Six’s maximum and minimum thermometer
This thermometer is popular among gardeners for use in greenhouses. Its purpose is to record the maximum and minimum temperatures reached since the thermometer was last read. Generally speaking, a minimum temperature occurs during the night
and a maximum during the day. Invented by James Six towards the end of the eighteenth century, the thermometer consists of a fairly large cylindrical bulb A which originally contained alcohol,
although oil of creosote is now more generall-yused. This is connected by a U-shaped stem to a second bulb nearly full of the same liquid (Fig. 14.5). The bend of the U contains a thread of mercury. Two scales are provided, one against each limb of the tube so that the temperature may be read against either of the mercury levels. Resting on each of the mercury surfaces are small steel indexes provided with light springs to hold them in position in the stem. Expansion or contraction of the liquid in A causes a movement of the mercury thread. Consequently, one or other index is pushed forward by the mercury and left in the extreme position reached. Thus, the lower end of the index on the left indicates the minimum and that on the right the maximum temperature attained. After readings have been taken a small magnet is used to bring the indexes back into contact with the mercury.