1882.] 



On a Deep Sea Electrical Thermometer. 



91 



connexions of the bridge are shown in figs. 1 and 2. The arm CD is 

 the comparison coil S made of the same wire as the resistance coil T, 

 and equal to it in resistance. This coil is immersed in a copper vessel 

 of 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. 



To avoid the error, which would be otherwise introduced by the 

 leads to the resistance coil, the cable was constructed of a double 

 core of insulated copper wire, protected by twisted galvanised steel 

 wire. One of the copper 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 connexion for both. 



The resistance coil and comparison coil were made of silk-covered 

 iron wire '15 millim. 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 W. Thomson's marine galvanometer with a mirror and 

 scale was employed to determine the balance of the bridge. 



Mr. J. E. Hilgard, assistant in charge of the United States Coast 

 and Geodetic Survey, has sent me the following results of Commander 

 Bartlett's experiments. 



The apparatus was set up on board the " Blake," at Providence, m 

 April, 1881, but owing to there being no ice machine on board, only 

 preliminary experiments were made until the following August. 



The " Blake " sailed from Charleston on August 4th, running a 

 line over known depths in the current of the Grulf Stream. A 60 lb. 

 sinker used in sounding was attached to the end of the cable near the 

 resistance coil, which was allowed to hang freely below. When well 

 in the strength of the stream a series of temperatures were taken by 

 the Miller- Casella thermometers on the sounding wire, and imme- 

 diately after the insulated cable was lowered to the surface, and water 

 from the surface placed around the comparison coil on deck. The 

 temperature of the attached thermometer read the same as that 

 determined for the surface by the thermometer attached to the hydro- 

 meter case. 



Under these conditions the pencil of light from the mirror was on 

 the zero of the scale. During the experiments the vessel was rolling 

 from 10° to 15°, and there was a moderate breeze from south-east. 

 The resistance coil was lowered to five fathoms below the surface, and 

 was allowed to remain five minutes, the circuit being closed, the 

 pencil of light remained at zero. Lowerings were then made to 10, 

 20, and 30 fathoms, and in each case five- minutes were allowed for 



