1 32 Royal Society. 



ence in the simultaneous changes of intensity at diiFerent places 

 which had already been discovered in the declination. The inge- 

 nuity shown in the invention of instruments and processes, the ma- 

 thematical skill employed in treating the observations, and the im- 

 portance and interest of the results, are well deserving of being ho- 

 nourably marked by the Royal Society, and the adjudication of the 

 Copley Medal to M. Gauss. 



The Council have also awarded a Copley Medal to Dr. Faraday 

 for his discovery of Specific Electrical Induction, published in the 

 eleventh series of his Experimental Researches in Electricity*. 



From the peculiar view which he had taken of the phenomena 

 of induction, Dr. Faraday was led to expect some particular relation 

 of this process to diiFerent kinds of matter, through which it might 

 be exerted. This relation he succeeded in establishing by the most 

 decisive experiments. 



The phenomena are shown in tlieir simplest form by an instru- 

 ment which he has named a Differential Inductometer. It consists 

 of three insulated metallic plates, placed facing each other; the cen- 

 tre one being fixed, and the other two moveable upon slides, by 

 which they may be approximated to or withdrawn from the centre. 

 Each end plate is connected with an insulated leaf of an electro- 

 meter. When a charge is communicated to the centre plate under 

 ordinary circumstances, the induction is equal on both sides, and 

 the gold leaves are not disturbed. But if after uninsulating them, 

 and again insulating them, a thick plate of shell-lac or sulphur be 

 interposed between two of the plates, unequal induction will take 

 place on the two sides, and the gold leaves will attract one another. 

 By these means Dr. Faraday ascertained that, taking the specific 



inductive capacity of air to be 1' 



That of Glass is 1 '76 



Shell-lac 2" 



Sulphur 2-24. 



The results obtained with spermaceti, oil of turpentine, and naph- 

 tha were higher than that of air, but their conducting powers inter- 

 fered with the accuracy of the experiments. 



By another form of apparatus he ascertained that all aeriform 

 matter has the same power of sustaining induction ; and that no 

 variations in the density or elasticity of gases produced any varia- 

 tion in their electric tension until rarefaction is pushed so far as that 

 discharge may take place across tliem. 



Hot and cold air were compared together, and damp and dry air, 

 but no difference was found in the results. 



The great importance of the discovery and complete establishment 

 of such a principle as that of specific inductive capacity, in all its 

 relations both experimental and theoretic, is so palpable, that any 

 comment must be superfluous ; and the Council have felt they can- 



• Dr. Faraday's Eleventh Series of Researches will be found in our last 

 volume, p. 281, et seq., and the Supplement to it in the preceding Number, 

 p. 34. 



