516 



SUBMARINE CABLE LAYING AND REPAIRING. 



resistance is available in the third arm R for adjustment. The 

 ordinary bridge balance is familiar in this form — 



.■=^ . R. 

 b 



It is well known that variations in resistance due to tempera- 

 ture in the arms a and h, which are always of the same alloy, 

 are the same percentage on each, and, consequently the ratio 

 between them is constant at all temperatures. 



Now, by expressing the balance in the equivalent form 



R 

 h 

 it is obvious that the same thing applies to the resistances R 



Fig. 287. — Gott's Bridge Standardising Arm. 



and 6 — that is, being of the same alloy and varying by equal 

 percentages with temperature, the ratio between them is a 

 constant at all temperatures. 



If, now, the arm a is constructed of manganin, having prac- 

 tically no variation over the ordinary range of temperatures, it 

 follows that the readings on the bridge will be self-correcting. 

 By applying the standardisirg arm to existing and old bridges 

 of german silver or other variable alloys they are at once 

 levelled up to the standard of the most modern manganin 

 bridges. 



Further, a bridge constructed in legal or standard ohms can 

 be at once converted to read in B.A. units by making the 

 standardising arm in B.A.. units. Or if the standardising arm 



