the car on the track wrong and the wheel hits the outside (-) rail and the center (+) rail the transformer light blinks because it is shorted. I hope you can understand my confusion.”
A short circuit is an electrical path around the intended load. Usually we assume that the resistance of the wires is zero, even though it is actually just very small compared to the normal load. But even a short circuit has some resistance. Likewise, we assume correctly that the load resistance is greater than zero. But the load resistance of one high-current circuit might be less than the wire resistance of another low-current circuit.
To measure the resistance of a circuit, in order to decide whether it has a short circuit, you need to have an idea what the normal resistance should be and to be able to measure a resistance of that value with some confidence. I just measured the resistance of a number-53 lamp with my ohmmeter. On the Rx1000 scale, it measures zero. On the Rx10 scale, it measures 15 ohms. On the Rx1 scale, it measures 40 ohms. So I cannot use the Rx1000 scale to decide whether there is a short circuit in a lighting circuit–it is too insensitive. (Could this be what you are doing?) I could use the other scales for that: The Rx1 scale would be best.
To understand why the latter two scales give different results, you need to know that incandescent lamps have a resistance that increases with voltage, in fact, approximately as the square-root. Since the Rx1 scale puts more voltage on the lamp (albeit far less than enough to light it up), it