SINGLE NEEDLE TELEGRAPH. 



effect of which is, that the current transmitted through the gal- 

 vanometer has a tendency to deflect both needles in the same 

 direction. The indicating needle, however, need not be magnetic. 

 If it be sufficiently light, being free from magnetism, it will be 

 carried by the axis to the right or left against the studs, 

 by the deflections of the galvanometric needle which plays 

 within the i coils of the galvanometer, -to 'which it is always 

 parallel. 



In connection with the instrument there are, as usual, an alarum 

 and a galvanic battery. 



By the commutator, the current produced by the* battery may 

 be transmitted upon the line- wire, or suspended or reversed in its 

 direction, according to the position given to the handle. If the 

 handle be vertical, as represented in the figure, the current is sus- 

 pended, the arrangement of the commutator being then such as to 

 cut off all communication between the battery and the line- wire. 

 If the upper arm of the handle be turned to the right, the battery 

 will be connected with the line-wire, on which accordingly the 

 current will be transmitted. If the upper arm be turned to the 

 left, the battery will still be connected with the line-wire, but 

 with its poles reversed, so that the direction of the current on the 

 line-wire will be reversed. 



The mechanical form of the commutator, by which these changes 

 of connection are made is different from that explained in (111), 

 but the principle is the same, and the variation of the details are 

 unimportant. 



To comprehend the practical operation of the instrument, we 

 are to consider that similar instruments, with similar accessories, 

 are placed at each of the stations, between which dispatches are 

 to be transmitted. To render the explanation more clear, let s 

 and s', fig. 67, be the two stations, o and o' the dials, c and c' 



the handles of the commutators, and B and B' the galvanic 

 batteries. If it be intended to send a dispatch from s' to s, the 

 arm of the commutator, c, is left in its vertical position, so that 

 no current can pass from the battery, B, to the line-wire, L. 



11 



