Question

when the length of the wire is doubled then the ammeter reading decreases..explain the statement in an equation ?

Answer 1

The resistance will be doubled:

We know that
V = IR
Ammeter displays current
I = V/R
If R becomes twice
I` = V/2R
​I` = (1/2) V/R = (1/2)I
So, ammeter reading will be halved

Answer 2

By ohms law

V=IR

I=V/R

Now length of wire in doubled then

R becomes 2R as resistance is directly proportional to length of the wire

So I=v/2R

I=1/2[v/r]

We know I=v/r so

I=i/2

So ammeter reading decreases to half the former value...