Voltage of a lemon cell

A lemon cell is formed by inserting Copper and Zinc plates in to a lemon, or any dilute acid. The voltage developed across the electrodes is very small and the cell has a very high internal resistance. Due to this the voltage drops when a load resistor is connected.

Objective

Make a lemon cell and explore it's internal resistance.

Procedure

Connect the electrodes to A1 and Ground
Enable Checkbutton for displaying the voltage at A1
Connect a 1000Ω resistance across the electrodes.
Measure the voltage again, to note down the reduction in it.

Discussion

Voltage across the Copper and Zinc terminals is nearly .9 volts. Connecting the resistor reduces it to 0.33 volts. When connected, current will start flowing through the resistor. But why is the voltage going down ?

What is the internal resistance of the cell ?

Current is the flow of charges and it has to complete the path. That means, current has to flow through the cell also. Depending on the internal resistance of the cell, part of the voltage gets dropped inside the cell itself. Does the same happen with a new dry-cell ?