726 COMMUNICATION BETWEEN TWO STATIONS. 



called before sending the message by pressing down 

 the key at /Si several times in rapid succession, whereby 

 a clicking of the armature is produced at S 2 ; the clerk 

 thereupon turns the catchy, fig. 363, sets the clockwork 

 in motion, sends back a clicking or other signal to /Si 

 that he is ready, and the printing begins. Several ad- 

 ditional pieces of apparatus are required in the actual 

 working of the telegraph, which, however, cannot be 

 considered here. 



A useful application of electro-magnetism often 

 used is the electric bell in hotels, manufactories, etc. 



The electro -magnet e 2 m fig- 366 consists of two 

 bobbins of copper wire with small cores of bar iron 

 fixed by screws or solder to an iron bar t. The bar , 

 which replaces the bend of a horse-shoe magnet, has 

 three projections, 6, <e, and d. The first, &, serves for 

 attaching t to the small wooden board which supports 

 the whole contrivance; c carries the bell; and to d a 

 flat metallic spring, /i, is fixed. The spring /i carries 

 the armature a a, to the end of which the hammer k is 

 fixed, which strikes the bell. To the back of the arma- 

 ture another flat spring, / 2 , is attached, which presses 

 gently against the pointed end of the brass screw s. 

 The nut m of this screw is connected by a wire with the 

 binding screw h : and one of the ends of the spiral of the 

 electro-magnet is similarly connected with the binding 

 screw ij h and i serve for connecting the apparatus 

 with the terminals of the battery. The second end of the 

 spiral is clamped by a screw-head to the flat spring /i. 

 The whole is usually suspended to the wall by rings, n n, 

 and protected against dust or damage by a small 

 wooden box (left out in the figure), with two apertures, 



