1885.] Electric Resistance of Platinoid. 341 



influences. Accordingly he patented the process, and registered the 

 alloy under the name platinoid. 



Platinoid is practically German silver with the addition of a small 

 percentage (1 or 2 per cent.) of metallic tungsten. The tungsten is 

 added in the form of phosphide of tungsten, a considerable percentage 

 of which is in the first place fused with a portion of the copper. The 

 nickel is then added ; and then the zinc and the remainder of the 

 copper. The mixture requires to be re-fused more than once, and 

 during the process the phosphorus and a considerable portion of the 

 tungsten originally added is removed as scoriae. In the end there is 

 obtained a beautiful white alloy, which is platinoid. When polished 

 the alloy is scarcely distinguishable in appearance from silver. To 

 test the quality claimed for it as to being untarnishable, I have for 

 some weeks been keeping ornamental specimens lying exposed to the 

 ordinary town atmosphere ; and I have satisfied myself that the alloy 

 has a very remarkable power of resisting the tarnishing influence of 

 the air of a large town. 



It is, however, with the electric resistance of platinoid that I have 

 chiefly interested myself. German silver wire has proved of great 

 use in the construction of galvanometer coils and of resistance coils, 

 on account of two important properties, viz., its very high resistance, 

 and the smallness of the variation of its resistance with change of 

 temperature. Both these properties are possessed in a still higher 

 degree by platinoid alloy. 



The resistance of German silver differs considerably in different 

 specimens. It is commonly stated to be 21'l7xlO -6 B.A. ohms 

 between opposite faces of a centimetre cube at 0°C.*; or, reducing to 

 legal ohms, 20935 X 10 -6 legal ohms between the opposite faces of a 

 centimetre cube. The table on page 342 shows the resistance of a 

 number of specimens of platinoid wire. 



It appears from these results that the specific resistance of 

 platinoid is about one and a half times that of German silver. 



The experiments on the variation of resistance of platinoid with 

 temperature were carried on in the following way. The specimen of 

 platinoid to be tested was wound on a wooden bobbin, on the surface 

 of which a screw had been cut, and the spires of the helix were kept 

 separate by lying between the threads of the screw. This coil was im- 

 mersed in a bath of oil, and was connected in series with a known wire 

 of German silver, the temperature of which was kept constant, and with 

 a single Daniell's cell. The differences of potential between the two 

 ends of the platinoid wire and the two ends of the German silver wire 

 were determined by applying the electrodes of a high-resistance 



* Griven by Prof. Fleeming Jenkin, F.R.S.,as expressing the results of Mattkies- 

 sen's experiments. 



