444 Dr. C. W. Siemens on Electrical Resistance. [Apr. 27, 



In measuring temperatures not exceeding 100° Cent., the instrument is 

 so arranged that two similar coils are connected by a light cable containing 

 three insulated wires. One of these coils, "the thermometer-coil," being 

 carefully protected against moisture, may be lowered into the sea, or buried 

 in the ground, or fixed at any elevated or inaccessible place whose tempe- 

 rature has to be recorded from time to time ; while the other, or " com- 

 parison-coil," is plunged into a test-bath, whose temperature is raised or 

 lowered by the addition of hot or cold water, or of refrigerated solutions, 

 until an electrical balance is established between the resistances of the two 

 coils, as indicated by a galvanoscope, or by a differential voltameter, de- 

 scribed in the third part of the paper, which balance implies an identity of 

 temperature at the two coils. The temperature of the test-solution is 

 thereupon measured by means of a delicate mercury thermometer, which 

 at the same time tells the temperature at the distant place. 



By another arrangement the comparison-coil is dispensed with, and the 

 resistance of the thermometer-coil, which is a known quantity at zero tem- 

 parature, is measured by a differential voltameter, which forms the subject 

 of the third part of the paper ; and the temperature corresponding to the 

 indications of the instrument is found in a table, prepared for this pur- 

 pose, in order to save all calculation. 



In measuring furnace temperatures the platinum-wire constituting the 

 pyrometer is wound upon a small cylinder of porcelain contained in a 

 closed tube of iron or platinum, which is exposed to the heat to be mea- 

 sured. If the heat does not exceed a full red heat, or, say, 1000° Cent., 

 the protected wire may be left permanently in the stove or furnace whose 

 temperature has to be recorded from time to time ; but in measuring tem- 

 peratures exceeding 1000° Cent., the tube is only exposed during a mea- 

 sured interval of, say, three minutes, to the heat, which time suffices for the 

 thin protecting casing and the wire immediately exposed to its heated sides 

 to acquire within a determinable limit the temperature to be measured, but 

 is not sufficient to soften the porcelain cylinder upon which the wire is 

 wound. In this way temperatures exceeding the welding-point of iron, 

 and approaching the melting-point of platinum, can be measured by the 

 same instrument by which slight variations at ordinary temperatures are 

 told. A thermometric scale is thus obtained embracing without a break 

 the entire range. 



The leading wires between the thermometric coil and the measuring in- 

 strument (which may be, under certain circumstances, several miles in length) 

 would exercise a considerable disturbing influence if this were not elimi- 

 nated by means of the third leading wire before mentioned, which is 

 common to both branches of the measuring instrument. 



Another source of error in the electrical pyrometer would arise through 

 the porcelain cylinder upon which the wire is wound becoming conductive 

 at very elevated temperatures ; but it is shown that the error arising through 

 this source is not of serious import. 



