If; 



Fig. CS. THE DOUBLE NEEDLE TELEGRAPH. 



THE ELECTRIC TELEGRAPH. 



CHAPTER Y. 



109. Current controlled by making and breaking the contact of con- 

 ductors. 110. Instruments for controlling the current commutators. 

 111. General principle of the commutator. 112. Its application 

 to telegraphic operations. 113. To transmit a current on the up line 

 only. 114. On the down line only. 115. On both lines. 116. To 

 reverse the current. 117. To suspend and transmit it alternately. 

 118. How to manage a current -which arrives at a station. 119. To 

 make it ring the alarum. 120. Station with two alarums. 121. 

 Notice of the station transmitting and receiving signals. 122. When 

 signals not addressed to the station the current is passed on. 123. 



LARDNER'S MUSEUM OF SCIENCE. H 177 



No. 38. 



