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Tuning fork
Frequency alignment tool

Adjusting the speed, and hence the data rate (baudrate) of a teletypewriter, was commonly done with the help of a tuning fork that was often supplied with the machine. It is resonant at a fixed frequency — commonly 125 Hz — but other frequencies are known to have been used as well.

A tuning fork [1] is an acoustic resonator in the form of a two-pronged fork, with the prongs (tines) forming a U-shaped bar of elastic metal. When excited, it resonates at a specific constant pitch which is subject to the length and mass of the two prongs. It is often used as a standard of pitch (frequency) for tuning musical instruments.

When adjusting the speed of a rotating object, such as a motor axle or – as in this case – a tele­type­writer, a modified form of a tuning fork is used, which has two overlapping plates attached to the end of the prongs, each with a narrow slit.
  
140 Hz tuning fork with wooden storage box

In most cases, a circular disc with an alternating black and white pattern, also known as a strobo­scoping disc, is attached to (or driven by) the common axle of the rotating object. By looking at the rotating disc, through the overlapping slits of an excited tuning fork, it is possible to calibrate the frequency. If you see a rapidly alternating black and white pattern, the frequency is far off.

If you see a (very) slowly alternating black and white pattern, you are getting close. If you only see the black OR the white field, you are spot on. The frequency of the stroboscopic disc now matches the frequency of the tuning fork. The actual frequency of the axle depends on the number of alternating fields on the strobo disc.

The image on the right shows the overlapping metal plates at the end of the prongs of a tuning fork. The slit in one of them is clearly visible here. This construction is also known as shutter or diaphragm. The one shown here is 140 Hz.
  
Two Diaphragms with slit

In practice, the machines – or actually the stroboscopic discs – were constructed in such a way that an industry standard 125 Hz tuning fork could be used for speed aligment. Early electronic devices could sometimes also be aligned with a tuning fork, although they do not contain any moving parts. In such cases, a frequency derived from the main oscillator clock was used to drive a neon lamp that generated the stroboscopic effect. An example is the Ecolex II cipher machine.

Wooden box with tuning fork 140Hz tuning fork inside wooden storage box 140 Hz tuning fork with wooden storage box 140Hz tuning fork Two Diaphragms with slit Frequency Siemens 125 Hz tuning fork Frequency
A
×
A
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Wooden box with tuning fork
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140Hz tuning fork inside wooden storage box
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140 Hz tuning fork with wooden storage box
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140Hz tuning fork
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Two Diaphragms with slit
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Frequency
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Siemens 125 Hz tuning fork
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Frequency

Frequencies
In Europe, the frequency of a tuning fork in commonly given in Hz (Hertz). In the US, it is often specified in VPS (vibrations per second). The following frequencies were commonly used:

  • 87.6 Hz
    Teletype
  • 120 Hz
    Teletype
  • 123.5 Hz
    Teletype
  • 125 Hz
    Siemens, RFT
  • 140 Hz
    Creed
  • 180 Hz
    Teletype
  • 440 Hz
    Musical instruments (A above middle C)
References
  1. Wikipedia, Tuning fork
    Retrieved March 2016.
Further information
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Crypto Museum. Created: Friday 30 March 2018. Last changed: Friday, 30 March 2018 - 13:03 CET.
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