40 SUBMAEINE CABLE LAYING AND EEPAIRING. 



Or, putting It the other way, if the working speed on the 

 1,500-naut cable is 153 letters per minute, the speed on the 

 2,000-naut cable under equal conditions would be 



153x4-7 720 lom ^^ 



=-77- =120 letters per mmute. 



6 D 



In each case it will be noticed that the numerator is the same 

 although the figures are for different cables. Hence it is a 

 constant, and is usually termed the "speed constant" of a 

 cable. In the above example the speed constant is 720, but 

 in practice it may be anything from about 600 to 900, accord- 

 ing to the conditions of working as affected by battery power, 

 capacity of signalling condensers and sensitiveness of receiving 

 instrument. Although the fundamental and principal factor 

 determining speed is the KR of the cable, the above-named 

 working conditions have a great deal to do with the actual 

 speed attainable on any given cable. There are considerable 

 limits in the adjustment of the recorder suspension to give the 

 best definition, and the speed constant can be raised by skilful 

 and experienced adjustment to suit the particular cable with 

 which the instrument is in use. Increased battery power will 

 usually give increased definition and increased speed, but there 

 are practical limitations under this head on account of the 

 cable itself when of great length and on account of the instru- 

 ment adjustment on shorter cables. Generally speaking, the 

 battery voltage must be in proportion to the conductor resis- 

 tance of the cable. Of course it is not practicable to work 

 an old cable with as relatively high battery power as a new 

 one. The degree of curbing is also a most important factor 

 in the improvement of speed. The higher the speed constant 

 attained the more efficiently is the cable concerned being 

 worked, but it will be seen from the foregoing remarks that 

 this is not dependent alone upon the KR. That is, it is 

 not dependent upon the cable alone, but also upon the con- 

 ditions of working. 



The speed constant on a given cable is the number obtained 

 by multiplying the KR of that cable by the speed in letters 

 per minute working simplex and dividing by 10"^. 



That is ^^^^^^ = speed constant, . . . (3) 



