276 Dr. F. T. Trouton on the Theory of Use of a 



With these values, equations (11) and (12) give the equi- 

 valent resistance and capacity of parallel circuits containing 

 resistance and capacity only. 



Physical Laboratory of Cornell University, 

 July 1S92. 



XXIX. On the Theory of the Use of a permanently Magnetized 

 Core in the Telephone. By Feed. T. Trouton, M.A., D.Sc* 



IN the development of the Bell Receiver it was found in 

 practice that a louder effect could be obtained by employ- 

 ing a magnetized core instead of a plain soft-iron core. This 

 appears to be at variance with ordinary experience, it being 

 generally supposed that for a maximum effect the core in 

 electromagnets should be of soft iron. 



An explanation of this which seems to be commonly received 

 attributes the improved results to the diaphragm of the receiver 

 being kept permanently in a taut condition, it being supposed 

 to be thus for some reason in a better condition for taking up 

 the vibrations than when left relaxed. However, no very 

 clear ideas on the subject seem to be current. 



For some time past I have been experimenting with the 

 object of clearing up this point ; the result of which is to 

 almost entirely refer the improved effect, obtainable by per- 

 manently magnetizing the core of the telephone, to the fact 

 that the mechanical force on the diaphragm, or armature of 

 the electromagnet, is proportional to the square of the mag- 

 netic force. Suppose, for example, H to be the magnetic force 

 at the diaphragm due to a permanent magnetization in the core, 

 we may take the mechanical force on the diaphragm to be a 

 constant multiplied by H 2 ; and should H vary by an amount 

 SH through the passage of a current of ordinary telephonic 

 dimensions, we have the corresponding variation (SF) in the 

 mechanical force, which for loudness in the telephone is the 

 important factor, dependent on the amount of the permanent 

 magnetization; thus SF = C.HSH. In this way it is seen 

 that the variations in the force on the diaphragm, and conse- 

 quently its amplitude of vibration, will depend on the per- 

 manent magnetization as well as on its variation produced 

 by the currents passed through the telephone. Or, to look at 

 it another way, it is important that initially the armature 

 have strong magnetic poles ready to be acted on by this 

 variation |. 



* Communicated by the Author. 



t This would suggest magnetized steel diaphragms for louder effects. 



