SUBMARINE TELEGRAPH CABLES 427 



known to the art which will operate on so small a power except at 

 uneconomically low speeds. For this reason it is necessary to insert 

 between cable and recorder an instrument which will amplify the 

 received signal. 



A cable signal-shaping amplifier must fulfill many severe require- 

 ments. With its associated apparatus it must be capable of correcting 

 the attenuation of the cable by equalizing the strength of all important 

 component frequencies of the signal and it must also be capable of 

 controlling in its output circuits a power many times as large as it 

 receives.* It must be as insensitive as possible to interfering currents 

 not included in the band of frequencies necessary to the signal and it 

 is very desirable that overloading, which may be caused occasionally 

 by these currents, should not permanently influence its adjustment or 

 destroy any of its elements. The strength of its output current 

 should be readily adjustable. It should be mechanically rugged, as 

 otherwise its maintenance will require too large a proportion of the 

 time of the staff at the cable station, and delays to traffic will be 

 caused. Finally it should be protected as well as possible against 

 local electrical fields and mechanical vibration and its operation 

 should not be affected by conditions of extreme humidity. 



Comparison of Mechanical Amplifiers and Vacuum Tube 



Amplifiers 



In recent years several satisfactory mechanical amplifiers (called 

 magnifiers in cable parlance) have been invented and their use has 

 led to radical improvements in the speed of transmission over non- 

 loaded cables. Most of these magnifiers utilize a sensitive moving- 

 coil galvanometer, which moves some device a small distance in order 

 to control a much greater powej than that which caused the original 

 motion of the coil. We may consider as typical of these the selenium 

 magnifier which causes a beam of light to move over one or the other 

 of two groups of selenium cells and thus varies their resistance, the 

 Heurtley hot wire magnifier which changes the resistance of two pairs 

 of almost microscopic heated wires by causing them to move relatively 

 to each other, and the electrolytic magnifier which changes the re- 

 sistances of a group of immersed electrodes. With all of these devices 

 the controlled power is obtained from a local battery, but it is so 

 small that it can do little more than operate a sensitive siphon recorder 

 or a delicate moving coil relay. The latter may of course control a 

 larger power which may in turn cause the operation of a comparatively 

 rugged electromagnetic relay and thus indirectly a considerable power 



* In practice the power amplification factor of the various types of aiiipHfiers 

 may range between five thousand and one hundred milHon. 



