THE MEASUREMENT OF CURRENT 95 



of the resistance of a portion of a conductor which may be carry- 

 ing a current and which must be measured in situ without being 

 opened. 



Suppose that, as a preliminary to measuring the stray current 

 in, for instance, a water main, it is necessary to determine the 

 resistance between two plugs which have been screwed into 

 the pipe to serve as potential terminals. 



The pipe is supposed to be traversed by stray currents of un- 

 known strength. The necessary connections are shown in Fig. 

 50. At Vi and V 2 are 2 millivoltmeters, with, perhaps, 10 milli- 

 volt scales; they are connected to potential points at 6, c and e, /. 

 These points are obtained by drilling into the pipe and firmly 

 inserting brass plugs to which the leads may be soldered. It is 



A' 



C 



v l v 



FIG. 50. Connection for measuring a direct current without opening 



the circuit. 



essential that the instruments be calibrated with the leads which 

 are to be used in the test. At B is a storage battery capable of 

 yielding enough current to give a good reading on V\ (100 or 

 200 amperes for a 15-in. iron pipe), at A is the ammeter by 

 which the current from B is measured, and K is the switch by 

 which the current is controlled. 



The spacing of the points a, 6, c, d is important, for the four 

 points 6, c, e, f should be on four equipotential planes through the 

 pipe. Therefore the distance between a and 6 and between c and 

 d should be great enough so that the current from B may spread 

 out and the stream lines become uniformly distributed before the 

 points c and b are reached; the lengths of ab and cd should be 

 about twice the diameter of the pipe, the points a and d being on 

 the top and b and c on the side. 



The positive direction of the currents may be assumed as 

 indicated; it is essential that such an assumption be made at the 



