After the 1920's all switchboards were of the positive clearing type.  What this means is that when an extension, which is on a call, hangs up; the operator gets an indication that this has occurred.  Negative clearing is the opposite.  An extension on a call indicates all the time except when they hang up when the indication ceases.

This is confusing because there are multiple indications which mean different things and it's cheaper to not use multiple indicators. Positive clearing avoids this and it does not use additional indicators and clearing indication is logical.

In the pictures below there are indicators above the extension calling indicators.  These indicators are connected to the speech circuits.  Sometimes these are numbered and sometimes lettered.

If there are three speech circuits (as in the 2+4) then there will be three clearing indicators.

The switchboards below are all 2+4 variants

Picture dated 1907 - Front view

Picture dated 1907 - Top removed

Picture taken 1919.
The clearing indicators are at the top.  They are marked A, B and C and relate to A, B and C which can
be seen to the right of the keys.


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Last revised: November 10, 2021