The plates are revolving in opposite directions; thus if there be a charge on one of the conducting segments of one plate and an opposite charge on one of the conducting segments on the other plate near it, their potential will be raised as the rotation of the plates separates them.[2] [Illustration: FIG. 34.--The Wimshurst Electric Machine.] CHAPTER III THE ELECTRIC CURRENT The ordinary statement that an electric current is flowing along a wire is only a conventional way of expressing the fact that the wire and the space around the wire are in a different state from that in which they are when no electric current is said to be flowing. In order to make laymen understand the action of this so called current, it is generally compared with the flow of water. In comparing hydraulics and electricity, it must be borne in mind, however, that there is really no such thing as an “electric fluid,” and that water in pipes has mass and weight, while electricity has none. It should be noted, however, that electricity is conveniently spoken of as having weight in explaining some of the ways in which it manifests itself. All electrical machines and batteries are merely instruments for moving electricity from one place to another, or for causing electricity, when accumulated in one place, to do work in returning to its former level of distribution. The _head_ or _pressure_ in a standpipe is what causes water to move through the pipes which offer _resistance_ to the _flow_. Similarly, the conductors, along which the electric current is said to flow, offer more or less _resistance_ to the flow, depending on the material. Copper wire is generally used as it offers little resistance. The current must have pressure to overcome the resistance of the conductor and flow along its surface. This pressure is called _voltage_ caused by what is known as _difference of potential_ between the source and terminal. [Illustration: FIG. 35.--Analogy of the flow of water to the electric current. The water in the reservoirs A and B stands at different heights. As long as this difference of level is maintained, water from B will flow through the pipe R to A. If by means of a pump P the level in B be kept constant, flow through R will also be maintained. Here, by means of the work expended on the pump, the level in the reservoir is kept constant; and in the corresponding case of the electric current, by the conversion of chemical energy a constant difference of potential is maintained.] The pressure under which a current flows is measured in _volts_ and the quantity that passes in _amperes_. The resistance with which the current meets in flowing along a conductor is measured in _ohms_. =Ques. What is a volt?= Ans. A volt is that electromotive force (E. M. F.) which produces a current of one ampere against a resistance of one ohm. =Ques. What is an ampere?=