$$\require{cancel}$$
Two coils are wound on a common iron core. The primary coil is connected to an AC (alternating current) generator of (RMS) voltage $$V_1$$. If there are $$N_1$$ turns in the primary coil, the primary current will be proportional to $$V_1 /N_1$$ and, provided the core is not magnetically saturated, the magnetic field will also be proportional to this. The voltage $$V_2$$ induced in the secondary coil (of $$N_2$$ turns) will be proportional to $$N_2$$ and to the field, and so we have
$\dfrac{V_2}{V_1}=\dfrac{N_2}{N_1}.$