1889.] Note on the Thermo-electric Position of Platinoid. 



287 



assumed considerable importance in the construction of electrical 

 instruments and resistance coils. Partly on this account, and partly 

 from present requirements, it became both interesting and necessary 

 to determine the thermo-electric constants for a specimen of this 

 alloy. 



Platinoid is in composition very similar to German silver. In the 

 manufacture of the alloy, however, phosphide of tungsten is em- 

 ployed ; and although an exceedingly minute quantity of metallic 

 tungsten remains in the alloy, yet the properties of the substance are 

 in many respects remarkable. The metal is capable of being polished 

 so as to be almost as beautiful as silver in appearance, having only a 

 slightly darker and more steel-like colour; and when it has been 

 polished it remains absolutely untarnished, even in the atmosphere of 

 a large town, for years at any rate. It has very remarkable proper- 

 ties as to electric resistance. It possesses a very high resistance, 

 while at the same time it has a much lower temperature variation of 

 electric resistance than any other known metal or alloy. It has also, 

 as Sir William Thomson has found, very excellent elastic qualities. 



Although it is not proposed to use the platinoid with any metal 

 other than platinum in the investigation on thermal radiation above 

 referred to, it nevertheless seemed advisable, when these experiments 

 were being undertaken, to determine its position with respect to some 

 other metals. It was accordingly tried as a pair with platinum, iron, 

 aluminium, and with two specimens of copper. 



A low-resistance Thomson's reflecting galvanometer was specially 

 prepared for the purpose of these experiments. The mirror was a plane 

 parallel mirror of very excellent quality by Steinheil of Munich. 

 Its deflections were observed by means of a telescope with cross-wires 

 and scale, instead of by a lamp and scale. To avoid any influence of 

 the suspending fibre (which even though of single cocoon silk fibre 

 does with short fibres give an appreciable torsional resistance) the 

 mirror was suspended by spider line. The suspending of a mirror, 

 weighing with its magnet 02 gram, by a single spider line is a matter 

 of some nicety and difficulty ; but when it has been accomplished the 

 result is so thoroughly satisfactory that it is easily admitted to be 

 well worth a morning's labour. 



To make the suspension two small pieces of very thin bristle or 

 of hard-spun silk fibre or split horsehair are attached to the ends of a 

 suitable length of spider line recently spun by a good large* spider. 

 By means of these attachments, which are easily seen, the spider line 

 can be handled. It is then brought over the galvanometer mirror ; 

 and great assistance is experienced in these operations, and in opera- 

 tions with single silk fibres, by performing them on the top of a piece 

 of looking-glass laid on the table. The illumination from beneath of 



* The body about as large as a pea. 

 VOL. XLVI. T 



