160 THE SCIENTIFIC PAPERS OF 



THERMOMETRIC RESISTANCE COIL. The simplest of these is 

 shown in Fig. 3, Plate 12, and consists of a spiral of insu- 

 lated wire wound upon a cylindrical piece of. wood or metal 

 enclosed in a cylindrical silver casing, the two extremities of the 

 wire being soldered to thicker insulated wires, a third thicker 

 wire being joined to one of the other two, the three forming 

 a light cable. This instrument I use for measuring ordinary 

 temperatures on land, and in this form the apparatus would, 

 I conceive, be useful to the physiologist or the medical 

 man for ascertaining the temperature of the human body 

 under certain influences without disturbing it. The instru- 

 ment is extremely sensitive, and temperatures may, with a good 

 Wheatstone balance, be read off to within a tenth of a degree 

 Fahrenheit.* 



In this arrangement of apparatus the indications of the ther- 

 mometric resistance coil, or instrument described, are read off by 

 direct comparison with a mercury thermometer, which latter will 

 represent the exact temperature of the former at a distance, it 

 may be, of several miles. 



The principle is as follows : 



When two similar thermometer coils have different temperatures, 

 they have also different resistances, and, therefore, in order to 

 make them equal, the temperature of the one in the room must be 

 made equal to that of the other at a distance. A plan of the way 

 in which this is arranged is shown in Fig. 4, Plate 12. The two re- 

 sistances, A and B, forming the left-hand side of the parallelogram, 

 consist of coils of silk-covered German silver wire, each of 500 units, 

 and both wound upon the same bobbin, so as to have the same 

 temperature. The resistance thermometer, T', of about 500 units, 

 is placed at the distant point, whilst the comparison thermometer, 

 T, precisely equal in respect of material and resistance to T', is 

 placed in the testing room, and these are connected with the other 

 resistances by the two leading wires, 1 and 1'. The lower end of 

 is put to earth at T', but the corresponding end of 1' is connected 

 with one side of the resistance thermometer, T', and then with the 



* An instrument similar in arrangement to the one here mentioned was described 

 by me before the Physical Section of the British Association at Manchester, in 

 1861 ; and a modified arrangement for measuring deep sea temperature was pre- 

 sented, in the ioint names of Dr. Werner Siemens and myself, to the Berlin 

 Academy in 1863. 





