What are ideal cell, ideal voltmeter and ideal ammeter?

If graph shown in figure represents the $V-I$ characteristic of a device. Identify the region, if any, over which this device has a negative resistance.

The current voltage graphs for a given metallic wire at two different temperatures ${\mathrm{T}}_1$ and ${\mathrm{T}}_2$ are shown in Fig. The temperature ${\mathrm{T}}_2$ is greater than temperature ${\mathrm{T}}_1.$ State whether this statement is true or false, giving reasons in brief to support of your answer.

Draw graphs to show the variation of resistance of a metal as a function of the diameter, keeping length and temperature constant.

How a battery is to be connected so that the shown rheostat in Figure. Will behave like a potential divider? Also indicate the points about which output can be taken.