Ringing machine

A ringing machine is a motor-generator set that was used by telephone companies to create the current needed to ring the bells in subscriber's telephones (referred to as machine ringing), and to create the call progress tones that a subscriber might hear while making a call. These sounds usually include dial tone, ringback tone (known as audible ringing) or busy tone.

Origins

Early ringing machine from an 1882 Western Electric Catalog. Note the pulley used for belt drive.

The origins of the ringing machine date back to hand cranked magnetos used in early subscriber sets and switchboards. In the late 1800s, larger offices with available power sources made use of motor powered generators to supply ringing current for their switchboards and subscribers. These earliest machines did not produce call progress tones, since progress was usually announced by the operator who was facilitating the call. By the early 20th century, some machines were supplied with attachments that generated audible tones, as well as machine ringing.

P-Type Machines

A P-type ringing machine (c. 1923) from the 803C Ringing and Tone Plant exhibit at the Connections Museum. The lamp is temporarily wired up to the 90 volt 20 Hz AC output of the ringing generator.

By the 1910s, full service ringing machines of the P-type were common in larger Bell System offices. These machines were powered by AC or DC motors, and included a ringing generator, several tone generators, and interrupters that created the proper cadence from the output of the machines. (Although the term "P-type" refers only to the ringing generator itself, the term had been expanded to refer to the entire machine.) P-type machines originally included tone generators that were simple commutators attached to the shaft of the ringing machine. The rapid interruption of the current as the commutator spun under the pickup brushes produced the audible tone.

These were later replaced with the tone alternator, which instead uses three wheels with various "tooth" cutouts, similar in appearance to a gear. The wheels spin in an excited field, and induce a varying current in pickup heads placed around the periphery of the wheels. This in turn produces various audible tones that are then passed to subscribers.

Commercial Type Machines

A commercial ringing machine from 1962, made by General Electric for the Bell System. This one is powered by an AC motor (left).

By the end of the 1920s, it had become apparent that it was no longer cost-effective to continue manufacturing the old-style P-type machines. Their basic structure had not changed since the late 1800's, and newer technologies and manufacturing methods could contribute to a significant cost savings. Thus, Western Electric opted to turn to General Electric to produce ringing machines to replace the old P-type apparatus for all future large central office needs. This production continued at least until the mid 1960s.

Smaller Machines

In addition to the large city ringing machines, several smaller types of motor generators were sold for use where high power output and a 24/7 duty cycle was not required. These uses included Private Branch Exchanges (PBX's), small Community Dial Offices (CDO's) and the like. These smaller machines produced many of the same general outputs as their larger counterparts, but often sounded slightly raspier, and were not built for continuous use.