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Re: Photo-Electric Effect Experiment





On Mon, 5 Aug 1996 greenslade@kenyon.edu wrote:

I would like to compare notes with others who use the Welch photoelectric
effect experiment.

We use the tube provided by Welch, and have disconnected all of the
associated potentiometers, thus giving us a direct connection to the tube. The
current is being measured with a Keithley Model 177 multimeter and the voltage
with a Keithly Model 614 Electrometer. The latter sounds like overkill, but we
wanted to use a very high input impedance voltmeter.

Our results run consistently low by at least 20%, and usually more. When we
do this in the introductory laboratory, we use a Keithly model 169 multimeter,
with an input impedance of 10 Mohms, as the voltmeter and get similar results.

The light source is a bright mercury arc, and we use the filters supplied by
Welch. A black cloth is used to eliminate stray light, and the room lights are
off.

We have used the usual method of measuring the phto-current as a function of
the retarding potential, and use the break point in the curve as the
appropriate stopping potential.

Tom Greenslade



I recently (about two years ago) reworked our PE setup. I got
noticable improvement by using a monochromator with the Hg source, and
shading the anode. I also used a very stable supply and a Keithley
electrometer with shielded cable for current meaurement.However, the most
dramatic change came from the data analysis. You need to drive the current
into the reverse current region until it saturates. You will notice that
each line has a different saturation current value and your data needs to
be normalized to one of those. This makes a large difference in the result.
Also, determining the zero value is tricky. I ended up fitting the
saturation current slope and then subtracting those values from the data.
For more details on these techniques look in Melissinos' Experiments
in Modern Physics. All of this is done in our sophomore-junior level lab
course. When we do the exp. in the intro. course with a setup more
similar to yours 20% error (or greater) is not uncommon.

This is still a great experiment as it even now requires some
effort on the part of the experimenter to wring a value out of the data.


Mike Monce
Connecticut College