96 THE ELECTRIC TELEGRAPH. 



instrument, it becomes necessary to employ a commutator or 

 current director to put the apparatus at pleasure in the cir- 

 cuit of the up or down line in order the meet the require- 

 ments of the service. At such a station the apparatus must 

 be so arranged as to be able to assume either of these three 

 positions : 



(1.) When the intermediate station is entirely cut out of 

 circuit, and the end or distant stations on opposite sides 

 correspond directly through the line. 



(2.) When two end or distant stations on opposite sides 

 correspond with each other, and the intermediate station 

 receives the despatch, at the same time. 



(3.) When the intermediate station wishes to communicate 

 with a station up or down the line whilst it has notice of 

 currents arriving from the other side. 



To avoid the inconvenience of altering continually the con- 

 nections to suit these various positions of the apparatus, com- 

 mutators are employed. Various forms of these instruments 

 are given by Siemens, Nottebohm, Borggreve, and others. 



66. One of the completest is that of Nottebohm. It consists 

 of six bars of metal screwed on to a wooden base, cut out in 

 seven holes to receive contact pegs between them, so as to 

 bring them in metallic contact with each other. 



Fig. 50 gives the commutator in half-size, and Fig. 51 the 

 contact peg in full-size. L and L are the terminal screws to 

 receive the line wires coming from the galvanoscopes. R and 

 RU the ends of the electro -magnet coils of the relay. T is 

 connected with the back contact, 2, of the key, and E with 

 earth. Between the front contact of the key and the bar R O 

 the line-battery is inserted. The beam of the key is also to 

 earth, according to the second plan mentioned above (Fig. 49) 

 for arranging the Morse system. 



In the first position, when the intermediate station is to 

 be cut out of circuit to let two other stations correspond 

 direct, the contact peg is put into the hole 3. The current 

 coming from the left-hand side passes over G', L, 3, L, G 2 , 

 and so on, to the other line. The employe can see, by the 

 deflections of the needles of his two galvanoscopes, GJ and G 2 , 



