The term _circuit_ used frequently in this work, has reference to the wire, which, commencing at the positive pole of the battery, goes to any distance and returns to the negative pole of the battery. When its going and returning are continuous or unbroken, the circuit is said to be _closed_ or _complete_. When it is interrupted, or the wire is disconnected, the circuit is said to be _broken_ or _open_. When a magnet or key or battery is spoken of as being _in the circuit_, it has reference to the use of the wire belonging to the key, magnet or battery, respectively, as a part of the circuit. There are three modes of arranging the wires, so as to communicate between two distant stations. Two of these modes are _inferior_, as they furnish but one circuit for the termini, and consequently obliging one station to wait, when the other is transmitting, both stations not being able to telegraph at the same time. These two modes are called the _dependent circuits_. The first mode is, where two wires are used, of which figure 12 is a diagram. B represents Baltimore, and W Washington; _m_ is the magnet or register; _k_ the key, and _bat_ the battery, all at the Baltimore station; _m′_ is the magnet or register; _k′_ the key at the Washington station. The lines, represent the wires upon the poles, connecting the two stations, and are called the east and west wires. In this arrangement of the wires and also in the second, the key (which has been explained in a preceding figure, 11, and shown at 6 and 7 to be open) must be closed at both stations, in order to complete the circuit, except at the time when a communication is being transmitted.[6] For the purpose of closing the circuit at the key, a metallic wedge is used, which is put in between the anvil 8 and the hammer 7, and establishes the circuit. Supposing the battery is in action, and B has a communication for W: he opens his key, by removing the wedge, and sends his message. The galvanic fluid leaves the point, P, of the battery, and goes to _k_, to _m_, along the east wire to _k′_, to _m′_, and back by the west wire to N pole of the battery. In the same manner it proceeds along the wires, if W is writing to B. In this arrangement, the direction of the galvanic current is the same, whether B or W is communicating, unless the poles of the battery are reversed. [6] At this time the key is opened at the station from which the communication is to be sent. [Illustration: FIG. 12.] [Illustration: FIG. 13.]