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Art. XVI.—On a new Thermometer for Lecture Purposes.
[Read before the Philosophical Institute of Canterbury, 1st October, 1874.]
A thermometer of simple construction that would show variations of temperature to a large number of persons at once, would be very useful in the lectures on heat and kindred subjects. It would be of especial value in schools, where but little time can be spent in preparing experiments. A thermoelectric pile and galvanometer is a most valuable piece of apparatus,
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but there are many cases to which it does not apply well, nor is it well seen in flat rooms, in which most elementary teaching is carried on; it is also expensive, and requires some skill to use satisfactorily. These considerations, and the necessity I have often felt for such an instrument, induced me to design the thermometer described below. The thermometer depends upon the different coefficients of expansion of metals, and is constructed as follows:—
About a dozen pairs of thin strips of zinc and steel, about 4 inches long, are riveted at their alternate ends, the two outside strips are continued on beyond one end of the bundle so formed. Each of these two pieces have a steel pin soldered across its end; these two pins work in holes in the end of a long index finger, which at the mean temperature of the range of the thermometer is in a straight line with the bundle of metal strips. Any increase of temperature causes each of the zinc strips to expand more than the steel, and the sum of their difference is at once transmitted to the pins, one of which is thus thrust beyond the other, and so causes the index to move on one side. When the temperature is lowered the index moves to the opposite side. The bundle of strips is filed up into a cylinder and placed in a copper tube, the bottom of which is closed. At the top of the tube is fixed a fan-shaped piece of metal, on which is marked the scale of temperature. The index moves backwards and forwards in front of this scale. The accompanying sketch shows the construction of the thermometer.
When the thermometer is used before a large audience the size of the scale may be increased by fixing to the back of the permanent scale a quadrant of black paper having white figures on its circumference. The metal index may also be lengthened by a strip of white paper pasted on it. With these additions the movement of the index produced by an alteration of a few degrees of heat may easily be seen by an audience of any number. I have used the thermometer to illustrate the following facts:—1st. It offers a good means of showing, by the principle of its construction, the difference of expansion of different metals. 2nd. The constant temperature of fusion and boiling. Thus the thermometer is placed in ice at −10° C., heat is applied, the index moves up to zero, it then remains still till all the ice is melted, it then goes on moving up to 100° C. 3rd. A pair shows well the different rate of cooling of water in polished and blackened tin tubes. 4th. The cold produced by the fusion of solids on ordinary freezing mixtures. 5th. The cold produced by evaporation, and of course a pair would show the difference of wet and dry bulb thermometers.
It might also be used to show the heat developed by friction by placing it
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in a block of wood made to revolve rapidly. It would also do well to show the constant boiling point of the different compounds in a substance subjected to fractional distillation, and for many other experiments. Mr. Ward, of the School of Mines, has suggested that it would be useful for fractional distillation at high temperatures, for which the mercurial thermometer is not available. I have had a thermometer máde with brass instead of zinc for the purpose of making experiments extending over some time in order to ascertain if the instrument is reliable for purposes of observation at comparatively high temperatures. When the thermometer is required to be sensitive to a very small alteration of temperature I have used two such bundles side by side; these two bundles are connected together by a diagonal piece of steel from the top of one bundle to the bottom of the other. The two pieces supporting the pins are brought up together, one from each of the other ends of the bundle of strips. I may mention to any one wanting such thermometers that Mr. Ladd, London, and Mr. Noble, Christchurch, have made them, and are consequently conversant with their construction. The dotted lines on the cut represent zinc, and the continuous ones near them steel.