Brown. — Action of Fusible Cutouts. 365 



Kilgour's law, and the approximate values of e calculated 

 from my results. The experimental values obtained by 

 Ayrton and Kilgour are shown by circles. 



It will be seen that my results agree fairly (considering a 

 possible difference between the materials tin and platinum) 

 with Ayrton and Kilgour's experimental results, but do not 

 agree with the extrapolation indicated by the law of Ayrton 

 and Kilgour given above, and that Sir \V. Preece's constant 

 value is incompatible with both. 



My curve, it will be seen, agrees fairly well with a straight- 

 line law from 157 mils to 20 mils diameter of wire of the 

 following form — 



e = 0-000314 + 0031 D- 1 , 157 to 20, 



the maximum error being about 8 per cent, at 97 mils (only 

 4 per cent, if the fusing-current is calculated from the for- 

 mula, since the current is proportional to the square root of 

 the emissivity). The figures are only provisional, but it is 

 interesting to compare them with the Ayrton and Kilgour 

 formula. However, I do not think that emissivity can be a 

 simple function of the diameter. As the result of a consi- 

 deration of the factors involved I fancy that the true curve 

 will be found wavy or " kinky." 



Experiments upon samples A and B, while giving results 

 hardly distinguishable from each other, both agree in suggest- 

 ing a kink in the curve at 97 mils. 



Of the further work remaining to be done with fuses it 

 will be observed — (1.) That a standard is required for the 

 specification of tin wire (or whatever metal is selected) in the 

 particulars of specific resistance, annealing coefficient, and 

 temperature coefficient (or specific resistance at a tempera- 

 ture near fusing-point) and the fusing-point. At any rate, 

 it should be decided whether a guarantee of " commercial 

 purity" of metal is sufficient. (2.) That the cooling effect 

 of terminals needs inquiry. I hope shortly to publish figures 

 accounting for this factor ; but a full theory of this phe- 

 nomenon involves amongst other things thermal conductivity. 

 This quantity, however, can, I think, be calculated in a form 

 suitable for the purpose from the cooling-effect results. 

 (3.) The effect of change from a horizontal to a vertical 

 position of the fuse- wire must be studied. This should, 

 however, be a small matter after the phenomena for hori- 

 zontal wires have been determined. Corrections in the form 

 of small percentages should cover it in practical cases. 

 (4.) That then the phenomena of fuses in their porcelain 

 boxes may be studied with some hope of success. This is 

 the problem which I set out to solve ; it is the practical 



