is one

are two

are three

are numerous

primary to secondary

secondary to primary

primary to secondary and from secondary to primary

none of these

copper-loss remains low

eddy-current-loss remains low

hysteresis-loss remains low

both copper and eddy-current losses remain low

it works on the principle of mutual induction

it is a static instrument

its windings remain immersed in the mineral oil

there is no loss of electric power in it

low copper-loss

low iron-loss

low copper and iron losses

high copper and iron losses