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Re: Work function. Was: Re: LSF method vs. averaging



You've got it. This method, which I introduced to the UCSC Advanced
Lab., resulted in the experiment's demotion to the intermediate lab.
[Too easy.]

Agilent makes a suitable meter > 10^10 Ohms, pricey tho. (~ $1k; 6 1/2
digits). Any of the old Keithley electrometers (610) will do.

The load resistance needed depends on the internal R of the photo cell,
which depends on the illumination intensity. One can estimate from the
time constant, If you use a solid state cell, I think, the source R is
lower.

The present theory is the old method has problems, because of the odd
geometry of the Leybold and other vacuum tubes. It may be just because
it's a poor, dicey, difficult, time consuming, etc. method.

bc

p.s. one must still take care to avoid stray light, that the short
wavelength end of the line, or whatever, is well defined, the envelope
doesn't fluoresce, anode not illuminated, etc.

Roger Haar wrote:

Hi,

OK, I will admit my ignorance and ask how the
terminal EMF as a function of wavelength method
works. My first thought is to connect a Voltmeter
between the anode and the cathode, but unless this
meter has a huge internal resistance, I see
problems.

Thanks
Roger Haar

************************************************************
Bernard Cleyet wrote:


p.s. I've found finding the terminal (as in end point) EMF as a function
of the incident photons' energy, instead of the variable stopping
potential method, results in a much better result; besides it's easier!