434 POPULAR LECTURES AND ADDRESSES. 



take too long, to occupy your attention on matters 

 of detail, and to explain minutely hew it is 

 that resistance is to be measured by a velocity. 

 It seems curious, but you will form a very general 

 idea of it in this way. Suppose you have two 

 vertical copper bars and a little transverse 

 horizontal bar placed so as to press upon those 

 two bars. Let the plane of those two bars be 

 perpendicular to the magnetic meridian, and 

 place a little transverse bar, like one step of a 

 ladder, across the two vertical bars. Let this 

 bar be moved rapidly upwards ; being moved 

 across the line of the horizontal component of 

 the earth's magnetic force, it will, according to 

 one of Faraday's discoveries, experience an in- 

 ductive effect, according to which one end of it 

 will become positively electrified, and the other 

 negatively. Now, let the two bars upon which 

 this presses be connected together : then the 

 tendency I have spoken of will give rise to a 

 current. That current may be made, as in 

 Orsted's discovery, to cause the deflection of a 

 galvanometer needle. Now, you will see how 



