SCIENCE AND PRACTICE. 311 



meter N is inserted. A Daniell's element is inserted between 

 the lever of the key and the brass stage A. 



The galvanometer used with this bridge is one of Weber's 

 construction, with a polished steel mirror. With a single 

 Daniell's element, when the currents of the system are 

 balanced, if the contact rollers be moved 0,1 millimetre out 

 of their place, the galvanometer mirror shows a deflection, 

 represented by five divisions of the reflected scale passing 

 before the fibre of the telescope. 



The resistance of the platinum wire is about twenty units. 

 It is seldom procurable absolutely cylindrical, although 

 drawn with great care through stone ; the conicalness, how- 

 ever, rarely exceeds an amount which throws the resistance 

 middle-point above 0,2 millimetre from the middle-point of 

 the length. 



The resistance of passage from the wire to the clamps E E 

 is a source of some little annoyance in using this really 

 beautiful apparatus, and renders it inapplicable, with the 

 same degree of accuracy, when the contact rollers are far 

 from the middle. 



The position of the roller is read off by a nonims carried 

 by the waggon, B, along the metre scale, m m. One reading 

 only is necessary for giving the value of x : 



1,000 a 



a being the length read off by means of the nonims from 

 the index to one end of the bridge. A second reading is, 

 however, usually made, by inverting the wire by means of 

 the commutator s, and reading the length d with the rollers 

 on the other side of the middle point of the wire, by which 

 we get 



If no appreciable resistance exists in the junction of the 

 wire with its clamps, and the electrical and geometrical 

 middle points of the wire coincide, a + a = 1000 ; but as 



