Peel the potatoes and cut them into 1.5 cm (¾ inch) slices, then again into batons for chunky chips. For French fries, cut them into slender batons, not much thicker than matchsticks.
Soak the chips in cold water for about 30 minutes to remove any excess starch and stop them sticking together. Drain thoroughly and pat dry between layers of kitchen paper – they must be totally dry before frying, otherwise the oil will splatter.
Put some of the chips into the deep-fat fryer basket and lower them into the preheated fryer, 160°C (320°F). (Don't cook too many at a time or they will turn soggy.) Cook for 5 minutes or until they start to soften but not brown. Raise the basket and let the chips drain for 5 minutes. Increase the temperature to 190°C (375°F) and lower the chips back into the pan. Fry for about 6–8 minutes, depending on the thickness of the chips, until golden brown.
Drain thoroughly on kitchen paper and sprinkle with salt.