38 SUBMARINE CABLE LAYING AND KEPAIRING. 



the length of the cable {I), and similarly the total resistance of 

 the conductor is the resistance per naut in ohms (r) multiplied 

 by the length of the cable. Therefore the KR of a cable may 

 be expressed as M x rl, which is equal to krP. That is the 

 product of the capacity per naut in microfarads, the resistance 

 per naut in ohms and the square of the length in nauts. For 

 future reference this is put in the form of the equations : 



KR = krP, (1) 



KR 

 f^r = jr (2) 



The comparison of speeds of signalling must be reduced to 

 a common basis to be of any value. It is usual now for speeds 

 to be reckoned on the basis of "letters per minute," but it 

 must be explained that this figure is not arrived at by counting 

 the number of letters transmitted in one minute in any kind 

 •of message, because the useful space taken up per letter varies 

 in different letters, and the word spacings also vary with the 

 length of the words used. To form some letters on the recorder 

 instrument it takes four impulses or " contacts," while other 

 letters take only three, two or one, and the time occupied in 

 transmitting them is strictly in proportion to the number of con- 

 tacts made in forming each letter. A message taken at random 

 might be composed of words having letters of few impulses, and 

 such a message could be transmitted quicker than one composed 

 of words having letters taking longer to form. Hence, to calcu- 

 late the number of letters per minute on a common basis, the 

 average number of contacts per letter must be arrived at by 

 taking some representative traffic matter, adding all the 

 necessary contacts together and dividing by the total number 

 of letters. This has been carefully done on the Eastern and 

 other systems, and the result shows an average of 2*6 contacts 

 per letter (see The Electrician, April 23, 1897). It is further 

 necessary to allow for the spaces between letters, which is 

 represented by one contact. The space taken by the average 

 letter with its space is therefore represented by 3*6 contacts. 

 Mr. E. Raymond-Barker has independently arrived at this 

 identical figure from examination of 1,000 words of ordinary 

 language (see Electrical Review, Vol. XL., pp. 517 and 600). 

 The space between words is also taken account of and may be 



