Plank's Constant kit 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 Planck's Constant Kit contains a photoelectronic unit, with six colour filters and a BNC to 4mm plug lead. The main 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 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. Items in the kit are available separately, and also a set of three narrow band filters for mercury violet, green and yellow lines from the spectrum of a mercury discharge lamp. These give a similar graph of the same gradient. Contents include one of each of the following (which are also available separately):. Planck's Constant Unit with screened lead (B8H28615). Colour Filters set of six (B8R05407). Set of 3 Narrow band filters (B8R05408). Plus the following metering:. Easy-Read Meter (B8R02652). 20V d.c. Attachment (B8H30919). Spacesaver Picoammeter (B8G85756). PP3 Battery available separately: B8R05410.
