State ohm's law. How can it verified experimentally? Does it hold good in all conditions? Comment.

Explanation

For statement of Ohm.s law see point Number 13 under the heading" Chapter At A Glance" The circuit diagram of the experimental set up has been shown in Fig.12.23. Here ,XY is a resistance wire, an ammeter and V a voltmeter. A battery of four cells is being used as a current source and K is a plug key.
Initially use one cell only. Put plug in key K and note current and voltage by noting ammeter and voltmeter readings respectively. Let these to be `l_1 and V_1`.
Then connect two cells in the circuit and note current `l_2 ` and potential difference `V_2` across the resistances. Similarly take readings with three cells and four cells in the circuit. From our observations, we find that
`V_1/I_1=V_2/I_2=V_3/I_3=V_4/I_4=a` constant =R
IF we plot V-I graph the graph comes out to be a straight line as shown in fig.12.24. It experimentally verifies Ohm.s law. Ohm.s law does not hold good under all conditions. It is true for metals and alloys only provided that temperature does not change during the course of experiment.