Matt, there is also a subject that we have not broached yet...
Station Busy lamps !!!
These are lamps, either free-standing indicators or spare lamps in a tel set, that light when a particular station goes off-hook. The station in question can be a SLT or a key set. A pick up key need not be pressed to activate the lamp. Just the fact that the phone is ROH is sufficient.
The scheme uses the A ground circuit, as it passes through the SWHK contacts of the victim's phone. The A ground in the phone is protected against a sneak path by putting a blocking diode in series with the SWHK and the Hold key A ground lead. Then, an additional A ground goes out from the SWHK through a lead in the mounting cord (usually the Y/Br lead), and is then cross-connected to a lamp which has -24 DC waiting on its other side.
The application for this is generally a secretary who needs to know when a boss's phone is ROH.
ITT used to make a box that would mount on the wall of an office, that had rear-lit numbered discs, one for each honcho in the office. The receptionist could then determine which boss (for instance partners in a law firm) could take the next call.
