360 Transactions. — Chemistry and Pliysics. 



figures, reduces the discrepancy to 28 per cent. ; and, secondly, 

 that unless the effect of time is carefully studied the observed 

 values may be too great. As an instance, I may give my own 

 experience. By applying current by slow increments (or what 

 I then considered slow) to a wire of 100 mils I obtained a 

 value for fusing-cmrent of 44 amperes, against the value 31 

 which I subsequently found to be the true one, and 52 as given 

 by Sir W. Preece's formula. Not only that, but I obtained a 

 number of results all agreeing within about 2 per cent., which 

 I, at that early stage, considered highly satisfactory. Upon 

 going into the subject mathematically, however, I found that 

 time was of the greatest importance, and, as indicated, con- 

 victed myself of the surprising error of 42 per cent, due to 

 " personal equation." I had discovered this before testing the 

 smaller size, unfortunately, but will hazard the calculation 

 that I would have erred 15 to 20 uer cent, wdth a 40-mil fuse. 

 Sir W. Preece, so far as I have observed, makes no mention 

 of this source of error in his papers. 



I have not yet quite finished my work on the cooling 

 effect of terminals, but have got far enough to be able to give 

 the length of fuse at which Sir W. Preece's values apply. 

 The following are these lengths : — 



lengths used, or rather longer, so that in practice, and unless 

 the fuse is confined or the terminals get hot, Sir W. Preece's 

 table gives a value of the fusing-current that will not fuse the 

 wire. This is, I believe, the explanation of the fact that the 

 error of Preece's tables has not hitherto been detected by 

 practical men, who seem to have been satisfied that the 

 troubles that have arisen have been due to faults due to the 

 material tin. 



Many engineers — not, I think, insurance inspectors — 

 prefer copper as a fusing material. It is significant to note 

 that in my preliminary experiments I tried copper fuses up 

 to about 30 amperes, and found my figures agree with 

 Sir W. Preece's, and that the cooling effect of the terminals 

 is much less than with tin. This was to be expected, since 

 the experimental figures go up to 53 amperes. 



A practical point I should like to mention is the remark- 



* Very long wire ; 5 amperes, = 2 in. 



