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 cores is connected with 



V^ 



Fig'. 13. — Siemens' 8 

 Sinker and Resistance 

 CoU. (Bartlett, U.S. 

 Coast Survey. ) 



