Two resistors of 6Ω and 3Ω are connected in series to a 9V battery of negligible internal resistance. An ammeter of resistance r is to be inserted in the circuit to measure the current.
(i) Calculate the current flowing in the circuit before the ammeter is inserted.
(ii) Derive an expression for the new current I′ in terms of r after the ammeter is connected in series with the resistors.
(iii) Using your expression, find the value of I′ and the percentage reduction in current when r=0.3Ω.
(iv) Based on the above calculation, discuss why the resistance of an ammeter should be kept as low as possible.