WENNER AND WEIBELI THE THOMSON BRIDGE 471 



from one side to the other of the branch connection, or to cut out 

 sections on one side and insert corresponding sections into the 

 other side. 



In use the ratio-set is connected in parallel with the potenti- 

 ometer to be tested in such a way as to form two arms of a bridge, 

 while the potentiometer forms the other two arms. This arrange- 

 ment constitutes a Matthiessen and Hockin bridge. The ratio- 

 set is adjusted so that a balance of the bridge is obtained for 

 each setting of the dials of the potentiometer for which correc- 

 tions are desired. The corresponding readings of the ratio-set 

 (when connected for known errors in its resistance sections) 

 give data from which the corrections b, e, ai, a2, etc., of equation 

 (1) can be calculated. Certain adjustments are made such that 

 the corrections are obtained almost directly from readings of 

 the potentiometer and ratio-set. 



PHYSICS. — Adjustments of the Thomson bridge in the measure- 

 ment of very low resistances. F. Wenner and E. Weibel,^ 

 Bureau of Standards. 



The purpose of this paper is to describe two new procedures 

 for carrying out those adjustments which make the correction 

 terms of the Thomson bridge equation negligibly small. 



Both procedures require the use of a variable double ratio- 

 set (so adjusted that, for any reading of the dial switches, the 

 lack of equality of the two ratios, a/ a, (fig. 1) is so small that no 

 appreciable error is introduced on this account) and variable 

 low resistance to be used in the connections, Xx and Xo, between 

 the terminals of the ratio-set and the potential terminals of the 

 four-terminal conductors under comparison. 



(1) With the test current supplied through the terminals 

 b and b' , the bridge is balanced by an adjustment of the dial- 

 switches of the double ratio-set. 



(2) With the test current supplied through the terminals 

 n and o, the bridge is balanced by an adjustment of the variable 

 low resistance fomiing a part of Xi. 



1 Paper to be published in the Bulletin of the Bureau of Standards. 



