514 SUBMARINE GABLB LAYING AND REPAIRING. 



and by similar formular derived from (4), p. 442, 



= 8,485 + ^^-^^-1^ 

 ' 240,000 



= 8,485+^i^^=8,485 + 14-2 = 8,499-2. 

 0-24 



The true CR is then the mean of the above corrected CR's — 

 namely, 8,498-6 ohms, which agreed very closely with the last 

 recorded CR in the splice list of 8,500 ohms. 



Temperature Corrections of Bridge Readings. — Bridge boxes 

 are marked with the temperature at which the readings are 

 correct, and when the surrounding air is at a higher or lower 

 temperature the readings must be corrected accordingly. Tbe 

 wire used for bridge resistance coils is very largely of platinum- 

 silver, for which the rise or fall per degree Fahrenheit is 

 0-000155 of the resistance (or 00155 per cent.) and 0-00028. 

 of the resistance per degree Centigrade (or 0'028 per cent.) 

 The temperature at which resistance boxes are standardised is 

 usually 60°F. (equivalent to 15'5°C.) but sometimes not exactly 

 at this temperature but somewhere near. In tropical countries 

 the temperature in the open air In the shade is 80°F. to 90°F., 

 but inside a testing room it may be from 90 to 100 deg. Con- 

 sequently, in these climates there is a considerable difference 

 between the resistance read off the bridge and its correct value 

 at the temperature of the room. It is therefore most impor- 

 tant that the temperature correction is not overlooked. No 

 correction is required for the ratio arms because they both 

 vary alike and the ratio between them is constant for any tem- 

 perature. The correction is applied only to the adjustable 

 part of the bridge, commonly known as the third arm. 



Suppose, for instance, the CR of a cable was balanced at 

 5,290 B.A. units on a platinum-silver bridge correct at 60°F., 

 the testing room temperature being 95°F. As the surround- 

 ing temperature is 35°F. higher than the corract temperature 

 of the bridge, the reading 5,290 is too low, and we must add 

 to it the increase of resistance for 35°F., which is 



5,290x0-000155x35= 28-6 units. 



