What factors could lower the voltage of a galvanic cell Recently I conducted a lab in which the voltage of a galvanic cell between two metals was recorded. Both metals were placed in an solution of their own ions with nitrate and a salt bridge was made by dipping some paper in salt water. The only issue is the voltage recorded in the cell was lower than that calculated using standard reduction and oxidation potentials. Does anyone have any ideas as to why this might be? I known the metals were not super clean although I don't think that would have an affect on the voltage.
Although it seems simple to make a galvanic cell ( i.e. a battery) it is really fairly complex. 


The theoretical voltage is given by the Nernst equation.
The actual measured voltage also depends on the current. The equation
$V=iR$ still applies. So various restance sources in the cell cause voltage drops. 


  
    
    The current must be very low so a high impedance voltmeter must be    used. (Not really a problem these days with modern semiconductor
    voltmeters as opposed to galvanometers.)
    The salt bridge offers resistance, so it has a voltage drop.
    Impure metals for the anode or cathode can cause a voltage change.
    Surface coating can cause a voltage drop.
    The acid or base concentration can cause a voltage change.
    The ionic strength of the solution can change the voltage.