Ohm's Law experiment with varying cells/materials

Four cells, a thin wire AB, a voltmeter, an ammeter, a plug key and some thick connecting wires are taken. The circuit is set up as shown below.

The key is inserted into the plug and readings of ammeter ($I$) and voltmeter ($V$) are noted. Then the plug key is removed. Now two cells are connected in the circuit in series. Plugging in the key the new values of '$V$' and '$I$' are noted. The experiment is repeated by connecting three cells in series and then four cells. In each case '$V$' and '$I$' are noted.

Why was the plug key removed after each measurement of '$V$' and '$I$' and inserted again for the next step?

What change takes place in the reading of the voltmeter when a new cell is added in series in the circuit? Give reason.

In which case will the readings of voltmeter and ammeter be maximum in the above experiment? What conclusion can be drawn from the straight line graph between '$V$' and '$I$'?

OR

Suppose this experiment is carried out using only one cell but wires of same length & thickness but made of different materials like nichrome wire, fuse wire, copper wire and iron wire which are replaced one by one. Out of V and I which will change and why?