Let's get down to "Earth"

Ever so often people who work with electrical and electronic gear talk about connecting this or that to "Earth" or "ground" or (even more confusing) "earthing" something or other. Mostly there is not even any "earth" present. What does it mean?

When we talk or write we often use expressions that go back many, many years. There would have been a meaning to it when the expression first came about, but then the expression carried over into similar processes while in our time it does not really make any sense. Let me explain what I mean:

One way of connecting telephones over some distance needs only one wire between the two telephones. The other 'conductor' is a metallic stick for each phone driven into the ground until it reaches the moist layers down below. The second terminal of the telephones then was connected to their stick and so the earth itself becomes the second conductor to complete the circuit.

Unfortunately, this arrangement was somewhat unreliable. Not always does one feel like hitting rods into the ground, then the earth is not always a good conductor for such low signals and all sorts of static noises could be heard. It became obvious that it is much safer to use, instead of one single wire, two. That is the system we use all over the world. But the expression: 'earth' remained. So, what do we mean when we say 'earth'?

Whenever we talk about a voltage, this voltage needs a point to which it refers. Imagine this: you are up on the 6 meter high platform of a swimming pool, ready to jump down into the water. At that moment you are standing on the board (in other words, referred to the board) at zero height. In reality, however, you are 6 meters up, and referred to ground level (or at least the water surface) 6 meters is your real height. Likewise, if we talk maybe of +24 V, is has to relate to something that is not 24, but nought. That is often simply called: "Earth" or "ground", even if it is only the chassis of a TV set or maybe the body of a car, although the car is insulated from true earth by the rubber tyres.

In Fact Sheet 7 we talked about circuit diagrams. Any of the three different symbols in the picture below can be used to represent earth in such diagrams. In most cases, earth is the same as the minus terminal of the power supply.

Come to think of it, a more correct way would have been to bring the whole earthing issue up to date. We may call it: 'Zero Potential', for example. After all, that is what it really is. But then, it is easier to say: "Connect this to earth!". What do you think?

In our next Fact Sheet No 9 we will talk about conductors. See you then.

If there is anything I might have forgotten or not explained fully, or if you have any questions you may send them to my mail address.

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Copyright © 1997-2010 Peter Schmedding, Child Development Projects, Canberra, Australia.