

ON SOME MARVELS IN TELEGRAPHY. 253 



Hitherto we have considered the action of the electric 

 current in deflecting a magnetic needle to right or left, a 

 method of communication leaving no trace of its transmission. 

 We have now to consider a method at once simpler in 

 principle and affording means whereby a permanent record 

 can be left of each message transmitted. 



If the insulated wire is twisted in the form of a helix or 

 coil round a bar of soft iron, the 

 bar becomes magnetized while 

 the current is passing. If the 

 bar be bent into the horse-shoe 

 form, as in Fig. 6, where A c B 

 represents the bar, a b c d e f 

 the coil of insulated wire, the 

 bar acts as a magnet while the 

 current is passing along the coil, 

 but ceases to do so as soon as FIG. 6. 



the current is interrupted.* If, 



then, we have a telegraphic wire from a distant station in 

 electric connection with the wire a b c, the part e f descend- 

 ing to an earth-plate, then, according as the operator at that 

 distant station transmits or stops the current, the iron A c B 

 is magnetized or demagnetized. The part c is commonly 

 replaced by a flat piece of iron, as is supposed to be the 

 case with the temporary magnets shown in Fig. 7, where this 

 flat piece is below the coils. 



So far back as 1838 this property was applied by Morse 

 in America in the recording instrument which bears his 

 name, and is now (with slight modifications) in general use 

 not only in America but on the Continent. The principle 



* I must caution the reader against Fig. 348 in Guillemin's Applica- 

 tion of the Physical Forces, in which the part c d of the wire is not shown. 

 The two coils are in reality part of a single coil, divided into two to 

 permit of the bar being bent ; and to remove the part c d is to divide 

 the wire, and, of course, break the current. It will be seen that c d 

 passes from the remote side of coil b c, Fig. 6, to the near side of coil de. 

 If it were taken round the remote side of the latter coil, the current 

 along this would neutralize the effect of the current along the other. 



