Table 3-2.NBS Radio Propagation Coding
Poor to fair
Fair to good
What government agency is responsible for monitoring our primary frequency standards?
MECHANICAL ROTATION AND VIBRATION METHODS
There are many instances when you are very much concerned with the question of rotational or
vibratory speeds. Knowledge of rotational speeds is necessary where the output of a direct current
generator has fallen below a minimum desired output or where the speed of a motor (such as the motor in
a teletypewriter or radar antenna) must be maintained at a constant value. There are many instruments that
you can use for this purpose, such as tuning forks, stroboscopes, vibrating-reed meters, and
electromechanical counters. The oscilloscope and the frequency counter are two of the other devices
which may be used, but their use may require the employment of accessory equipment.
Tuning Fork Methods
A tuning fork is generally used in conjunction with the measurement of the rotational speed of a
teletypewriter or facsimile motor but is not limited to this application. However, you must remember that
the tuning fork can be used at only one frequency, the frequency of vibration for which it was
manufactured, and therefore cannot be used on variable-speed motors. To use the tuning fork, you direct a
source of light upon the point to be observed. In the case of a teletypewriter, a black-and-white segmented
target is painted on the outer circumference of the motor governor. Radial spokes in a flywheel could be
used equally well. Permit the motor to reach operational speed under normal load conditions; otherwise,
the motor will slow down considerably when the normal load is applied. Strike the tuning fork against the
side of your hand to set it into vibration. Then observe the target through the slots in the plates attached to
the tines of the fork. The correct speed is obtained when the segments of the target appear to be
stationary. If the segments seem to move backward, apparently against the known motor rotational
direction, the speed is too low. If the segments seem to move forward, the speed is too high. There is also
the possibility that the target segments will appear to jump back and forth or to disappear suddenly. Such
erratic action is often because of governor malfunctioning. The correct speed adjustment is reached when
the targets appear to be stationary.
What is the primary measurement application for tuning forks?