Unexpected change in potential difference across a parallel circuit

[physicsalex]

Hello, I have just completed a physics lab in which I was trying to prove that the potential difference across a parallel circuit remains constant regardless of how many resistors are connected in the circuit. The potential difference of the battery is 6V. As I connected the resistors, starting with one and increasing to four, the potential difference dropped. With one resistor is was about 5.6 V, with two it was about 4.7 V, with three it was about 4.5 V, and with four it was about 3.9 V. I know this is not supposed to happen. I made sure that I connected everything correctly and I did.

Can anyone help me understand why this happened?
Thank you

 

[Doc Al]

Consider the internal resistance of the battery and the increased current drawn as you add resistors.