Plancks Constant Apparatus

The Planck’s Constant Unit can be used:

• As a direct method that gives a reasonable order of magnitude value for Planck’s constant, h
• For the demonstration of photoelectric effect and the principle of applying a backing potential difference to reduce the anode current to zero

The unit contains a photo emissive Caesium Antimony cathode, with a spectral response of 185-650nm. When light of frequency, f, strikes the cathode, electrons of energy E=hf-W0 are released and would normally reach the anode, producing a current which can be registered on a very sensitive instrument such as a Picoammeter (B8G85756) or an Electrometer.

The potential difference from an internal PP3 (6F22)-type cell can be adjusted to a value V where the anode current is just reduced to zero. V is proportional to the frequency of light used. This apparatus enables values of V to be found for various frequencies of light selected by placing each of the colour filters in turn over the cell. A plot of V vs. f gives a reasonable straight line where the gradient is h/e.

The Planck’s Constant Unit requires the following to operate:

• Spacesaver Picoammeter (B8G85756)
• PP3 Battery (B8R05410)
• Easy-Read Meter (B8R02652)
• 20V d.c. Attachment (B8H30919)
• Colour Filters set of six (B8R05407)
• Set of 3 Narrow band filters (B8R05408)

All of the required equipment for Planck’s Constant can also be purchased together in the Planck’s Constant Kit (B8H76701)