CHAP. III.] 



ELECTRIC TELEGRAPHY. 



555 



disconnecting apparatus. By the aid of the commutator, telegraphic 

 communication may be kept with the stations interested, and the 

 service continued independently between all the others. 



In the two needle, as in the single needle, telegraph the deflections 

 are limited by two little ivory pins, which have the further advantage 

 of enabling the ear to catch the number of beats by the little blows 

 of the needle upon the ivory. Other inventors have constructed 

 different systems of needle telegraphs which have worked with 



Earth 



FIG. 354. Bain's I and V telegraph, 1843. 



success. We may refer to a few of them, simply noting the principle 

 of their construction. 



We mention first the two needle telegraph of M. Glcesener, which 

 is nothing more than a modification of Wheatstone's. This modifica- 

 tion consists principally in the addition of two electro-magnets to the 

 multiplier of the receiving instrument, each of which acts upon a 

 different pole of the three magnetized needles composing the galvano- 

 meter. The magnetizing coil of these electro-magnets is the continuation 



