280 



Mr. W. H. Preece. 



[Dec. 22, 



December 22, 1887. 



Admiral Sir GEORGE HENRY RICHARDS, K.C.B., Vice- 

 President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. " On the Heating Effects of Electric Currents. No. II." 

 By William Henry Preece, F.R.S. Received November 

 24, 1887. 



On March 19th, 1884, I submitted to the Royal Society a paper on 

 the heating effects of electric currents,* showing the strength of 

 current necessary to fuse the fine platinum wire employed for pro- 

 tecting submarine cables from the ill effects of atmospheric electricity. 

 The paper proved that the law that regulates the production of heat 

 is one which can be expressed by the formula C = ad sl2 , " a" being 

 a constant dependent on the metal used, and " d " the diameter of the 

 wire. The current observed was that which heated the wire up to 

 the point of self-luminosity (525° C). 



Since "cut-outs" of the same character as the cable lightning pro- 

 tector have become an essential feature of all electric lighting 

 installations, to act as safety fuses when from accident or design an 

 excess of current is allowed to pass through the conductor, it became 

 most desirable to determine the current that would fuse wires of 

 different diameters, and of different materials, so as to determine the 

 coefficient a for all metals. The best material to use and the proper 

 dimensions of the fusible wire to be employed for the protection of the 

 electric light Conductors would thus be easily deduced. 



My source of electricity was a large secondary battery of 52 cells. 

 I could regulate the current flowing at will by a rheostat of thick 

 iron wire, and by varying the number of cells. The current strength 

 was calculated by measuring the potential difference at the ends of a 

 thick flat platinoid bar, whose resistance was 01822 <o , inserted in the 

 circuit, and so large that it would not perceptibly warm up nor have 

 its resistance appreciably increased with any current used. The sizes 

 of wire experimented upon were limited by the current. It is not safe 



* ' Hoy. Soc. Proc.,' 1884, So. 231. 



