1890.] on Welding by Electricity. 189 



I have said that the heating effect of an electric current depends 

 upon the quantity of the current, and upon the drop or reduction in 

 electrical pressure. We have four lamps here upon the table, and I 

 think you will see when I turn on the current they are glowing uni- 

 formly; not giving much light, however, for they are glowing very 

 badly, but all of them uniformly bad, no one better than the other. 

 Now I will ask you to remember that if we are introducing a given 

 quantity of electric current here, at the first lamp, at a pressure of say 

 100 volts, and it is leaving the fourth lamp at zero, we are introducing 

 the same quantity of electricity here at the second lamj) as at the first, 

 but the pressure is only 75 volts, we have therefore dropped 25 volts 

 between the two. We are also introducing the same quantity of current 

 to the third lamp, but at 50 volts, we therefore drop another 25 volts. 

 We are introducing it to the fourth lamp at 25, and using this pressure 

 up in this lamp, we come down to zero. What we have done is this : 

 wo have destroyed an equal amount of electrical energy in every lamp 

 by these reductions, and have turned it into heat, making the lamps 

 glow, and it is, as you will have seen, a matter of absolute indifier- 

 ence as regards heating effect, whether we have done this by taking 

 25 out of 100 and leaving 75, or by taking 25 out of 25 leaviuo- 

 zero. If we change the switch and throw one of the lamps out of 

 circuit, we have now the same initial electrical pressure, i. e. 100 

 volts, but there are only three resistances instead of four, and we 

 are consequently now dropping by 33 volts at each lamp. You 

 observe the increase in brightness, but still the three lamps all glow 

 alike. If we switch another lamp out we have only two lamps' re- 

 sistance to overcome, and are dropping by 50 instead of by 33 volts. 

 We therefore get a further increase of brightness'. We take the third 

 lamp out, and may thus destroy the one remaining, for we now have 

 the whole drop of 100 volts occurring at this one lamp, and you see 

 the intense glow that results, although fortunately it has not oiven 

 way. 



But the increases in the heating effect have not varied in the mere 

 ratios of 25 to 33^, 25 to 50, or 25 to 100 ; but have varied as the 

 squares of these ratios, and have done so for this very simple reason. 

 Daring our experiment, we have always commenced with the same 

 electrical pressure of 100 volts ; but when we used only three 

 lamps in the circuit, instead of the fOur which I first showed you, the 

 resistance was only that of the three lamps, i.e. three-fourths of the 

 four, but the pressure being the same the current became four-third 

 times that which it originally was when each of the four lamps was 

 used. The drop in voltage was 33J- volts for each lamp, instead of 

 25, that is to say, this was also four-third times as much as before. 

 The disappearance of electrical energy was therefore four-third 

 times four-thirds = sixteen-ninths, or, in other words, each of the 

 three lamps was heated to ly-th times the heat which was generated 

 in each one of the four lamps. When we had only two lamps in the 

 circuit, the resistance was one-half that which it was when we had 



