In general, electronic signals are passed around by applying potential differences between places and persuading electric currents to flow around closed
loops. The absolute potentials of places in a circuit do not normally matter. However, there is one important exception to this, for places meant
to be at earth potential.
This problem often appears when you have to connect equipment together using coaxial cable (co-ax). Co-ax essentially consists of two wires, one wrapped
inside the other. The outer wire - often called the screen or shield - surrounds the inner wire - often called the hot or
signal lead. The purpose of this arrangement is to prevent external signals from radiating on to the inner wire. The inner wire carries the signal
voltage and the outer wire is connected to earth via the mains socket.
There are two very good reasons for using co-axial cables.
They prevent any dangerous voltages from inside the electronics from ever appearing in a place where they can be touched.
They stop any interference (radiated signals from elsewhere) getting on to the wire which is carrying the signal voltage.
This is why co-ax is very useful in things like hi-fi systems for keeping sounds ‘clean’.
When linking equipment it is essential that the earthed wires are connected together and the hot signal wires are
connected together, i.e. we have to connect the inner leads together and the outer leads together. This ensures that the signal
is carried from place to place correctly.
If we get the wires crossed the signal coming from the source is lead through the screen wire directly to ground.
The signal destination never gets a chance to see the real signal, instead its hot lead is connected to zero (ground)
through the screen of the source's cable. Hence the signal vanishes!
Most laboratories use croc clips (shaped like the jaws of a crocodile) to connect co-axial cables together. These are
usually colour coded. The inner hot wire is usually red, and the outer screen wire is usually green or black.
So to avoid crossed wires connect red-to-red and green/black-to-green/black.
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Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)