96 



THE VOYAGE OF H.M.S. CHALLENGER. 



CD is the comparison coil S, made of the same wire as the resistance coil T, and equal to 

 it in resistance when the temperatures of both are the same. This coil is immersed in a 

 copper vessel with double sides, 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 this will also be 

 the temperature of the resistance coil T. To avoid the error which would be otherwise 

 introduced by the leads to the resistance coil T, the cable was constructed of a double core 

 of insulated copper wire, protected by twisted galvanised steel wire. One of the copper 



Fig. 32. — Siemens' Electrical Thermometer. 



cores was connected to the arm BC of the bridge, and the other to the arm DC, and the 

 steel wire served as the return (earth) connection for both. The resistance coil and 

 comparison coil were made of silk-covered iron wire 0*15 mm. diameter, and each about 

 432 ohms resistance at a temperature of 66° F. To allow the resistance coil to be readily 

 affected by changes in the temperature of the water, it was coiled on a brass tube with both 

 ends open, allowing a free passage to the water. Sir William Thomson's marine galvano- 

 meter, with a mirror and scale, was employed to determine the balance of the bridge. 

 Several more or less successful observations were made with this instrument during 



