Why is plus on top?

[neil.johnson at camcon.co.uk]

I have a suggestion which is open for comment.

I think it dates back to valves.  Most valves have the anode at the top of the
envelope mostly, I think, due to the construction of the device.  This leads to
the natural step of drawing the valve with the anode at the top and the cathode
at the bottom.  Then you have to make the anode positive WRT the cathode, so the
natural place for the positive rail is at the top of the drawing.  And from this
the convention for drawing the positive rail at the top and the negative rail at
the bottom of a schematic was born.

When transistors first started appearing people finally accepted that the "real"
current flows from negative to positive, and in the transition period schematics
were drawn with the negative rail at the top.  However, I believe that so strong
was the convention for top rail positive from the valve days that eventually the
convention won, and so today we draw schematics with the positive rail at the
top.

This convention is reinforced by virtually all electronics test equipment - an
ammeter indicates positive "current" flow from positive to negative, a voltmeter
shows that the positive terminal has a greater voltage than the negative
terminal, and so on.

Its not really a case of one is right and the other is wrong, and maybe the
"hole" is a nice concept to help explain why we still use a certain convention,
but there we go.  I think "current flows from positive to negative" is a
factoid, and the minor irregularity certainly hasn't impeded progress in this
technology!

Tuppence spent.

Neil
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|     Neil Johnson  MSc CEng MIEE MIEEE     |
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