356 Transactions. — Chemistry and Physics. 



Art. XLIV. — Besearches into the Action of Fusible Cutouts. 



By E. G. Brown, A.I.E.E. 



[Read before the Wellington Philosophical Society, 12th March, 1901.'] 



Plate XIV. 



The following is an account of the general results of researches 

 (as yet incomplete) which I have been making into the be- 

 haviour of tin wires exposed to the heating effect of electric 

 currents. These researches are the outcome of my experiences 

 as fire underwriters' electrical inspector here, it having been 

 borne upon me that the action of fusible cutouts (or fuses) 

 was not at all satisfactory. 



Since the underwriters have become responsible for the de- 

 sign of these fittings it appeared desirable to make a few ex- 

 periments on the subject. These soon led to the conclusion 

 that the tables at present in use (quoted in the rules I am 

 working under as well as in all the electrical pocket-books I 

 have seen, and due to Sir W. H. Preece,* and commonly 

 called Preece's tables) are erroneous, the error with the 

 larger wires being, according to my experiments, as much as 

 90 per cent., or probably more with very large wires, the error 

 being that the fusing-currents are given too great. This ap- 

 pears an extraordinary statement, but I think I shall be able 

 to show how it is that this error has hitherto escaped the 

 notice of practical men. It will be also suggested, as the re- 

 sult of experiments, that tin is probably a thoroughly reliable 

 material for the construction of fuses, under proper design, of 

 course. 



I found it necessary to establish the physical properties of 

 the tin I was using, the more especially as practical men as- 

 sured me that my wire was made of an alloy of tin and lead. 

 Accordingly I submitted the fuse-wires A and B to a careful 

 approximate analysis. Taking Sn0 2 as 78 - 66 per cent, metal, I 

 obtained, with both A and B, a percentage of tin of 99-5 per 

 cent. ; the impurities, roughly estimated as sulphides con- 

 taining two-thirds of metal, gave 03 per cent, impurities 

 with sample B, and about half this amount in the case of 

 sample A. Sample A is certainly the purer of the two, and it 

 will be noticed in the table of specific resistances that it has 

 the higher conductivity — about 3 per cent, higher. It also 

 has a higher fusing-poiht ; but, still, the values of the fusing- 

 currents observed were practically identical. 



* Proc. Roy. Soc, 1884, No. 231 ; 1887, December ; 1888, April. 



