350 Sir W. Snow Harris on a Genei'ul Law of 



(5.) I have shown, that, under certain conditions, the best con- 

 ductor may become the most heated, because it can transmit a 

 greater quantity of electricity*. 



15. In the Philosophical Transactions for 1827 will be found 

 an experimental investigation of the relative conducting powers 

 of different metals. The results are in accordance with the best 

 general deductions of both the old and modern electricians, di- 

 stinguished by their inquiries in this department of physics. It 

 will be seen in this paper last referred to, that the heating effect 

 of the electrical discharge on a metallic wire of a given diameter 

 is precisely the same as that upon four wires of half the diameter 

 and of equal length ; that is to say, in elongating the wire to 

 four times its length by the ordinary mechanical means, and 

 placing it in the ball of the thermometer under the form of four 

 small wires. 



Now in this experiment we may perceive, that since the dia- 

 meter of each of the smaller wires is one-half the diameter of the 

 large wire, and that each of the smaller wires, in transmitting 

 one-fourth part of the charge, contributes one fourth-part of the 

 total effect, it follows, that if the whole charge were transmitted 

 by one of the smaller wires singly, the heating effect on that 

 wire would become sixteen times as great, since it would transmit 

 four times the quantity of electricity, the heating effect being as 

 the square of the quantity (14). Let, for example, the total 

 effect upon the larger wire, or upon the four smaller wires, be 

 16 degrees of the thermometer scale. In this case we have 4 

 degrees for each of the small wires considered alone. Now in 

 discharging all the electricity through one of these small wires, 

 we should have a heating effect equal to 64° ; that is to say, four 

 times the effect on the original or large wire. The comparative 

 heating effects, therefore, on these two wires are in the inverse 

 ratio of 1 : 4, whilst their respective diameters are directly as ] : 2. 

 The heating effects, therefore, are reciprocally proportional to the 

 squai'es of their diameters or to the squares of their radii, that 

 is, inversely as the area of the section. Here is again another 

 result of the practical application of my instrument such as I 

 constructed it, quite in accordance with well-known laws of con- 

 duction since determined, as also in all the subsequent researches 

 of M. Iliess. 



16. I might quote many other examples, all confirming the 

 accuracy of my instrument as an instrument of quantitative elec- 

 trical research, such as I constructed if, and its singular opera- 

 tion in jiroducing results which are now received as general laws 

 of electricity. How then can it be said, with any degree of just- 

 ice, as announced by M. De la Rive on the autliority of M. Iliess, 



* Edinb. Phil. Tiaus. vol. xii. 



