handle of the commutator at A is turned in the other direction, _b_ and _c_ are connected, as also _a_ and _d_; the current from P passes along _a d_ to the ground plate G, thence to G´, along _d´ b´_, round the needle _n´_, back by the wire to the station A, where, after circuiting the needle _n_ in the same direction as the needle _n´_, it travels by _b_ and _c_ to the negative pole N. The upper end of the needle, at both stations, is deflected to the left by the passage of the current in this direction. It is easily seen that, with two wires and one battery, two needles can be worked at both stations, either one moving alone, or the other alone, or both together; but for the two to move differently, two batteries must be used. The systems by which either the movements of a single needle, or of a pair of needles, may be made to indicate the various letters of the alphabet, numerals, and so on, need not here be described. They are of course altogether arbitrary, except only that the more frequent occurrence of certain letters, as _e_, _t_, _a_, renders it desirable that these should be represented by the simplest symbols (as by a single deflection to right or left), while letters which occur seldom may require several deflections. One of the inventions to which the title of this paper relates can now be understood. [Illustration: FIG. 5.] In the arrangement described, when a message is transmitted, the needle of the sender vibrates synchronously with the needle at the station to which the message is sent. Therefore, till that message is finished, none can be received at the transmitting station. In what is called duplex telegraphy, this state of things is altered, the needle at the sending station being left unaffected by the transmitted current, so as to be able to receive messages, and in self-recording systems to record them. This is done by dividing the current from the battery into two parts of equal efficiency, acting on the needle at the transmitting station in contrary directions, so that this needle remains unaffected, and ready to indicate signals from the distant station. The principle of this arrangement is indicated in Fig. 5. Here _a b n_ represents the main wire of communication with the distant station, coiled round the needle of the transmitting station in one direction; the dotted lines indicate a finer short wire, coiled round the needle in a contrary direction. When a message is transmitted, the current along the main wire tends to deflect the needle at _n_ in one direction, while the current along the auxiliary wire tends to deflect it in the other direction. If the thickness and length of the short wire are such as to make these two tendencies equal, the needle remains at rest, while a message is transmitted to the distant station along the main wire. In this state of things, if a current is sent from the distant station along the wire in the direction indicated by