Maximum and Minimum Thermometers are thermometers which automatically record the highest or lowest temperatures to which they have been exposed during• a given period. In the Rutherford maximum thermometer the capillary stem of the instrument is placed nearly horizontal and as the mercury rises it pushes before it a tiny index of iron or steel, placed within the tube; and the index, being left at the most extreme position attained by the mercury, indicates the highest temperature to which the instrument has been exposed. In the Ruther ford minimum thermometer a similar index is used, but thermometric column is here com posed of alcohol and the index lies within the alcohol. When the temperature falls, the free end of the column of alcohol in the stem ad heres to the index and drags it toward the bulb; but when the temperature rises again, the alcohol flows around the little index (which does not fill the capillary tube), and so leaves it in the position to which it had been drawn at the moment when the temperature was low est. In both forms of thermometer the index is returned to a suitable position for making a new observation by the aid of a small mag net. In the Negretti and Zambra maximum thermometer the capillary tube is partially ob structed near the bulb so that although the mercury flows outward readily enough as the temperature rises, a fall of temperature at any moment causes the mercury thread in the stem to break at the obstruction, so that the maxi mum temperature to which the thermometer has been exposed can be .read directly, in the usual manner. The broken thread can easily be returned to the partially empty bulb by jar ring the instrument, or by whirling it sharply in a circle.
Clinical Thermometer.—A form of the Ne gretti and Zambra maximum thermometer, which is used by physicians for determining the temperature of the human body.
The graduation on these instruments is fine, so that the temperature can be read to the 10th of a degree or so; and the entire interval coy ered by the graduation rarely extends below 95° F., or above 115° F., the normal tempera ture of the body being about 98° F. In using the instrument, the bulb is placed under the patient's tongue or in the arm-pit.
Radiation Thermometer.—A form of ther mometer designed to indicate the intensity of solar or terrestial radiation. 'The solar radia tion instrument consists of a thermometer with a blackened bulb, the stem being sealed into an exhausted sphere of glass, so that the black ened bulb comes in the centre of the sphere. When sunlight is allowed to fall upon this thermometer and also upon a similar one with a bulb that is silvered and polished, the black bulb absorbs most of the radiant heat, while the polished one reflects most of it. The dif ference in the readings of the two instruments is assumed to indicate the intensity of the radi ant energy falling upon them.
Upsetting Therrnometer.—A form of thermometer provided with a constriction in the stem similar to that used with the Negretti and Zambra maximum thermometer, and so designed that when the instrument is inverted the mer cury thread brealcs at the constriction and runs down into the stem. These instruments are
graduated so as to read correctly when they are held upside down. By upsetting a ther mometer of this kind by means of clocicwork, the temperature that prevails at any particular hour can be recorded.
Deep-Sea Therrnometer.—An instrutnent commonly of the upsetting type, for observing temperature at various depths in the sea. It is enclosed in a very strong case, and is reversed at the depth at which the temperature is de sired. At moderate depths the reversal is ef fected by sending a weight down along the sounding wire; bnt at greater depths the up setting mechanism is usually actuated by a small propeller which is arranged so as to begin its rotation when the thermometer starts on its return to the surface of the sea.
Registering Thermometer.—Any thermom eter which automatically records its own read ings.
Dew-Point Thermometer.—A thermometer adapted to the determination of the tempera ture at which dew will be deposited from the air. The most accurate form of the instrument is that devised by Regnault. Thig consists of a pair of thin receptacles of polished silver, shaped somewhat like ordinary chemical test-tubes. A thermometer is placed in each of these, and one of the tubes is then partially filled with ether, or some other volatile liquid. When a current of air is passed through the ether by means of an aspirator, the rapid evaporation cools the silver tube and its contents (including the ther mometer) ; and the observation consists in not ing the temperature of the ether, when the polished exterior of the silver tube containing It is first dimmed by the deposition of dew. The second tube of silver, which is not cooled, assists the eye in judging when the dew is first deposited upon the other one; and the thermome ter that the uncooled tube contains is used merely to record the temperature of the air at the time of the experiment.
Differential Thermometer.—An instru ment for measuring or detecting differences of temperature, without reference to the absolute vahies of the temperatures that are compared. Sir John Leslie's form, as improved by Rum ford, consists of a horizontal tube, turned up ward at the two ends, and there provided with a pair of equal bulbs of considerable size. The bulbs are filled with air, and a small quantity of colored liquid is placed in the horizontal tube which joins them; the liquid serving to separate the air masses that the bulbs contain, and also as an index for reading the instrument. So long as the temperatures of the two bulbs remain equal, the pressure of the air will be the same in each, and the liquid index will not move. If one of the bulbs is warrned slightly more than the other one, however, the air that it contains expands and forces the liquid index toward the cooler bulb; the amount of this displacement indicating the difference in the temperatures of the bulbs. This form of dif ferential thermometer is not used to any great extent at the present time, the thermo-pile (see THERMO-ELECTRICITY) and the platinum resist ance thermometer (see THERMOMETRY) having almost entirely displaced it.