Planck's constant got from experiment lower than accepted value

[lkl1226]

Homework Statement

I did a photoelectric experiment using the PASCO Model AP-9368 mercury lamp. The stopping potential I got was very low: 1.476V for ultra violet, 1.303V for violet, 1.181V for blue, 0.788V for green and 0.707V for yellow. And the Plansk's constant I got from this set of values was about 4.005e-34J.s which is nowhere close than the 6.626e-34J.s

But when I did this experiment again about four days later, the stopping potential values I got changed: I got 1.964V for ultra violet, 1.686V for violet, 1.488V for blue, 0.840V for green and 0.728V for yellow. And this time the Planck's constant was 6.184e-34J.s which is better.

And after another two days when I did it again, the stopping potential values changed again to the lower ones. I checked the battery for the apparatus and the battery level was the same all three times. Could anyone please give me an idea why this is happening? I'm so confused? Thanks!

 

[dauto]

Experimental uncertainty and inaccuracy due to limitations and intrinsic bias of the equipment is always a good bet. User error is another possibility. hard to tell without looking at the experimental set up.

 

[TheAustrian]

Don't worry about Introductory Level Physics labs. During your UG course you will not be expected to find anything new in Physics.

 

[lkl1226]

Experimental uncertainty and inaccuracy due to limitations and intrinsic bias of the equipment is always a good bet. User error is another possibility. hard to tell without looking at the experimental set up.

Thank you for the ideas. The experiment set up is like the image below.