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.