PRINCIPLES OF DESIGN. 39 



reckoned on the basis of a five, eight or ten-letter word as may 

 be agreed upon. This spacing is equivalent to two contacts, 

 one of which belongs to the proper space after each letter 

 (allowed for by the above figure). Tnere is, therefore, one 

 extra contact to be allowed for spaces between words, and if 

 the average word is considered as of five letters, the part of a 

 contact per letter representing word spaces would be | = 0"2 

 and the total contacts per letter 3 6 + 0*2 = 3-8. Again, if a 

 ten-letter word is taken as a basis we have to add J;j = 0'l and 

 the contacts per letter become 3*6 + 0-1 = 3'7. 



In the auto-transmitter each of the centsre holes in the per- 

 forated slip represents one contact, and, therefore, the speed in 

 "letters per minute" on this basis can readily be observed 

 where these instruments are in use by counting the number of 

 centre holes per minute and dividing by the number 3 8 or 3*7 

 for five or ten-letter words respectively. 



Wich standard conditions as to battery power, receiving and 

 sending condensers and sensitiveness of receiving instrument, 

 the speed of signalling on all submarine cables, of whatever 

 size, type or length, varies In inverse proportion to the respective 

 KR of each. 



Take, for Instance, two cables, one of 1,500 nauts, with 

 a conductor resistance of 6 ohms per naut and capacity of 

 0-35 microfarad per naut, and the other of 2,000 nauts with 

 0*3 microfarad capacity and 5 ohms resistance per naut. The 

 KR in each case by formula (1) is respectively 



0-35 x6x (1,500)2 = 4-7 xlO^ or 4-7 millions, 



and 



0-3x 5 X (2,000)2 = 6 xlO^ or 6 millions. 



The theoretical speeds are in inverse proportion thus : 



Speed on l,5Q0-naut cable 6 x lO^ 6_ 



Speed on 2,000-naut cable " 4'7 X 106~4-7" 



The factor 10^ cancels out and does not appear in any calcu- 

 lations of relative speeds. 



Assuming the speed of working simplex on the 2,000-naut 

 cable to b3 120 letters per minute, then the speed on the 

 1,500-naut cable under the same conditions would be 



120x6 720 _„ , ,, , ^ 



— -^-^ — = -r-;^ = 153 letters per minute. 



4-7 4-7 ^ 



