The lactometer was standard issue to sanitarians for most of the last century. It is used to measure the density of cow’s milk by measuring the amount of water in the milk, thereby adjudging the purity of the milk. The specific gravity of milk does not give a conclusive indication of its composition since milk contains a variety of substances that are either heavier or lighter than water. By using a lactometer, however, the deeper the bulb sinks, the more dilute or impure the milk is.
The lactometer has correction factors sealed in the glass. For every change in temperature by 10 °F, there is corresponding change of a 1.0 lactometer reading. After applying a correction factor, the reading is known as a corrected lactometer reading.
The instrument pictured (circa 1920–1930) with its wooden, internally padded carrying case (not shown), is marked “N.Y. Board of Health Lactometer temp: 60° Fahrenheit.” This statement indicates that milk tested at that temperature requires no correction factor.