THE MEASUREMENT OF RESISTANCE 



187 



The second arrangement is about 23 times as sensitive as the 

 first. However, as the total bridge current is about 44 times as 

 great, the possibility of overheating a low-resistance arm of the 

 bridge should be kept in mind. This is seen from the following, 

 where E' has been assumed as 2 volts. 



Watts dissipated in the bridge arms, 



With the first connection. . . 

 With the second connection. 



If 



0.000004 

 0.9 



N 



0.004 

 0.0009 



A' 



. 00004 

 0.09 



p 



0.04 

 0.00009 



In the second case there is a possibility that the arm M may 

 be overheated if the current be kept on continuously. 



1000 W 



100 w f 6 P 



IB - "2T Approx. 



r E 'S P 



l <> = 440,000 



7T* ' 



10.200.000 



FIG. 106. Showing effect of relative positions of battery and galvanometer 

 on the sensitiveness of a Wheatstone bridge. 



Sensitiveness Attainable with the Wheatstone Bridge. The 

 sensitiveness of a bridge arrangement may always be increased 

 by increasing the bridge current. The limit to this increase is 

 fixed by the carrying capacity of the bridge arm which will 

 safely stand the least current. 4 



With coils of like construction but of different resistances 

 the allowable energy losses are the same, that is, 



(I')*R = k or I'\fR = \/k, a constant. (9) 



/' is the allowable current and k the allowable heating loss in 

 watts-; k depends upon the construction and manner of using the 

 coils. 



In planning new work it is frequently of importance to calculate 



