WENNER AND WEIBEL: TESTING OF POTENTIOMETERS 469 



Proper choice should be 

 made between IV and IVa 

 or between V and Va so 

 that the logarithm of C will 

 be positive. The expo- 

 nential curves contain a 

 flexure. On one side of the 

 minimum (or maximum) 

 they extend out indefinitely y 

 from both axes; on the 

 other side they reflex ^nd 

 tend to become parallel to 

 one axis. The end without 

 the flexure is intermediate 

 in curvature between a 

 parabola and a rectangu- 

 lar hyperbola. Fig. 1 shows 

 the character of thse curves. 

 They can be made to pass 

 through any three given 

 points of which at least two must be on the non-reflex portion. 



PHYSICS. — The testing of potentiometers. Frank Wenner 

 and Ernest Weibel, Bureau of Standards. ^ 



For precise measurements with a potentiometer the errors 

 in the adjustment of the resistance sections or coils in the appa- 

 ratus must be known. These errors can be determined by 

 measurements of the resistances of the various sections but, 

 without some systematic method of applying corrections, they 

 are not easily allowed for in the use of the potentiometer. 



We have found that it is convenient to express the relation 

 between the known electromotive force, the unknown electro- 

 motive force, the readings and the corrections by the formula 



E =f [(ei + Gi) + (62 + ao) + etc.] (l -^ b + c + d) 



(1) 



1 Presented at the Washington, 1914, meeting of the American Physical Society. 



