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XLIIL An Addition to the Wheatstone Bridge for the Deter- 

 mination of Low Resistances. By J. H. Reeves, M.A., 

 City and Guilds of London Central Technical College *. 



WHILE it can be assumed with fair certainty that 

 even in moderately equipped laboratories there will 

 be found one sensitive galvanometer and a good set of resist- 

 ance-coils arranged in the form of a Wheatstone bridge, it is 

 by no means so common to find a convenient method for 

 determining low resistances, such as, for example, a Kelvin 

 bridge. The piece of apparatus which forms the subject of 

 this paper is a comparatively cheap addition to the ordinary 

 bridge, and enables the resistances of exact metre-lengths of 

 even thick wires to be directly measured with almost as much 

 ease as larger resistances can be determined with the ordinary 

 Wheatstone bridge. The method also possesses the advantage 

 that all the measurements are made in terms of a standard 

 wire with fixed contacts, and, therefore, not subject to the 

 wear which accompanies the frequent use of a slider ; further, 

 in the case of copper wires no temperature-correction is 

 needed. 



The apparatus is represented in fig. 1. On a mahogany 



Fig. 1. 



G H 



baseboard are stretched close to one another two wires — one, 

 ABCD, being the standard of comparison, while the other, 

 EFGH, is the wire to be tested. 



A and E are two massive pieces of brass which can be 

 joined together by a plug P. D and H are smaller pieces of 

 brass with binding-screws attached. 



The standard BC is permanently fixed to A and D, whilst 

 two clamps F and G fixed respectively to E and H form the 

 means of fixing in its place the wire to be tested. 



KL and NM are two brass springs which pass over but do 

 not touch BC. They are provided with binding-screws at 

 K and M, and with two knife-edges L, N exactly one metre 

 apart, which press on the wire FG when in position, while 



* Communicated by the Physical Society : read March 13, 1896. 



