110 PYEOMETER thermo-electric currents produced by heating the junction of two different metals, as Bec- querel's; 8, those which determine tempera- tures by the measurement of changes, pro- duced by heat, in the electrical resistance of a length of platinum wire, as Siemens's ; 9, those which use the expansion of the wave length of a sound, which traverses a tube placed in the furnace whose temperature is to be measured, as Mayer's. We will restrict our detailed de- scription to the three pyrometers which ex- perience has shown to be most trustworthy, viz. : Dtiniell's pyrometer, the air pyrometer, and Siemens's pyrometer. Of the others we will give only general descriptions of the prin- ciples on which they depend. 1. The first py- rometer based on the expansion of solids ap- pears to have been invented by Mnsschenbroek about 1730. This instrument, which he called a "pyrometer," was formed of a metallic bar, fixed at one end, and connected at the other with wheel work which multiplied the motion of elongation caused by the elevation of its temperature. This was improved by others, who directed their efforts principally to the mechanism by which the motion was commu- nicated to the index. Many of these contri- vances are described in the article " Thermom- eter and Pyrometer " in vol. ii. of the " Natural Philosophy " published in the " Library of Use- ful Knowledge " (London, 1832). Daniell's py- rometer, called by its inventor "the register pyrometer," was first described in the " Transactions of the Royal So- ciety" for 1830. It consists of two parts, the register, fig. 1, and the scale, fig. 2. The register is a solid bar of black-lead earthenware, A, highly baked. In the axis of this a hole is drilled, reaching from one end of the bar to within half an inch of the other extremity. In this cylindrical cavity is placed a rod of platinum or of iron, a a, 6$ in. long. Upon the top of the bar rests a cylindrical piece of porce- lain, c c, long enough to project a short distance beyond the extrem- ity of the black-lead bar, to serve as an index. It is confined in its position by a ring or strap of plati- num, d, passing round the top of the black-lead tube, which is partly cut away at the top ; the ring is tightened by a wedge of porcelain, e. When it is exposed to a high temperature, the expansion of the metallic rod, a a, forces the index forward to a distance equal to the difference in the amount of expan- sion between the metallic rod and the black- lead bar, and when cool it will remain pro- truded to the same distance, which will be greater or less according to the temperature ; the exact measurement of this distance is ef- fected by the scale, fig. 2. This scale is in- Fio. 1. DanielPs Pyrometer, Register. dependent of the register, and consists of two rules of brass, f g, joined together by their edges accurately at a right angle, and fitting square upon the two sides of the black-lead bar. Near one end of this double rule a small brass plate, A, projects at a right angle, which f Fic. 2. Daniell's Py- rometer, Scale. when the instrument is used is brought down upon the shoul- der of the register, formed by the notch cut away for the pla- tinum strap. To the extremity of the rule nearest this brass plate is attached a movable arm, D, turning at its fixed extremity upon a centre, ', and at the oth- er end carrying an arc of a cir- cle, E, the radius of which is exactly 5 in., accurately divided into degrees and thirds of a de- gree. Upon this arm, at the cen- tre, , another lighter arm, C, is made to turn, carrying upon the extremity of its longer limb a vernier, II, which moves on the face of the arc, and sub- divides the graduation into minutes. The shorter arm, which is half an inch in length, crosses the centre, and terminates in an obtuse steel point, m, turned inward at a right angle. To use the instrument, the metallic rod is placed in the register, and the index is pressed firmly down upon its extremity and secured tightly by the platinum strap and the wedge. The position of the index is then read off on the scale, by placing the register in the reentering angle for its reception, with the cross piece firmly held against the shoulder, and the steel point, wi, resting on the top of the index, in a notch cut for it, which coincides with the axis of the rod. A similar observation, made after the instrument has been heated and allowed to cool, gives the value of the expansion. The scale of the pyrometer is compared with that of the mercurial thermometer, by observing the amount of expansion between two fixed points, such as the melting of ice and the boil- ing of mercury. In this pyrometer the tem- perature to which its register has been exposed is deduced from the amount of elongation of its metallic bar, on the supposition that the amount of elongation for an elevation of the same number of- degrees is the same whether these degrees occur in the lower or in the high- est regions of the thermometric scale. We now know, however, that the coefficient of expan- sion of a solid is not constant throughout the
