190 Sir Frederick Bramwell [April 18, 



the four, thus the current was doubled, the drop of pressure per 

 lamp was also doubled, giving, therefore, the double of the double, 

 or four times the heating effect ; while when we had only one lamp 

 in the circuit the current and the drop were each four times as great 

 as when the four lamps were in, so that in this case we had four 

 times four, or sixteen times the heat generated. 



Here is another way of showing this effect. We have in this 

 machine two hoops, side by side and shaped like small croquet hoops. 

 They are made of similar iron wire, but one is double the length of 

 the other. The electrical pressure being the same at the two ends 

 of each of the two hoops, we shall have quantities of currents coming 

 through which will be in proportion to the lengths of these hoops. 

 That is to say, you will have half the current coming through the 

 long one which comes through the short one. The short one there- 

 fore ought to glow more brightly, because it will be hotter than 

 the long one. Now I turn on the current and the short one 

 begins to glow, but you cannot see any light at present from the 

 long one. Now I can see a feeble light appearing in the long 

 one, but probably those who are not so near as I am cannot do 

 so yet. I am sorry that we have not sufficient horse-jDower in our 

 engine here to enable us to do that which I did at the Institution of 

 Civil Engineers — to go on with an increased current till the short 

 one is fused. Now the long one is glowing fairly, and the short hoop 

 is very bright indeed. There you see an instance where, the electric 

 potential being the same at each end of two bars, but one being double 

 the length of the other, and carrying therefore only half the amount 

 of current, shows hardly any light at all, while the other has a con- 

 siderable amount of luminosity. 



It is obvious that if we were using a perfect conductor of 

 electricity we could have no electric heating whatever, because 

 a perfect conductor would not destroy any of the electrical 

 energy of the current passing through it, and, therefore, no heat 

 would be produced. It is equally obvious that if we had sub- 

 stances absolutely impermeable to electricity, so that no current could 

 pass through it, we could not heat such a substance. "What we want, 

 therefore, is something between the two. Fortunately for us, both 

 iron and steel hold a very happy position in respect of their electrical 

 conducting power, or to use the converse term which I have hitherto 

 employed — their resistance. At the ordinary temperature of 60° 

 Fahr. a piece of wrought iron, 1 foot long and 1 square inch in 

 section, would need half a volt to drive 10,000 amperes through it, 

 and in doing this 3700 foot-pounds of electrical energy would be 

 destroyed in every second of time, equal, therefore, to the production 

 of 4f units of heat in the conductor. If we had a similar length 

 and area of German silver, as it is so much worse as a conductor, 

 we should in the same time destroy rather more than double the foot- 

 pounds of electrical energy, namely, 7700. Tliis is equivalent to the 

 production of a little over ten units of heat in each second. A 



