'permanently Magnetized Core in the Telephone. 211 



Another advantageous result arising from permanently 

 magnetizing a soft-iron core by the addition of a steel magnet 

 (as in a common arrangement of the telephone) probably is 

 that the permeability of the soft iron is increased thereby: so 

 that our SH itself (that is to say, the magnetic force at the 

 armature arising from the current) is greater than were the 

 current to act on an initially unmagnetized core. This effect 

 probably is always unimportant compared with the last. 



The effect here alluded to may perhaps be better understood 

 by reference to the well-known curves of magnetization (fig. 1), 



Fte.l. 



where it will be at once seen that equal small variations in the 

 magnetic force (horizontal direction) produce effects in the 

 magnetization (vertical direction) dependent in amount on 

 the initial position on the curve. 



An apparatus found convenient in these experiments is 

 shown in fig. 2. It consisted of an ordinary tambourine, 



Fig;. 2. 



gssggg 



armed at the centre with a little piece of iron, and thus could 

 be made to vibrate by an alternating current passed through 

 an electromagnet with a soft-iron core placed telephonewise 

 close up to the little piece of iron. The core could be perma- 

 nently magnetized to the extent desired, either by a current 

 passed through an additional coil wound on for the purpose, 

 or by bringing up in line with the core a permanent magnet 

 to a variable distance. 



