Perpetual Top




  • Top contains motor and off-balance rotor with rotational wobble.
  • The wobbling rotational impulse provides top impetus to spin in clockwise direction. For additional info, see: P3-3503_DS.
  • Top also contains a red/green/blue LED. Persistence of vision mixes primary colors as top spins, allowing eye to see various secondary colors depending on LED flashing pattern.
  • Located in L03, section D4.


measuring H

Measuring h with LED

measuring H1

How to measure Planck’s constant using an LED

  • Measure LED threshold voltage using voltmeter and ammeter.
  • LED converts electron energy to photons. Energy lost by electron = energy gained by photon = (electron charge) times (voltage).
  • So, if threshold voltage is 2.2 volts, energy of emitted photon must be 2.2 eV.
  • Use  E=hc/lambda, and measure lambda using Red Tide spectrometer with Logger Pro (see below pic), to determine h. Take data using LEDs of 4 different colors and fit data to line with slope h.
  • Hand held spectrometer can be used to view LED spectrum.

measuring H
measuring H4

measuring H3

measuring h

measuring H2