408 



ELECTRICAL MEASUREMENTS 



When very small inductances having a magnitude of, for 

 example, 0.001 henry are to be measured, the residual induct- 

 ances of the bridge coils must be considered. These coils are 

 wound non-inductively, in the usual understanding of the term, 

 but either the inductance or the capacity effect may preponderate. 

 The high resistance coils will give the most trouble. 



The Mutual Inductance Bridge. If variable or alternating 

 currents be used, a Wheatstone bridge which has three non- 

 inductive arms and one inductive arm connot be made to balance, 

 for the potentials at the two ends of the detector circuit can 

 never be in the same time phase. A balance can be obtained, 

 however, by the addition of an adjustable mutual inductance, 

 or air-core transformer of variable ratio, the secondary of which 



FIG. 238. Mesh diagram for Hughes bridge. 



is connected in series with the detector while the primary is 

 placed in one of the leads from the source of supply to the bridge. 

 The primary, therefore, carries the entire bridge current, and 

 the mutual inductance introduces into the detector circuit a 

 small e.m.f. which is in quadrature with that current. 



An apparatus so arranged was used in 1886 by Professor 

 Hughes and the results obtained were given by him in his in- 

 augural address on assuming the presidency of the British Insti- 

 tution of Electrical Engineers. The discussion 23 which fol- 

 lowed the presentation of this paper should be read by every 

 student who has any doubts on the question of practice vs. 

 theory plus practice. It is sufficient to say here that on account 

 of an inadequate theory of his bridge Professor Hughes misinter- 

 preted the readings which he obtained. H. F. Weber, Rayleigh 



