358 Transactions. — Chemistry and Physics. 



ance of 1 unit 

 betwen 0° and 

 100° C 



Matthiessen ... -4264 \ nu 



-,-,,- AAO Change in resist- 



lleming ... -443 



Lorenz ... '432 



Benoiit ... -460 



My own value ... -4478 :; 



Matthiessen' s value for the temperature coefficient is 

 usually given as 00365 per degree at ordinary tempera- 

 tures. This value, as may be seen by differentiating the 

 formula I have given, is the rate of change of po at 15° C. 



Further research is evidently required to set up a standard 

 for "pure tin." I am at present engaged in testing some 

 samples of commercial tin, some results for which will be 

 found in a table printed herewith. 



To add to the confusion, I find that at a temperature be- 

 ween 170° and 190° the resistance of a sample of tin (hard- 

 drawn sample A) increases 36 per cent., the highest tem- 

 perature reached being 2007° C. Upon reheating to 2148° a 

 further small increase (0 - 6 per cent.) occurred, making a total 

 of 4-2 per cent, on two " annealings." 



Upon another sample the effect of fusing the wire (laid flat 

 on asbestos) was tried. The resistance at 15° C. increased 

 51 per cent. It will be observed, however, that commercial 

 tin does not vary much in the different samples I have tested. 

 This will be noted as a point of great importance in estimating 

 the reliability of tin as a fuse material, other essential points 

 being thermal conductivity, emissivity, and fusing-points. I 

 have not yet examined either of the former two quantities 

 very closely ; but, as far as sundry fusing-tests indicate, the 

 change in thermal conductivity is very small after " anneal- 

 ing " the hard-drawn wire. As to emissivity, I hope to have 

 something to say about this later on, meanwhile I include 

 figures showing the effect on the fusing-current for one or two 

 sizes of wire of shellacking and of thoroughly oxidizing the 

 surface of the wire (by dabbing with nitric acid). It will be 

 seen that a 10-per-cent. margin would about cover the effect of 

 shellac or white oxide with these sizes. This is another point 

 to be noted with regard to reliability. 



As to fusing-points, a table is given of sundry samples, but 

 the results are, owing to uncertainty in temperature stan- 

 dards, not absolute. Callendar and Griffiths give the fusing- 

 point as 231 - 68° C. Still, the comparison shows that the 

 variation which we may expect is very small. The standard 

 thermometer has been sent to Kew. 



The foregoing leads to the conclusion that there is no 

 standard to which I can refer my wire for comparison with 

 that used by Sir W. Preece. The only way in which the 



* Thermometer errors included ; correct value probably leap. 



