An alcohol thermometer is a thermometer which utilizes the expansion and contraction of alcohol in response to temperature changes to measure the temperature. A number of different alcohols can be used, depending on the environment where the thermometer is being utilized, with ethanol being among the most common. This type of thermometer is very popular because it is nontoxic, unlike a mercury-in-glass thermometer, and the contents will not pose a threat to human health or the environment if the thermometer is broken.
Since alcohols are clear, a dye is usually added to the alcohol used in the thermometer. Red is a common choice for dye, although other colors may be used as well, and the backing of the thermometer is usually colored to provide contrast so that the meniscus of the fluid will be clearly visible, allowing for accurate temperature readings. The thermometer may also be housed in a casing which protects it from impact and fluke temperature fluctuations, and to make it easier to handle.
Alcohol thermometers work by enclosing a narrow capillary attached to a bulb of reserve fluid. As the temperature heats up, the alcohol expands, rising up the capillary. As the temperature goes down, the fluid contracts, dropping down the capillary. Markers along the capillary indicate the temperature, with people reading the temperature by finding the marker which corresponds to the meniscus of the fluid inside the capillary. This can be tricky to do with a narrow capillary, as the thin thread of alcohol may appear almost invisible even with dye.
The earliest prototype of the alcohol thermometer appears to date to the 1600s. It is one among numerous devices which can be used to measure the temperature, and alcohol thermometers are utilized by a number of laboratories and scientific organizations for temperature readings. Different types of alcohol are used for different conditions; an ethanol alcohol thermometer, for example, cannot perform very well at temperatures too far over the boiling point of ethanol.
As with a mercury-in-glass thermometer, it is easy to skew the reading of an alcohol thermometer by heating or chilling the bulb which holds the reserve fluid. For this reason, it's important to avoid handling the bulb when working with this type of thermometer, and to avoid exposing the bulb to temperature extremes when trying to get an accurate reading. For example, an alcohol thermometer being used to measure indoor temperature should not be placed in a sunny spot in the house or in an area next to a stove or heater.