# Contact electricity and photoelectric effect

Most universities provide an experiment about the photoelectric effect to determine $h$ by measuring the stop voltage against the light frequency and calculating the slope $h/e$.

But mostly they also talk about the contact voltage, which just changes the offset, but not the slope.

I have several related questions regarding this contact voltage in combination with the photoelectric effect:

• Why do they talk about contact voltages at all if it's irrelevant for the slope?
• Why are the photo cells build with two different materials at all? The same material would give no contact voltage and everything would be fine.
• Books and Wikipedia tell me contact voltage is the voltage between two materials that are connected directly. But the photo cell's electrodes are not contacted directly, but normaly through copper cables and with several devices inbetween. Doesn't this change the contact voltage inpredictable?
• Are there papers or books or something that explains contact electricity from ground up?
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## 1 Answer

• They talk about contact voltages because it affects “stop voltage” as measured by their instruments, but it doesn’t affect $\Delta U_{\rm stop} / \Delta\nu$ since aforementioned contact voltages are assumed independent of illumination conditions.
• Because a photovoltaic cell made of a homogeneous piece of material won’t work. A piece of conductor will not produce any voltage, whereas you will be unable to extract the current from a dielectric.
• No, it doesn’t. You can also read an interesting discussion at Ambiguity on the notion of potential in electrical circuits?. BTW, where does Wikipedia tell you about the contact voltage? https://en.wikipedia.org/wiki/Volta_potential , or… ? A posting that vaguely refers to “books and Wikipedia” without any specificity demonstrates a shortage in Internet communication skills, especially given that the people refers to several different things as to contact voltages.
• I don’t known (I’m not an English speaker, usually).
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