English: Tuning fork frequency standard used by the U.S. National Bureau of Standards (now the National Institute of Standards and Technology, NIST) in 1927. It consists of a 1000 Hz tuning fork with its tines between the poles of two electromagnets connected to a vacuum tube electronic oscillator circuit. Feedback from the vacuum tube caused the fork to vibrate at its resonant frequency. This method of exciting the fork kept its frequency more accurate than striking it. This was a working standard; it was calibrated by comparing it to the primary standard for time and frequency, the swinging pendulum of a precision Shortt free pendulum clock. An accuracy of a few parts per 100,000 was achievable.