Philosophical Transactiom. 379 



tion of twelve batteries similar to those above mentioned ; 

 several trials shewed that it barely discharged six batteries. 



4. Having therefore determined that there is a limit to the 

 quantity of electricity which wires are capable of transmitting, 

 it became easy to institute experiments on the different conduct- 

 ing powers of different metallic substances, and on the relation 

 of this power to the temperafure, mass, surface, or length of 

 the conducting body, and to the conditions of electro-magnetic 

 action. 



The leading result of these researches was, that " the con- 

 ducting power of metallic bodies varied with the temperature, and 

 was lower, in some inverse ratio as the temperature was higher." 

 Thus a wire of platinum of one •220th of an inch diameter, and 

 three inches long, discharged the electricity of two batteries 

 when kept cool by oil ; but when in air it barely discharged one 

 battery ; and some apparently paradoxical results depend upon 

 this circumstance. Thus, let a fine wire of platinum be heated 

 red hot in the galvanic circuit; then apply a spirit lamp to any 

 part of it, so as to heat that part to whiteness; the consequence 

 is, that the rest of the wire ceases to remain red hot, in conse- 

 quence of the bad conducting power of the white hot portion : 

 again, for the converse illustration, apply a piece of ice to a 

 part of the wire, and the remainder, previously only red hot, 

 will now become heated to intense whiteness, in consequence of 

 the improvement in conducting power occasioned in the cool 

 part, enabling a larger quantity of electricity to traverse the 

 wire. 



Another important fact developed in these researches relates 

 to the great difference in the conducting power of different me- 

 tals : in experiments with 6 inches of wire of -^^j inch diameter, 

 silver discharged the electricity of 65 pairs of plates ; copper 

 and lead that of 56 ; tin of 12 ; platinum of 11 ; and iron of 9 ; 

 all the wires being kept cool by immersion in water. These 

 facts lead to others connected with the magnetic energies of the 

 different wires which are not a little curious. If wires of the 

 different metals be placed successively in the voltaic circuit 

 the best conductors become most powerfully magnetic ; tliat is, 

 they take up the largest quantity of iron filings ; so that in this 

 way a silver wire becomes an infinitely more powerful magnet 

 than one of iron. In a powerful voltaic circuit, (the wires not 

 being capable of carrying off the whole of its electricity) two 

 inches of silver wire of l-30lh inch diameter took up 32 grains 

 of iron filings ; a similar wire of copper only attracted 24 grains ; 

 of platinum II ; and of iron only 8-j'*,. . 



There are various other important topics treated of in this 



paper; more especially the production of heat in various media 



by the transmission of electricity. That different wires beconu; 



very differently heated when placed in tlie voilnio i irenil was 



2 ( : 2 



