OF ARTS AND SCIENCES. 113 



XL 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXXIII. —THE STRENGTH OF THE INDUCED CURRENT 

 WITH A MAGNETO TELEPHONE TRANSMITTER AS IN- 

 FLUENCED BY THE STRENGTH OF THE MAGNET. 



By Charles R. Cross and Arthur S. Williams. 



Presented November 14, 1888. 



It is a well-known fact in practice, as well as an evident consequence 

 of theoretical considerations, that the effectiveness of a magneto tele- 

 phone when used either as a transmitter or as a receiver varies with 

 the strength of the magnetism of the core. But the relation of the 

 one to the other has never been studied, so far as we are aware. 



Our investigations include a study of the changes in strength of the 

 current produced by a magneto transmitter under varying conditions 

 of magnetization, and of the magnitude of the momentary changes in 

 the magnetic condition of the core of the receiving telephone when 

 subjected to the action of undulatory or other brief currents, as influ- 

 enced by the strength of the primitive permanent magnetization of the 

 core. The present paper contains only the results of a series of ex- 

 periments relating to the first of these, that is, to the effect of varying 

 strength in the magnet of the transmitter, the study of the allied 

 problem of the receiver being still in progress. 



The apparatus employed consisted of a cylindrical bar of soft iron 

 about 4| inches in length and £ of an inch in diameter, around one 

 end of which was placed a coil of fine wire similar to thai used in ordi- 

 nary telephonic practice. The resistance of this coil was 100 ohms. 

 It was placed in circuit with a ballistic mirror galvanometer, from 

 whose deflection the momentary current produced in the coil by any 

 variation in the strength of the core could be determined. The dia- 

 phragm, which was in all cases 2-& inches in diameter, was in its usual 

 place opposite the end of the magnet about which the wire coil WM 

 wound, and about T jfr of an inch from that end. By means of a rod 



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