Wet Bulb thermometer whose bulb is covered with thin wet muslin, and which is used for determining the amount of moisture in the air. In practice, the wet bulb thermometer is used in connection with a similar thermometer having a dry bulb, the two being whirled through the air together, or hav ing a current of air directed upon them by a fan, or otherwise. The evaporation of the moisture about the wet bulb causes that instru ment to become cooler than the other one; and the difference in the readings of the two ther mometers, when talcen in connection with the reading of the dry one, enables the observer to determine the degree of saturation of the air at the time the experiment is made. Tables for this purpose are published by the Weather Bureau.
Weight thermometer consisting of a bulb provided with a capillary outlet in the place of the usual stem. In using this instrument, the bulb is first weighed while empty, and again when filled with ice-cold mer cury. It is next heated to the boiling point of water, and the mercury which escapes from it on account of the expansion is collected and weighed. These data enable the observer to cal culate the fraction of the original weight of ice-cold mercury that is lost upon heating the bulb to the boiling point. To determine any other temperature, he fills the bulb, as before, with ice-cold mercury, and then exposes it to the temperature that is to be measured (this ten;iperature being a.ssumed to be higher than the freezing point of water). Collecting the
mercury that runs out of the bulb, and express ing its weight as a fraction of the weight of cold mercury that was present at the outset, he has only to compare the fraction so obtained with the fraction obtained in the first experi ment, in order to be able to calculate, by a simple proportion, the temperature desired. The weight thermometer is not a convenient instru ment to use, but it is simple. in theory, and is free from certain of the errors to which ordi nary thermometers are liable.
Metallic Thermometer.—An instrument in which temperature is determined by noting the change of form or of length that a metallic strip experiences when it is heated. In Bre guet's instrument, three thin strips composed respectively of platinum, gold and silver are rolled together into the form of a single rib bon, the gold being in the centre. The ribbon is then coiled into a spiral, with the silver on the concave side. When one end of such a spiral is fixed, a rise of temperature causes the spiral to partially unwind, owing to the fact that the coefficient of expansion of silver is greater than that of gold, while the coefficient of gold is also greater than that of platinum. The free end of the spiral is caused to actuate a pointer, by which the temperature is indicated.
Platinum Resistance Thermometer.—An instrument for determining temperature, by noting the variation of the electrical resistance of a wire or strip of platinum. (See THER MOMETRY).