JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. II, FEBRUARY 4, 1912 No. 3 



ELECTRICITY. — The four-terminal conductor and the Thomson 

 bridge. Frank Wenner. Communicated by E. B. Rosa. 

 To appear in the Bulletin of the Bureau of Standards. 



In the measurement of low resistances the resistances of con- 

 nectors and contacts are often as large as the resistances with 

 which we may be concerned. It is therefore necessary, if a high 

 precision is desired, to use a method of measurement by which 

 the effects of the resistance of the connecting leads and contacts 

 are eliminated. That is the method must be such that we can limit 

 the resistance measured to that of a part of a conductor which con- 

 tains no variable connecting resistances. The first requisition then 

 is that the conductor have four terminals suitable for making 

 electrical connections to other conductors or leads. Usually two 

 of the four terminals are designed for making connections to cur- 

 rent leads while the other two are designed for making connections 

 to the potential leads. 



When a current enters through one of "the current terminals 

 and leaves through the other, there is a difference in potential 

 between the potential terminals. The ratio of this potential 

 difference to the current is the resistance of the four-terminal 

 conductor. 



When such a conductor is to carry a large current, as is often 

 desired, the current terminals must be large so the current distri- 

 butions in the conductor depends to some extent upon the way in 

 which the current leads are connected. Where this is the case 

 we are not surprised when we find the resistance of the conductor 

 somewhat indefinite. Where, however, this matter is given due 

 consideration in the design the resistance can be made definite 

 within the limits of measurement, even where the conductor is 

 made of sufficient section to carry 1000 amperes or more. 



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