PRINCIPLES OF DESIGN. 41 



and ^^^^ speed constant ^^Qe. .' . . (4) 



speed 

 As an instance take the '94 Anglo-American Atlantic cable, 

 the length of -which is 1,847 nauts, capacity 0*420 mfd. per 

 naut, and resistance 1*682 ohms per naut. 

 The KR is by formula (1) 



O-4-20X 1*682 x(l,847)'=2'4x 106. 

 A speed of 249 letters per minute on regular traflBc has been 

 attained on this cable working simplex auto transmission on 

 duplex conditions, and reckoning 3-7 impulses per letter and 

 two extra spaces for every word of five letters (Mr. Patrick B. 

 Delany, in The Electrician, November 30, 1894). The speed 

 constant for this cable is, therefore, 



2-4xlO«x249 _gg>, 



or approximately 600. 



The speed constant is a figure denoting the speed at- 

 tainable on a given cable. It includes every condition of 

 working — namely, the KR of the cable, battery power used, 

 sensitiveness of instrument, sending and receiving con- 

 denser?, curbing, &c. — in fact, it is a comprehensive figure 

 representing the commercial value of the cable taken together 

 with all its transmitting and receiving apparatus', and account- 

 ing for the skill of the operators who work the instruments. 

 The recorder is more capable of working at maximum obtain- 

 able speed on a long cable than on a short one. In fact, on a 

 high KR more than the theoretical speed can be obtained on 

 this instrument, but on a short cable of low KR the speed 

 attainable is less than the theoretical. The theoretically 

 possible speed is, of course, enormously higher on short cables 

 of a few hundred miles than on cables of over a thousand miles, 

 and the difference in the capability of the instrument to 

 respond is noticeable. A higher speed constant may therefore 

 be attained on a long cable than on a short one, and figures 

 which agree fairly well with practice are 600 on cables of low 

 KR to 900 at high KR. 



In any given cable the speed constant is a direct indication 

 of the speed. For instance, if by introducing some improve- 

 ment in the working of a cable the speed constant be raised 

 from 600 to 630 — that is, by 5 per cent. — the speed is also 

 increased by the same percentage. And similarly, in compar- 

 ing this cable with any other cable of the same KR, the 



