THE LOCALISATION OF BRKAKS AND FAULTS. 515 



The correction is therefore in this case quite considerable, 

 as the reading, instead of being 5,290, is really 

 5,290 + 28-6 = 5, 318-6 units. 



If the temperature of the room is lower than the standard 

 temperature of the bridge, the correction is worked out in 

 exactly the same way, but subtracted from the bridge reading. 



For example, a balance of 2,712 units is obtained on a 

 platinum silver bridge correct at 17°C., where the surrounding 

 temperature ia 8°C. The correction to apply in this case is for 

 a fall of 9-C. The variation of this alloy is 0-00028 of the 

 resistance per degree Centigrade, which is 



2,712x0-00028x9 = 6-8 units. 



As the temperature is lower than that at which the bridge 

 is right the correction must be deducted, and the true resis- 

 tance is therefore 



2,712 -6-8 = 2,705-2 units. 



Bridges in which the resistance coils are of manganin need 

 no correction as the variation of resistance in this alloy over 

 the ordinary range of temperature is only 0*00002 to 0'000025 

 of the resistance per degree Centigrade or 000011 to 0*000014 

 per degree Fahrenheit, which is quite negligible. It may be 

 useful to note that the coefficients given above, and in fact the 

 coefiicients for temperature variation in any metal or alloy apply 

 equally to all systems of units in which the resistances may be 

 expressed. For instance, they apply to B.A. units, standard 

 Board of Trade ohms, or any other units without alteration. 



Gott's Bridge Standardising Arm — A very useful adjunct 

 by which any bridge, withouc change of construction, can be 

 made self-correcting for temperature and to read in any desired 

 units has been devised by Mr. Jno. Gott, consulting electrician 

 to the Commercial Cable Co. {The Electrician, February 21, 

 1902). This consists of a separate manganin bridge arm con- 

 taining the usual coils of 10, 100, 1,000 and 10,000 ohms. 



This special arm is connected up in place of the arm a next 

 to the cable, as in Fig. 287. In the diagram the connections 

 are such that the ordinary ratio arms form together the arm 6, 

 thus giving a greater range in the ratios obtainable. Or if the 

 .galvanometer is connected at Tj instead of T, 11,110 ohms more 



ll2 



