72 FARADAY 



piece of wood and a piece of metal joined together, and put 

 it so that the flame shall play equally both upon one and the 

 other, we shall soon find that the metal will become hot 

 before the wood; for if I put a piece of phosphorus on the 

 wood and another piece on the copper, you will find that the 

 phosphorus on the copper will take fire before that on the 

 wood is melted; and this shows you how badly the wood 

 conducts heat. But with regard to the traveling of elec- 

 tricity from place to place, its rapidity is astonishing. I will, 

 first of all, take these pieces of glass and metal, and you will 

 soon understand how it is that the glass does not lose the 

 power which it acquired when it is rubbed by the silk; by 

 one or two experiments I will show you. If I take this 

 piece of brass and bring it near the machine, you see how 

 the electricity leaves the latter and passes to the brass cyl- 

 inder. And again: if I take a rod of metal and touch the 

 machine with it, I lower the indicator; but when I touch it 

 with a rod of glass, no power is drawn away, showing you 

 that the electricity is conducted by the glass and the metal 

 in a manner entirely different; and, to make you see that 

 more clearly, we will take one of our Leyden jars. Now I 

 must not embarrass your minds with this subject too much, 

 but if I take a piece of metal and bring it against the knob 

 at the top and the metallic coating at the bottom, you will 

 see the electricity passing through the air as a brilliant spark. 

 It takes no sensible time to pass through this ; and if I were 

 to take a long metallic wire, no matter what the length, at 

 least as far as we are concerned, and if I make one end of it 

 touch the outside, and the other touch the knob at the top, 

 see how the electricity passes ! It has flashed instantaneously 

 through the whole length of this wire. Is not this different 

 from the transmission of heat through this copper bar 

 (Fie. 42) which has taken a quarter of an hour or more to 

 reach the first ball? 



Here is another experiment for the purpose of showing 

 the conductibility of this power through some bodies and not 

 through ^thers. Why do I have this arrangement made of 

 brass ? [pointing to the brass work of the electrical machine, 

 FIG. 41.] Because it conducts electricity. And why do I 

 have these columns made of glass? Because they obstruct the 



