GRANULAR CARBON TELEPHONY

The attic of the Marshall Steam Museum became a storehouse for larger household items that three generations of Marshalls no longer needed or used but still held value. Among the historical treasure-trove of artifacts are a pair of Kellogg Switchboard & Supply Company wooden box wall telephones.

 

We know that the first telephone exchange for the Hockessin area was established December 30, 1899, by the Delaware & Atlantic Telegraph & Telephone Company. Located in Ball’s Drug Store, the switchboard initially served six subscribers, and within a couple months served a dozen Hockessin area subscribers, including the Marshall family and Marshall Brothers Company. We believe the pair of Kellogg Wooden Box Magneto-Battery, Grounding Key, Bridging telephones, as they were properly named, one possibly from the mansion and one from the mill office building (constructed 1895), were connected as a ‘private party line’ to the Hockessin Exchange.

 

To place a call to any other phone sharing the party line, all one had to do is turn the crank, which caused all telephone ringers on the party line to sing out. Ringing codes, a series of short and long rings between pauses, were used to signal which of up to a dozen party line phones needed to answer. The caller picked up the handset, and, placing it to their ear, then awaited to hear a familiar voice answer from afar.

 

1800s “telephony,”, as it was referred to in the late 1800s, was very simplistic and involved but few components. A small chamber of carbon granules within the transmitter (component one spoke into) transformed spoken word into an electrical signal. The handpiece held to the ear contained a horseshoe electromagnet to vibrate a disc for transforming the electrical signal back into sound for our ear to hear. Several dry cell batteries supplied power for the speaking circuit to operate.

 

That meant any telephone of the early era, wall mounted similar to the Marshall telephones or a candlestick desk phone, could only function one way. What was this unique operating characteristic of early 20th-century telephones?

Answer
The Marshall Kellogg telephones contain few components: the oak box everything was enclosed within, transmitter (part you talked into), receiver (part of the handset that was held to the ear for listening), induction coil (an audio transformer), magneto (generated up to 90 volts to operate multiple ringers), ringer (containing two saucer bells), batteries (large 1-½ volt carbon-zinc dry cells), hook (switching mechanism the handset (receiver) rested on when not in use). Two copper-coated steel wires interconnected multiple telephones on the same circuit in what was referred to as a “party line.”

In order for the transmitter to create an electrical signal that represented the sounds impinging on the transmitter disk, a capsule full of carbon granules was shaken by the attached diaphragm. The transmitter’s diaphragm and capsule’s end plates must be orientated vertically so that the capsule’s electrode ends internally contacted the carbon granules. Attached to the diaphragm, the capsule created a variable resistance reflecting the diaphragm’s vibrations. As the capsule vibrated, creating a constantly varying resistance, a variable electrical signal resulted, representing the vibrations of the diaphragm.

Early 1900s carbon capsule transmitters only operated if they were oriented so that the horn was horizontal! Position the transmitter’s horn vertically (i.e. transmitter horn pointed up or down) instead of horizontally, and the transmitter failed to function properly if at all. When the capsule was oriented horizontally, the carbon granules only contacted one end of the cylindrical capsule and a varying resistance could not be generated as the diaphragm vibrated! That is why candlestick telephones were always picked up using both hands; holding the handpiece to the ear and the candlestick base to the mouth. Wall phones, such as what the Marshalls had, were mounted with the transmitter at mouth height, and the handpiece was lifted off the hook and held to the ear.

Imagine today if capsules of carbon granules were still state of the art technology (they actually are still used for hazardous location microphones) and we could only use our mobile phones if the phone was held vertical! As the telephones aged, the carbon granules did break down by dusting, and the transmitter became less effective, requiring replacement of the transmitter capsule. It wasn’t until the later 1900s that the workhorse carbon microphone telephone transmitter was replaced with electronic microphones that did not rely on carbon capsules.

When the Marshall Steam Museum face lift is complete, and we’re permitted to host visitors at Steamin’ Days, the Kellogg telephones will be one of the new exhibit items to experience. Initially, due to COVID-19 museum touch guidelines, operating the Kellogg telephones will not be possible. Both telephones have been returned to full functionality nearly 125 years after they were originally constructed. Visitors eventually will be permitted to “ring up” the mating Kellogg phone on the museum’s “private party line” and hold a conversation with someone at the second Kellogg phone, much the same as Lizzy and Israel might have done in 1900 when Lizzy rang up Marshall Brothers Office to inform Israel and any guests that she was ready to serve dinner (what we call lunch today) or supper (what we call evening dinner today).