Trotobridge and Sheldon — X utradzation of Induction. 19 



Bergmann had anticipated us,* we were led to adopt the follow- 

 ing form of the instrument, which differed entirely from that 

 ^i these authors. Four coils were employed, as in the Hughes 

 form of instrument. One of the coils in the telephone circuit 

 was fixed upon a horizontal axis which was at right angles to 

 the axis of the coil. The coil could therefore be moved through 

 all positions, from perfect parallelism to its neighboring coil in 

 the interrupted circuit to a position at right angles to this coil. 

 The horizontal axis was provided with an index arm which 

 moved over a graduated circle. Calling 6 the angle of inclina- 

 tion of the axis of the movable coil with the axis of the fixed 

 coil in the interrupted circuit, and iTthe strength of the in- 

 duction current in the movable coil, we have evidently, on the 

 Supposition that the strength of the alternating current remains 

 constant, 



i^= constant X cosine 6. 



When the axes of the coils are at right angles, cosine 6=0, 

 and we should have silence in the telephone. Since adopting 

 this arrangement we have discovered that Dr. Bowditch, of the 

 Harvard Medical School, + has employed this arrangement of a 

 movable coil placed in front of a fixed coil as a modification of 

 Du Bois Reymond's apparatus for controlling induction cur- 

 rents so that they may be administered by known amounts for 

 physiological purposes. In Du Bois Reymond's apparatus one 

 induction coil was simply moved away from a fixed coil through 

 which an interrupted current w T as passed, much in the same 

 manner as the coils in Wiedemann's form of galvanometer are 

 moved. Here no minimum could be obtained. In Dr. Bow- 

 ditch's form of this apparatus, theoretically a minimum should 

 be obtained, that is, when cosine 6 = 0, or when the axes of the 

 coils were at right angles. An indication of an electrical cur- 

 rent is obtained even when the axes of the coils are at right 

 angles, on account of the windings of the coil not being per- 

 fectly at right angles to those of the stationary coil. 



That no minimum should be obtained when the axes of the 

 coils are at right angles, and when the induction arises from all 

 parts of the circuit, is evident upon an elementary considera- 

 tion of the subject. We have to deal in this form of instru- 

 ment with the mutual induction which arises between the fixed 

 coil and the movable one, and also with the self-induction 

 which arises between the spires of the movable coil in the tele- 

 phone circuit. The mutual induction can be reduced theoreti- 

 cally to zero by placing the movable coil of the telephone 

 circuit at right angles to the fixed coil. The self-induction can 



* Annalen der Physik. xxxvi, 1889, p. 783. 

 + Proc. At-). Acad , vol. xi, p. 281. 



