THERMOMETER. 



THERMOSTAT. 



by M. Renault Our liiuiU will not allow ui to do more than just 

 indicate the melhixU adopted. And fnl, as to the calibration of 

 the tab*. A short column of mercury (lea* than 1 inch in length) 

 baring been introduced into it, the tube i attached to the frame 

 of Pemmix's dividing engine, and, by means of flexible tubing, is 

 connected at both ends with india-rubber air-bag*, the pressure on 

 which is regulated by screws. The mercury is brought to that part of 

 the tube where the graduation is to be commenced. The cutting- 

 frame of the engine carries a small microscope with cross-wires in its 

 focus ; on turning the dividing-screw, the microscope wire is made to 

 coincide with the first extremity of the mercury, and the screw is then 

 turned forward until the wire reaches the second extremity ; so that 

 the length of the column is thus given in revolutions of the screw. The 

 mercury is then made to move along the tube by pressing on one of the 

 india-rubber bags until the first end again coincides with the microscope 

 wire, when the length of the column is again measured, and the 

 mercury again moved forward : this process is repeated until the column 

 has been measured for each length of itself through the whole extent of 

 the proposed scale. Permanent marks are made on the glass at the 

 points of commencement and ending of calibration. If the progress of 

 the numbers shows any considerable irregularity in the tube, and as a 

 verification of the first set of measures, the calibration may be re- 

 peated, commencing at a point one-half the length of the column in 

 advance of the original starting point. A series of measures inter- 

 polated from the two seta may then be adopted. In this operation 

 the mercury should be very pure and the tubes clean. Secondly, with 

 respect to the gradual inn, measured lengths of the column of mercury, 

 in its successive steps along the tube, correspond to equal volumes, so 

 that assuming the calibre of the tube not to vary throughout the 

 small length of the calibrating column, it is evident that by dividing 

 the spaces occupied successively by the mercury into an equal number 

 of parts, the divisions will represent the same capacity, although they 

 may be of different lengths. Before making the tube into a ther- 

 mometer, the divisions, of the scale may be verified by introducing a 

 longer column of mercury, and examining whether the column occu- 

 pies an equal number of divisions in different parts of the scale. 

 Should there be any irregularity, a table of corrections is to be formed, 

 but this is seldom required. The divisions are cut with a fine needle- 

 point upon a coating of engraver's varnish, and are afterwards etched 

 by means of hydro-fluoric acid. The required dimensions of the bulb 

 may be found approximately by weighing a measured length of the 

 mercurial column, and from the known expansion of mercury and its 

 specific gravity, computing the capacity of the bulb. Thirdly, as to the 

 determination of the scale co-efficients. The thermometer being pro- 

 perly filled with mercury and sealed, the divisions of the scale evidently 

 represent equal increments of the volume of the fluid, but their value 

 is quite arbitrary. The advantage of this plan is, that the divisions 

 on be tested before the instrument is converted into a thermometer. 

 The instrument is kept sufficiently long to allow of the settlement of 

 the freezing point The tendency of this point to shift, if the gradua- 

 tion take plaice soon after the filling, has already been noticed, but 

 Mr. Welsh has pointed out another peculiarity in this respect : after a 

 thermometer has been exposed for some weeks to the ordinary tempe- 

 rature of the air, if its freezing point be ascertained, and it be suddenly 

 exposed for a short time to the temperature of boiling water, and again 

 immediately placed in ice, the latter determination of the freezing 

 point will be lower than the former, by as much as from 0'l to 0'2, 

 and the freezing point does not recover its former position for some 

 time, probably two or three weeks. The freezing point is found by 

 placing the thermometer in finely pounded ice, from which the water 

 U drained off as it melts. The boiling point is taken at the temperature 

 of steam (from distilled water) of the same elasticity as that of the 

 atmosphere. The barometer is observed at the time, and the correction 

 to a uniform height of 30 inches (reduced to 32) is found from 

 Begnault's Table. The fixed points are determined in the position in 

 which the thermometers are to be used. 



In the graduation of mercurial thermometers, it is customary to con- 

 sider increments of volume as proportional to increments of temperature, 

 a plan which cannot be adopted in the case of spirit thermometers. 

 In testing some spirit thermometers 'graduated for low temperatures 

 (namely, to 75 Fahr.) intended for the Arctic Expedition under Sir 

 E. Belcher, Mr. Welsh proceeded to determine the rate of expansion 

 of alcohol in glass, as compared with that of mercury. The alcohol 

 had been carefully prepared by Prof. Miller, and its specific gravity at 

 80 was 0796. A tube was calibrated and divided with an arbitrary 

 wale, and its divisions were found upon verification to be of exactly 

 equal capacity throughout. The tube was then furnished with a bulb 

 of the same dimensions as those intended to be supplied to the 

 Admiralty, and wu filled with alcohol Comparisons were then made 

 between the readings of this instrument and those of a standard mercu- 

 rial thermometer through u wide a range as was found practicable. 

