Telephones Nos. 27 and 29

The main and extension phones of a Western Electric CB one-line intercom. This illustration of the switching unit is probably correct, as its circuit matches that in the 1914 Technicians' handbook fairly closely.

The main phone used a 3-position switch on the switchbox to connect to the extension, and had separate bells to signal calls from the extension and calls from the exchange. The extension bells are located just under the writing slope on this example.

The extension phone used a generator to signal the main phone, and could be any suitable magneto telephone.

The same switchgear was built into a separate box for use on automatic exchanges and for where the above telephone was not suitable. It was fitted with a rotary knob to select the exchange line or extension phone. It was more versatile, and some lasted into the 1980s.

Left: early version with drop shutter to indicate an incoming call from the extension.

Later model CB Extension Switch, also known as an Interswitch or Junction Switch and Bellset (JS&B). This probably dates from the 1920s.

For a detailed look at the unit, go to Bob Freshwater's Telephone File.

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