104 



THE ELECTRIC TELEGRAPH. 



(2.) Circular, the station participating in the despatches. 

 (3.) Corresponding, the station transmitting and receiving 

 by the lines. 



70. Battery commutator. When the insulation of the line 

 varies, or by any other reason as, for instance, when an end 

 station has to transmit to a near station with little line resist- 

 ance in the circuit it becomes necessary to alter the strength 



of the current, a battery com- 

 mutator is inserted, by which 

 one-third, two-thirds, or the 

 whole of the elements, may be 

 brought into service by simply 

 changing the place of a con- 

 tact peg. Such an apparatus 

 consists of four slabs, as in 

 Fig. 55. The copper pole of 

 the battery is connected to 

 the screw of the bar, B, and 

 connections from elements 

 one-third and two-thirds of 

 their number from the copper 

 pole, are brought to the screws 

 of the bars c and D respectively ; the zinc pole being con- 

 nected in the usual way with earth. The place of the stopper 

 in J,-<yor d determines the battery power used. 



When more than one line is worked with a single battery, 

 a bar-commutator (Fig. 56) is used. Three brass bars, 1, 2, 

 and 3, are screwed on a slab of dry wood or vulcanite, and, 

 above them, at right angles, three others, I, II, III. The 

 bars are insulated from each other when the holes are open, 

 but when a contact cone is inserted in one of them, the cor- 

 responding cross-bars are electrically connected. If the front 

 contacts of the keys are connected to the three bars I, II, 

 III, and three parts of the battery, as above, to the bars 1, 2, 

 3, the operator is able to work the lines with either of the 

 battery powers which may be necessary. 



71. Translation or re-transmission of Morse-signals. On 

 long telegraph circuits, where the wires are insulated by 



Fig. 55. 



