The First Atlantic Cables 



all the tube is distended, and its compression, or 

 secondary effect, really transmits the impulse. 

 A remedy for this is a condenser formed of alter- 

 nate sheets of tin-foil and mica, C ', connected 

 with the battery, B, so as to balance the electric 

 charge of the cable wire (Fig. 60). In the first 

 Atlantic line an impulse demanded one-seventh 

 of a second for its journey. This was reduced 





Fig. 60. Condenser 



when Mr. Whitehouse made the capital dis- 

 covery that the speed of a signal is increased 

 threefold when the wire is alternately connected 

 with the zinc and copper poles of the battery. 

 Sir William Thomson ascertained that these 

 successive pulses are most effective when of pro- 

 portioned lengths. He accordingly devised 

 an automatic transmitter which draws a duly 

 perforated slip of paper under a metallic spring 

 connected with the cable. To-day 250 to 300 

 letters are sent per minute instead of fifteen, as 

 at first. 



In many ways a deep-sea cable exaggerates in 

 51 



