202 THE ADDRESSES, LECTURES, ETC., OF 



doubled ; we have thus a conductor of double length and sectional 

 area, and therefore of four times the weight, and relying upon 

 this calculation it is argued that the weight of the conductor must 

 increase as the square of the distance : so that a conductor of 30 

 miles' length would require to be (60) 2 = 3600 times the weight 

 of the half-mile conductor, and this enormous increase in weight 

 would certainly be required if the object to be accomplished was 

 the working of one electric lamp by a dynamo-electric machine. 



My critics have, however, fallen into the error of overlooking 

 the fact that half a unit resistance is the same for a circuit capable 

 of working one lamp as it is for working 100 or 1000 lamps. 

 Electricity is not conducted upon the conditions appertaining to a 

 pipe conveying a ponderable fluid, the resistance of which increases 

 with the square of the velocity of flow : it is, on the contrary, a 

 matter of indifference what amount of energy is transmitted 

 through an electric conductor, the only limit is imposed by the 

 fact that in transmitting electrical energy the conductor itself 

 retains a certain amount proportional to that transmitted, which 

 makes its appearance therein in the form of heat. If this heat 

 was allowed to accumulate, the electrical resistance of the con- 

 ductor would increase in proportion to such increase, and a point 

 might be reached where fusion of the wire would ensue. 



I will now connect the spiral of platinum wire I hold in my 

 hand with the dynamo-electrical machine which is working a 

 hundred yards off, and you will discover in a moment that the 

 wire is red-hot, owing to the amount of electricity that has been 

 passed through a wire of so small a sectional area. 



The real power of transmission of an electric current depends, 

 therefore, upon its capability to discharge its heat to surrounding 

 objects, and it will be readily conceived that a wire of sixty times 

 the sectional area and sixty times the length of another wire is 

 capable of radiating away 60 ^60 = 460 times as much heat per 

 hour as the smaller conductor, and that 460 machines or lights 

 may be supplied through it without causing inconvenience. 



When, some weeks ago, I had occasion to use this argument 



before the Institution of Civil Engineers, your President,* who 



happened to be at the meeting, immediately recognised its force, 



and, with the fertility of mind for which he is so remarkable, 



* Sir William Thomson. 



