Common return

I have mentioned that my father had an attic full of OO-gauge railway and that this was my favourite place to be of a Sunday afternoon. The track was divided into sections, each section being powered via a switch on the control panel and thus from the speed-controller associated with that panel. There were three panels altogether for up line, down line and goods. I divided my circuit into a number of sections and added signalling and automatic changeover between two speed-controllers allowing me to run two trains independently on my railway circuit.  All this has been discussed elsewhere. The point in question is that all these electrical circuits had a common return. Some call this "ground" or "earth" - all the same idea. All potentials (fancy name for voltages) are measured with respect to this and, for convenience, the voltage of the common return is generally called zero.

Having a common return means that each section needed only a single wire to power it - the power returns via the common return. The idea is based upon the scientific fact that all points on a conductor at at the same potential. All straightforward stuff.

So when, in my growing up, I took notice of telegraph wires like in the picture and my father told me that each telephone required two wires, I could not understand why they didn't use a common return and thus almost halve the amount of copper required. And I think my father could not give a satisfactory reason.

It is, of course, because my "scientific fact" is only true if no current flows.  It would be true even then if the resistance of the conductor could be ignored - but only if the current was not changing, because a conductor has inductance and capacitance as well as resistance.  I could wax eloquent and try to explain these terms but this is hardly the place...

For many applications the idea of a "common return" is a good enough approximation to reality but, if large currents are flowing, or the wires are long, or if the current is changing rapidly, the approximation is no longer good enough.

Thus was ruined my early understanding of electricity.

In the process of time I progressed beyond simple switches and light bulbs and, my father's business office (being in full view and about 50m from my bedroom window) having been burgled, I decided it was time to build a burglar alarm. My alarm consisted of a microphone and pre-amplifier at the office end, and in my bedroom the receiving end being an audio detector with a knob to adjust the threshold and a suitably loud bell. A burglar, I figured, was bound to make some noise.

The clever bit was to avail of the earth common return. After all, early telegraph stations used this scheme (see Fig. 151) and thus halved the amount of wire needed and, for me, wire was always in short supply. My Christmas gift list for Michael always featured batteries and wire, though invariably I was given socks instead. What good are socks to man or beast?

Having built it and activated it for the first time, I set my weary head upon my pillow and slept the sleep of the righteous. But not for long. The bell woke me with a start: I rushed over to the window to get a view of the burglary taking place but nothing was visible. I imagined... but I could hardly go over and tackle the burglar(s) single handed and I wasn't that sure of my technology to wake my parents.

Thus was the demise of my one-wire burglar alarm. It was never activated again. Whether it was electrical noise in the earth common return, or a insect that chose to walk across my microphone diaphragm I shall never know.