11. _The Relay_ (Fig. 32).--Telegraphing from 3,000 to 10,000 miles under the ocean is full of difficulties not now to be explained. Of course when we attempt to telegraph many miles upon land we find that the resistance of the wire cuts down the strength of the current so that it will not move the sounder. This, however, is readily obviated by the relay devised by Morse. It simply serves as an automatic key to close a circuit. A diagram will make this clear (Fig. 33). Suppose the line wire to be very long and on account of its resistance the current is too feeble to operate a sounder. It is likely to be about .025 ampere where the local sounder may require .25 ampere or ten times as much. It is easily possible to wind a magnet (Fig. 33), _R_, such that .025 ampere will close the armature _a_, so that it may complete a local circuit when it would not make noise enough for a sounder. _B_ may represent a local battery of any desired strength which may operate the sounder _S_ of that station as loudly as may be desired. [Illustration: Fig. 33] [Illustration: Fig. 34] 12. _Annunciator_ (Fig. 34).--We live in a fifth-floor apartment. When we push the button to call the elevator a No. 5 appears in the annunciator in the elevator car. This tells the elevator boy where the call comes from. Take out two or three screws and the annunciator opens, revealing a series of electro-magnets like the one shown in Fig. 35. When an electric current passes around the coil it pulls back an iron catch and allows a number to drop so as to show through a small window. The elevator boy, having noted that the call is from the fifth floor, pushes up the number and the iron catch holds it until the coil is magnetized again by an electric current. [Illustration: Fig. 35] [Illustration: Fig. 36] [Illustration: Fig. 37]