. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. EQUIPMENT. 17 the descent of the thermometer. Such an apparatus has been devised by Sir WilHam Siemens. In the Bakerian Lecture for 1871, he showed that the principle of the variation with the temperature of the electrical resistance of a conductor might be applied to the construction of a thermometer, which would be of use in cases Avhere a mercurial thermometer was not available. The instrument he described has since been largely used as a pyrometer for determining the temperatures of hot blasts and smeltin
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. EQUIPMENT. 17 the descent of the thermometer. Such an apparatus has been devised by Sir WilHam Siemens. In the Bakerian Lecture for 1871, he showed that the principle of the variation with the temperature of the electrical resistance of a conductor might be applied to the construction of a thermometer, which would be of use in cases Avhere a mercurial thermometer was not available. The instrument he described has since been largely used as a pyrometer for determining the temperatures of hot blasts and smelting furnaces ; and it has been found that its indications agree very closely with those of an air-ther- mometer. He devised a similar instrument for measuring tem- peratures where a much greater degree of accuracy is required, as in the case of deep- sea observations; and during the autumn of 1881, this deep-sea electric thermometer was subjected to a series of tests on board the " Blake," by Commander Bartlett. The apparatus consists essentially of a coil of silk-covered iron-wire (Fig. 13), fifteen mil- Hmetres diameter, and about four hundred and thii'ty-two ohms resistance, attached to an insulated cable by which it can be low- ered to the required depth, and connected so as to form one arm of a Wheatstone bridge. The corresponding arm of the bridge is formed by a second coil, made precisely similar to the former one and of equal re- sistance. This coil is immersed in a copper vessel filled with water, and the temperature of the water is adjusted by adding iced or hot water until the bridge is balanced. The temperature of the water in the vessel is then read by a mercurial thermometer, and corresponds with the temperature of the resistance-coil. To avoid the error which would otherwise be introduced by the leads of the resistance-coil, the cable is constructed of a double core of insulated copper-wire, protected by tAvisted gal- vanized steel-wire. One of the copper
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