94 ELECTRICAL MEASUREMENTS 



action of the coils, which is proportional to the square of the 



current. 



The device of using the weight in two portions is adopted 

 because the beam can then be made twice as long as would other- 

 wise be the case. 



The current is calculated by the formula 7 = K 2V R, where 

 R is the displacement of the weight as read on the uniformly 

 graduated scale. A table of doubled square roots, supplied 

 by the maker, is of service in the calculation. 



For rough work, the position of the sliding weight may be 

 referred to the fixed inspectional scale, placed immediately 

 behind the scale beam. It is graduated to give the quantity 

 2 \/R directly. For the purpose of extending the range four sets 

 of weights are supplied with each instrument. 



The Kelvin balances are made in a variety of ranges up to 

 2,500 amp. Their particular field of usefulness is as secondary 

 standards in the calibration of alternating-current ammeters. 

 They are serviceable in laboratories where the circuit conditions 

 can be controlled, but are not adapted to the measurement of 

 fluctuating currents. 



MEASUREMENT OF CURRENTS IN PERMANENTLY CLOSED 



CIRCUITS 17 



Occasionally it is necessary to measure the current in a con- 

 ductor which cannot be broken to allow the introduction of an 

 ammeter or shunt. For example, such cases occur in the investi- 

 gation of the electrolytic deterioration of underground pipes for 

 water or gas, due, for instance, to the stray currents caused by 

 the use of a ground return, or imperfect bonding, in a traction 

 system. The damage occurs where the current leaves the pipe, 

 and may cause such a menace to health and property that large 

 expenditures of time and money are justifiable in locating the 

 source of the trouble and in its elimination. 



In this or similar cases, if the resistance between two potential 

 points on the pipe or other conductor can be determined, the 

 current may be measured by using this resistance as a shunt, a 

 millivoltmeter being used to determine the P.D. between the 

 points. The problem thus resolves itself into the determination 



