Construction of Platinum Thermometers. 105 



filaments for incandescent lamps undergo more or less rapid 

 deterioration. 



The Siemens pyrometer is a commercial and not a scientific 

 instrument. I nave myself examined some of the most recent 

 pattern, and I should have been surprised if they had not 

 been found to exhibit changes of zero when used at high 

 temperatures. The wire is wound on common clay, which is 

 apt to attack it, and is inclosed in an iron tube without 

 sufficient protection from the metallic and other vapours 

 which are sure to be present. 



In the case of lamp-filaments which are heated by a cur- 

 rent in vacuo, it might appear at first sight as though the wire 

 were perfectly protected from strain or contamination; but 

 this is far from being the case. The sudden heating and 

 cooling of the wire when the current is turned on or off, and 

 the intense radiation which keeps the surface at a lower tem- 

 perature than the central portions, must be a severe strain on 

 the wire. It is also evident that any crack or flaw in the 

 surface will tend to be intensified by the local development 

 of greater heat ; and if the wire is heated to a temperature 

 near its melting-point where it begins to be appreciably vola- 

 tile, this action must inevitably produce serious results. If a 

 wire which has been thus treated be examined under the 

 microscope, its surface will generally be found to be cracked 

 and scored in a manner which is of itself amply sufficient to 

 account for the increased resistance and brittleness. 



The wire of a platinum thermometer which is properly pro- 

 tected does not undergo any alterations of this kind, if treated 

 with reasonable care, I have recently succeeded in making 

 these thermometers of a very convenient and accurate form ; 

 and I have reason to believe, from inquiries which have reached 

 me from various sources, that a description of the pattern 

 which I have found to give the best results would be useful 

 to other observers who require a sensitive and trustworthy 

 thermometer. 



The simplest form of platinum thermometer is made by 

 fusing or welding a coil of fine wire to leads of relatively low 

 resistance. The coil and leads must be suitably insulated and 

 supported ; for most purposes it is convenient to inclose the 

 instrument in a tube of similar dimensions to an ordinary 

 thermometer. For use at temperatures below 700° C. the 

 leads may be of copper or silver, and the tube of hard glass. 



For rough work at temperatures below 1000° C. very fair 

 results may be obtained by the use of a wrought-iron tube. 

 The leads should also be made of iron. Copper and silver are 

 too volatile. Their vapours will attack the platinum, and 



