CB873

The CB873 is an example of a corded switchboard. They were known as a corded PMBX, which stands for Private Manual Branch Exchange. Many companies, hospitals, hotels etc used these switchboards for internal communication and to connect to the outside world. 

My PMBX has 20 extensions and 5 external lines. It has been professionally modified at a later stage, where the fixed transmitter was replaced by a handset and a dial was added. The original cabinet was probably produced in the early 1900s. 

The flap of an external line drops on an incoming call. This activates the buzzer. The operator responds by lifting the handset, connecting an answer cord to the exchange socket and switching the associated speak key. The operator then connects the call cord to the socket of the requested extension and turns the magneto handle. The called person lifts the handset which establishes the connection. The operator returns the speak key to neutral. The dolls eye of the called extension shows its number when the call ends. The operator unplugs the cords and returns the exchange flap manually. A night switch can be set to connect an incoming line to a dedicated extension.

When an extension lifts its handset the associated dolls eye shows its number. The operator connects an answer cord and either connects an internal line or dials an outside line, depending on the request of the caller.

I have connected just two telephones, arbitrarily picking extension numbers 2 and 8.