Seventy-five years ago this month, the September 1951 issue of the British magazine Practical Mechanics showed how to build this intercom system for use between a motorcycle and its sidecar. The author noted that the bike/sidecar was a practical means of transportation, but limited communication between the operator and passenger.

The result was this intercom constructed of surplus components. The battery was tucked away in the sidecar. A switch was included, although it was noted that this was not completely necessary, as it could be powered off simply by unplugging one of the connections. Those connections were made from old tube sockets. For the operator, a throat-strapped microphone was used.