Now suppose that P and N, Fig. 2, are the positive and negative poles of a galvanic battery at A, and that a wire passes from P to the station B, where it is coiled round a needle suspended vertically at _n_, and thence passes to the negative pole N. Let the wire be interrupted at _a b_ and also at _c d_. Then no current passes along the wire, and the needle _n_ remains at rest in a vertical position. Now suppose the points _a b_ connected by the wire _a b_, and at the same moment the points _c d_ connected by the wire _c d_, then a current flows along P _a b_ to B, as shown in Fig. 2, circuiting the coil round the needle _n_ and returning by _d c_ to N. The upper end of the needle is deflected to the right while this current continues to flow; returning to rest when the connection is broken at _a b_ and _c d_. Next, let _c b_ and _a d_ be simultaneously connected as shown by the cross-lines in Fig. 3. (It will be understood that _a d_ and _b c_ do not touch each other where they cross.) The current will now flow from P along _a d_ to B, circuiting round the needle _n_ in a contrary direction to that in which it flowed in the former case, returning by _b c_ to N. The upper end of the needle is deflected then to the left while the current continues to flow along this course. I need not here describe the mechanical devices by which the connection at _a b_ and _c d_ can be instantly changed so that the current may flow either along _a b_ and _d c_, as in Fig. 2, circuiting the needle in one direction, or along _a d_ and _b c_, as in Fig. 3, circuiting the needle in the other direction. As I said at the outset, this paper is not intended to deal with details of construction, only to describe the general principles of telegraphic communication, and especially those points which have to be explained in order that recent inventions may be understood. The reader will see that nothing can be easier than so to arrange matters that, by turning a handle, either (1), _a b_ and _c d_ may be connected, or, (2), _a d_ and _c b_, or, (3), both lines of communication interrupted. The mechanism for effecting this is called a _commutator_.