THE MEASUREMENT OF RESISTANCE 173 



binding post so that the gap may be closed by a strap of low re- 

 sistance or by a resistance coil, as is desired. Between e and / is 

 stretched a wire of high resistance. It is about a millimeter in 

 diameter and, in this form of bridge, intended to be just 1 meter 

 long. A slider, s, makes contact at any desired point along the 

 wire and its position may be read off on a scale divided into milli- 

 meters. It is intended that e and the zero of the scale shall be 

 coincident. The connections of battery and galvanometer are 

 generally as shown, and this is usually the more sensitive arrange- 

 ment. If the battery and galvanometer are interchanged, the 

 resulting arrangement will be less disturbed by thermal e.m.fs. 

 at the contact s. 



To make a measurement, the simplest process would be to close 

 the gaps at a and b by straps, place at d the unknown resistance 



s 



FIG. 98. Diagram for slide-wire bridge. 



X, and at c the known resistance S, the best value of which would 

 be about the same as X. The slider s would then be pressed 

 carefully down upon the wire at one point after another until 

 one was found where the galvanometer remained undeflected, 

 i.e., where opening and closing at s did not cause motion of the 

 galvanometer index. Let I represent the scale reading in milli- 

 meters, i.e., I = distance e's. As e'f = 1,000 mm., sf = 1,000 I. 



priQ of ^^* 



Then, by the bridge principle, X = S - FJ But the wire 



res. 01 l 



is assumed to be of uniform resistance per unit length, so 

 that the resistances of the two parts of the wire are pro- 

 portional to their lengths. Hence, if the resistances of the 

 leads from e r to c and from f to d are assumed to be zero, 



X = S (1>00 ~ 0. 

 i 



Resistances at Ends of Bridge. The leads at the ends of the 

 bridge may not be of negligible resistance, and there may be 



