DOUBLE NEEDLE TELEGRAPH. 



transmission from all stations except that to which it is exclusively 

 addressed, of the use of the alarum, &c., is applicable, without any 

 important modification to this form of telegraphic instrument. 



THE DOUBLE NEEDLE TELEGRAPH. 



181. This is nothing more than two single needle telegraphs, 

 such as has been just explained, mounted in the same case, their 

 indicating needles playing side by side upon the same dial, and 

 the handles of their commutators placed so that they can be con- 

 veniently worked at the same time, by the right and left hand of 

 the telegraphic agent. Each instrument is altogether inde- 

 pendent of the other, having separate accessories, and transmit- 

 ting its current upon a separate line-wire. 



The purpose of this form of instrument is merely to accelerate 

 the transmission of dispatches, by enabling the agent to produce 

 the signals expressing letters and figures in more rapid succes- 

 sion. In the single instrument there are only two signs made by 

 one deflection of the needles, viz., a deflection to the right and one 

 to the left. In the double instrument there are eight such signs, 

 viz., two with each needle, as in the single instrument, and four 

 obtained by combining the deflections of the two needles. Thus, 

 if o express the position of the needle without deflection, r, a 

 right hand, and / a left hand- deflection, and K the right hand, 

 and L the left hand needle, the following eight signals may be 

 made in the time of a single motion of either needle. 



With a single needle two deflections can only make four signals, 

 viz., rr, II, rj, Ir. But with two needles, these being combined 



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