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Western Electric Company,. n.d. “Western Electric News”.
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The Bell Telephone Company of Chicago,. n.d. “Bell Telephone News”.
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American Telephone and Telegraph Company,. 1935. “The Bell System Technical Journal, Vol. 14”.
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American Telephone and Telegraph Company,. 1932. “The Bell System Technical Journal, Vol. 11”.
Object, Instrument, Technology

Adjustable forks were used primarily as teaching instruments, although they may have had medical applications as well. These instruments seem to have been introduced in the 1890s and were common in 20th century high school laboratories. Adjustable forks could take the place of several individual forks and had the added advantages of being relatively inexpensive and quite durable. Moving the adjustable weight on each tine changes the tone by effectively changing the tines' length. Thus moving the weight down the tine raises the tone, and moving the weight up the tine lowers it.

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American Telephone and Telegraph Company,. 1934. “The Bell System Technical Journal, Vol. 13”.
Object, Instrument, Technology

Lord Kelvin’s harmonic synthesizer is basically Henrici’s harmonic analyzer in reverse. Originally designed as a tide predictor in 1873, the system can combine numerous component waves—in some devices, up to 64 separate components—into a single curve. It is based on the earlier pin-and-slot device, which produces simple harmonic motion with the turn of a crank.

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Campbell, George Ashley. 1910. “Telephonic Intelligibility”. Philosophical Magazine, 6Th Ser. 19 (109): 152-159.