WILLIAM SIEMENS, F.K.S. 31 



the height of the liquid in a tube is to represent electromotive 

 force or the height of this column represents the attraction of the 

 the zinc for the oxygen. If to one element I add a second similar 

 element, I produce the same effect as adding to one column another 

 similar column ; I superadd one tension to another, or I double 

 my electromotive force. 



Now, the liquid-column, shown in the diagram representing the 

 electromotive force, would discharge itself through a narrow pipe, 

 with a velocity depending upon the height of column, on the one 

 hand, and upon the frictional resistance or obstruction offered to 

 the discharge of liquid on the other hand. The analogous ob- 

 struction offered to the electric current within a conductor we 

 call electrical resistance. If the pipe is of a certain length, say 

 one metre,and of a certain size.say of one square millimetre sectional 

 area, we may call the obstruction which it offers to the flow of liquid 

 a unit measure. If we were to double the length of the pipe, we 

 should have exactly double the resistance ; and if we neglected 

 the influence of inertia, only half the amount of liquid would flow 

 in a given time. By increasing the length of pipe indefinitely, 

 we shall be able to ascertain its length in measuring the delivery 

 at the outlet. This is accomplished in dealing with electrical re- 

 sistances by means of the electrical balance which I shall describe 

 presently. 



Another important point for us to fix, before entering upon the 

 real subject of the evening, is that of charge, or electrical induc- 

 tion. If the liquid, represented in the diagram, was suddenly let 

 loose upon the pipe, it would not flow from it until the pipe itself 

 was filled ; and if the sides of the pipe were composed of a porous 

 substance, then its pores must also be filled beforehand ; but if 

 we suppose that this substance, besides being porous, is elastic, 

 that is to say, composed of cells containing air, then the amount 

 of liquid that would have to be absorbed into the walls of the pipe 

 would be proportionate to the height of liquid, or to the electro- 

 motive force, as we should call it. 



In these illustrations, analogous laws are involved to those which 

 we have to deal with in electricity. I must mention, however, 

 another. If the porous substance composing the pipe allowed of 

 a certain amount of liquid to ooze out on all sides, we should then, 

 not only have to deal with the liquid flowing out at the other end, 



