In recipes, low, medium, and high heat describe what is happening inside the pot, not a particular knob setting. High heat brings water to a full boil and makes oil sizzle immediately; medium heat produces a gentle simmer; low heat creates only an occasional bubble at the surface. The same knob setting can produce very different results on gas, electric, and induction cooktops, so judge the heat by what is happening in the pot.
On a gas cooktop, judge the heat by the flame’s height relative to the pot’s base. On high, the flame reaches the edge of the base but does not extend beyond it and climb up the sides; a flame climbing the sides heats the air instead of the pot, can burn the handle, and wastes energy. On medium, the flame covers roughly half the radius of the base. On low, the burner flame is at its smallest and its blue tips are barely visible. Gas has the advantage of changing heat instantly: as soon as you turn down the knob, the heat reaching the pot drops, which makes it the quickest type of cooktop for responding to a boil-over.
On electric coil and glass-ceramic cooktops, heat takes 1–3 minutes to respond after you turn the knob, and it continues for the same amount of time after you turn the cooktop off. So when a recipe says to lower the heat, turning down the knob alone may not be enough; you may need to move the pot off the burner for a few seconds. These cooktops also have a higher risk of boil-overs: if milk starts rising, turning off the knob will not stop it immediately, so lift the pot off the burner. When you start cooking on these cooktops, choosing a setting one level lower than you think you need is safer than increasing it later.
An induction cooktop generates heat in the pot’s own base, so it responds even faster than gas, and its settings are usually numbered from one to nine. As a rough guide, on a nine-setting cooktop, low heat is 1–3, medium is 4–6, and high is 7–9. This is only a starting point; every cooktop and pot behaves differently, so judge the heat by looking at the pot. Thin-bottomed pots brown food quickly on all three types of cooktop but do not hold heat well; thick-bottomed pots heat up and cool down more slowly.
The heat setting depends on the pan, stove, and food; water bubbles alone do not establish an exact temperature. Do not drop water into hot oil, as it can make the oil splatter. Measure the oil’s temperature with a thermometer suitable for frying.