the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium—typically five minutes in the mouth and ten minutes under the armpit. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction. When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction. Mercury Mercury-in-glass thermometers have been considered the most accurate liquid-filled types. However, mercury is a toxic heavy metal, and mercury has only been used in clinical thermometers if protected from breakage of the tube. The tube must be very narrow to minimise the amount of mercury in it—the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimise the effect of the temperature of the tube—and this makes the reading rather difficult as the narrow mercury column is not very visible. Visibility is less of a problem with a coloured liquid. It has been decided by many states to prohibit the use and sale of mercury thermometers due to the risk of handling and spilling, and the potential to cause mercury poisoning; the vigorous swinging needed to "reset" a mercury maximum thermometer makes it easy to accidentally break it and release poisonous mercury vapors. Mercury thermometers have largely been replaced by electronic digital thermometers, or, more rarely, thermometers based on liquids other than mercury (such as galinstan, coloured alcohols and heat-sensitive liquid crystals). Gallium One company markets a thermometer of this sort which it bills as "the first analogue thermometer without mercury," as it instead uses the liquid metal gallium. Gallium is considered non-toxic and when disposed of does not face environmental concerns. Like mercury, gallium is a liquid at body temperature (melting point 29.7 °C), but according to the manufacturer an alloy of gallium, indium and tin is actually used, resulting in a lower melting temperature. Phase-change (dot matrix) thermometers Phase-change thermometers use samples of inert chemicals which melt at progressively higher temperatures from 35.5 °C to 40.5 °C in steps of 0.1 °C. They are