An oil bath is a laboratory

Laboratory

A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

 heating device which uses boiling

Boiling

Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...

 oil

Oil

An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

 as the temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 regulator. Since different oils have different boiling point

Boiling point

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

s it is possible to obtain a temperature near to the desired temperature by selecting an oil with a boiling point as close as possible to the desired temperature.

An oil bath is essentially a container of oil heated by a hot plate, open flame or other heat source.

Another use of an oil bath is to filter particulates out of air, by leading the air stream through an (unheated) oil bath. This type of air filter

Air filter

A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. A chemical air filter consists of an absorbent or catalyst for the removal of airborne molecular contaminants such as volatile organic compounds...

was used in car engines, but has been replaced by modern paper air filters.