Voltmeter values drop w/ time when measuring elec chem cell?

[mizzmarymary]

I set up a zinc-copper elctrochemical cell by placing both electrodes in lemon juice to test the ability of lemon juice to conduct a current. I used a voltmeter and attached each lead to one of the metal electrodes. What I observed was that the voltage over time given on the voltmeter began to decrease and continued to decrease. I would be grateful if someone could explain what is happening? Thanks so much!
Mary

 

[mfb]

How long did the process take?
The voltmeter draws some current and the cell does not run forever.

 

[nasu]

See "electrochemical cell polarization".
What you see is normal for these home made cells.

 

[tech99]

See "electrochemical cell polarization".
What you see is normal for these home made cells.

These cells work better if you use copper sulfate solution instead of the lemon juice. Careful, it is toxic.