 The comparisons above the freezing point were taken in water : those 

 below 82' were taken in freezing mixtures of ice and salt, or chloride 

 of calcium. From these comparisons the law of expansion was 

 deduced, but for the details we most refer to Mr. Welsh's report. 



The apparatus used at Kew for comparing the indications of different 

 thermometers consists of a cylindrical glass vessel, 15 inches deep, and 

 84 inches diameter, together with a stand for supporting the ther- 



mometers, and an agitator or flat ring of tinned iron, fitting easily 

 within the vase, for agitating the water so as to preserve an equable 

 temperature throughout The stand is a vertical rod supported by a 

 small tripod, resting on the bottom of the vessel Hooks sliding on 

 this rod servo for suspending the thermometers. These are arranged 

 with their bulbs at the same height in a circle of 8 inches diameter 

 round the rod, and are kept fixed by means of elastic bands to a pro- 

 jecting six-rayed frame, attached to the supporting rod. In this way 

 six thermometers can be compared at once. The whole apparatus is 

 placed on a wooden revolving stand, and in taking the observations, 

 the observer first agitates the water briskly for some time, then turns 

 the revolving stand until each thermometer is brought opposite to his 

 eye, and he reads off the scales as quickly as possible to an assistant, 

 who takes down the numbers. The six thermometers can be read off 

 and recorded in 20 seconds. More than one set of readings should be 

 made for each temperature, and the order of observing the instruments 

 should be reversed, to avoid, as much as possible, the changes whic-h 

 may occur during the reading off. 



In graduating a thermometer for meteorological purposes, the 

 freezing point is determined as usual with melting ice or snow, and an 

 Upper reading is fixed, by placing the thermometer in water of a certain 

 temperature, together with a standard instrument. Instruments are 

 now sold which have undergone comparison with the standards of the 

 Kew Observatory, and as their price is moderate, there is no reason 

 why the observations made by amateurs in various parts of the 

 kingdom should not be accurate, and comparable with each other. The 

 scales are engraved on the tubes, so that no correction is required for 

 the scale. When the instrument is intended to record the external 

 temperature, it should be placed in the shade away from the reflected 

 and radiated heat of walls and buildings, and the bulb should be kept 

 perfectly dry. Convenient thermometer stands have been contrived 

 byjMr. Ulaisher, Mr. Lawson, and Dr. Drew, for sheltering the instru- 

 ments from disturbing influences. The best hours for observation are 

 9 a.m., 3 p.m., and 9 p.m. The readings should be taken rapidly, 

 especially in cold weather, to prevent the warmth of the body from 

 influencing them. 



THERMO-MULTIPLIER. [THEiuio-ELECTBicrrv.] 



THERMO-PHONE. When a bar of copper slightly curved and 

 furnished with a wire for a handle, is heated, and its convex surface 

 placed on a notch cut in the upper surface of a stout ring of lead rest- 

 ing on a sounding board, it will vibrate and evolve a variety of musical 

 sounds resembling those of an tcolian harp, and also the drone of the 

 bagpipes, changing in the most fitful and irregular manner, and varying 

 with the pressure applied by means of a point to the concave surface of 

 the copper. The sounds will continue until the temperatures of the 

 lead and the copper approach each other. Such an arrangement is 

 called a tkermo-phone. The experiment succeeds best with metals 

 whose conducting powers for heat are very different, such as copper 

 and lead, the conducting power of the former being = 845, and of the 

 latter =287 (silver =1000). The experiment, which is due to Mr. 

 Trevelyan, was investigated by Professor Forbes (' Phil. Journal '), who 

 showed that it could be performed with various other metals of dif- 

 ferent conducting powers, and the subject has been more recently 

 investigated by Professor Tyndal (' Phil. Trans.') 



THERMOSTAT, or heat-governor, is an apparatus invented ami 

 patented in 1831 by Dr. Ure, for regulating temperature in the 

 processes of vaporisation and distillation, in heating baths and hot- 

 houses, in adjusting the draught of stoves'and.furnaces, in ventilating 

 apartments, 4c. It acts upon the principle that when two thin metallic 

 bars, of different degrees of expansibility, are riveted or solder 

 by side, any change of temperature will cause a sensible flexure in the 

 compound bar ; the side consisting of the least extensible metal be- 

 coming concave, and the other convex. By this flexure of the com- 

 pound bar, which takes place with considerable force, a movement i* 

 effected, which, by the intervention of levers, may be made to opi-n or 

 close stop-cocks, dampers, ventilators, or any description of valves, 

 and thereby to regulate the flow of heated liquids or the admission 

 and emission of air. The principle of the thermostat may be applied 

 in many different ways, of which the following may serve as examples. 

 In fig. 1, a is the compound bar, firmly fixed at 4. To the other, or free 



Fig. 1. 



Fig. 5. 



end of the compound bar, i attached, by means of a connecting rod, 

 the short end of a lever mounted upnn the axis of a circular revolving 

 valve, or ventilator, c ; and from the longer end of the lever is BUS- 



