THEBMOMETER. 218 THERMOMETER. whose upper ends have respectively a hirge bulb full of alcohol and a small bulb containing air and vapor of alcohol. The lower portion of the legs of the U is filled with mercury and in each leg there floats an index. One of these indices is forced up by the expansion of the alcohol in the bulb and the index is left at the maxinuim tem- perature. The other index is raised by the con- traction of the alcohol and the consequent rising of the mercury which is pushed up by the ex- pansive force of the air pressure in the small bulb. Modifications of this thermometer are used in Austria, England, and America, but none of its forms is considered to be so reliable as the separate maximum and minimum thermom- eters previously described. The Upsetting Thermometers. These were invented by Negretti and Zambra and have been modified and improved from time to time. An ordinary thermometer has its tube so bent that when the thermometer is turned upside down the column of mercury above the constriction will run off into a separate portion of the tube, where it remains until the observer can make the neces- sary reading. The thermometer is especially adapted for recording deep ocean temperatures or temperatures at any hour of the night. Metallic Thermometebs. These depend for their action upon the differential expansion of metals. As made by Bregnet, 1817, or Herrman and Pfister, 1805, they consist essentially of two thin spiral bands of different metals soldered togetlier. Differences of temperature cause the spiral to coil or uncoil and the apparatus can easily be adapted for self-registration. Optical Thermometers. The plane of polari- zation of a quartz crystal is rotated about the axis of a beam of light by the varying tempera- ture of the quartz, increasing with great regular- it,v with the increase of temperature. This ap- paratus is reconunended by Cornu for the study of high temperatures when the gas thermometer fails because of the softening of the bulbs con- taining the gas or air. Electric Thermometers. Melloni's thermo- electric pile. Pairs of pieces of different metals are soldered together and the extremities com- municate by a wire with a needle galvanometer. Any difference of temperature between two con- secutive junctions causes the electric current to traverse the wire ; its strength is measured by the galvanometer. The absolute value of the galvanometer scale can be expressed in ther- mometric degrees by means of careful compari- sons. The thermo-electric thermometer can be made to give extremely reliable results. Resistance-Coil Thermometer. ( 1 ) Abso- lute resistance — the resistance experienced by an electric current flowing through an insulated wire — ordinarily increases with the temperature of the wire, and is shown by the deflection of a galvanometer needle. The resistance is not precisely proportional to the temperature, but if it has been determined at several temperatures the wire or coil can be used for all other tem- peratures. By using a coil of platinum wire, a wide range of temperatures may be measured with great accuracy. See Marvin, "Recent Prog- ress in Physical Science," Bulletin of Philosoph- ical Societjt of Washington, vol. xiv., p. 110. (2) Differential resistance. Two similar resist- ance coils are balanced against each other, one of them being in the region whose temperature is to be measured, the other in a l)ath of liquid whose temperature can be varied and can be ob- served at any time by means of a thermometer. If the galvanometer needle shows that the resist- ance of the two coils is the same, then it is understood that the temperatures must be the same. If the resistances differ, then the liquid surrounding the accessible coil is warmed or cooled until equality is attained. The Tiiermophone. This is a differential re- sistance thermometer, as just described, but com- bined with a telephone instead of a galvano- mctric needle. The equality of temperature due to the absence of an electric current is indi- cated by the absence of all sound when one listens at the telephone. Tliis instrument, the invention of G. C. Whipple, is both accurate and portable and very convenient for use in boats on lakes or at sea. Consult "The Thermophone," Teclh- nology Quarterly, vol. viii., p. 25 (Boston, 1895). Langley's Bolometer. Tliis is a differential thermometer in which the difference of tempei'a- ture of two delicate wires is shown by the de- flections of a galvanometric needle. See Bolom- eter. Deep Sea Thermometer. This is an ordinary or self-registering mercurial thermometer wholly inclosed within a very strong protecting case of glass. The space between the thermometer and its inclosure is partly filled with water or mer- cur}' above which some air remains. When this arrangement is lowered to the bottom of the ocean the great external pressure compresses the inclosure, but does not seriously affect the bulb of the thermometer within. Soil Thermometer. This was formerly made with a large bulb and wide tube many feet in length so that a bulb buried at a depth of even twenty feet would show the top of the liquid column above the surface of the earth. As these were expensive, sluggish, and liable to accident, the modern practice is to use an ordinary ther- mometer inclosed in a wooden tube and pushed down to the proper depth in a hole in the ground. Modern electric thermometers, especially the thermophone, are to be recommended. Nocturnal Radiation Thermometer. This is supposed to give the approximate temperature of a surface exposed to radiation during the night time. The bulb is of glass, sometimes, but not necessarily, blackened; it is fully exposed to the open sky. If all terrestrial heat is cut off by screens the thermometer may be made to give an indication of the clearness of the sky or the temperature of the air that occupies the dome of the sky. If, however, no attempt is made to cut off the radiation and conduction of heat, the thermometers give the temperature of the leaves on the trees. If the thermometer lies upon the up])er surface of a metal or wooden or stone object it gives approximately the tem- perature of the upper surface layer of that ob- ject. Integrating Thermometer. This is intended to sum up the temperature, or it may be the temperature changes during any given interval of time. A clock or chronometer whose pendulum or balance wheel is uncompensated or even in- tentionally over-compensated has a rate that varies with the temperature and becomes an ad- mirable integrator. A metal bar supported on
