The fundamental connections of the telegraph are shown in Figure 217. If the key _K_ is pressed down by an operator in Philadelphia, the current from the battery (only one cell is shown for simplicity) flows through the line to New York, passes through the electromagnet _M_, and thence back to Philadelphia. As long as the key _K_ is pressed down, the coil _M_ acts as a magnet and attracts and holds fast the armature _A_; but as soon as _K_ is released, the current is broken, _M_ loses its magnetism, and the armature is pulled back by the spring _D_. By a mechanical device, tape is drawn uniformly under the light marker _P_ attached to the armature. If _K_ is closed for but a short time, the armature is drawn down for but a short interval, and the marker registers a dot on the tape. If _K_ is closed for a longer time, a short dash is made by the marker, and, in general, the length of time that _K_ is closed determines the length of the marks recorded on the tape. The telegraphic alphabet consists of dots and dashes and their various combinations, and hence an interpretation of the dot and dash symbols recorded on the tape is all that is necessary for the receiving of a telegraphic message. The Morse telegraphic code, consisting of dots, dashes, and spaces, is given in Figure 218. [Illustration: |A .- |H .... |O . . |U ..- | |B -... |I .. |P ..... |V ...- | |C .. . |J -.-. |Q ..-. |W .-- | |D -.. |K -.- |R . .. |X .-.. | |E . |L --- |S ... |Y .. .. | |F .-. |M - - |T - |Z ... . | |G --. |N -. | | | FIG. 218.--The Morse telegraphic code.] The telegraph is now such a universal means of communication between distant points that one wonders how business was conducted before its invention in 1832 by S.F.B. Morse. [Illustration: FIG. 219.--The sounder.] 301. Improvements. _The Sounder._ Shortly after the invention of telegraphy, operators learned that they could read the message by the click of the marker against a metal rod which took the place of the tape. In practically all telegraph offices of the present day the old-fashioned tape is replaced by the sounder, shown in Figure 219. When current flows, a lever, _L_, is drawn down by the electromagnet and strikes against a solid metal piece with a click; when the current is broken, the lever springs upward, strikes another metal piece and makes a different click. It is clear that the working of the key which starts and stops the current in this line will be imitated by the motion and the resulting clicks of the sounder. By means of these varying clicks of the sounder, the operator interprets the message. [Illustration: FIG. 220.--Diagram of a modern telegraph system.]