permanently Magnetized Core in the Telephone. 279 



tance to the position of contact with the core. Not so with 

 an intermittent current. One pole effects, as before, a con- 

 tinuous increase in the intensity of the sound as it is brought 

 up from a distance to the position of contact with the core ; 

 but the other pole at first, as it is brought up, steadily dimi- 

 nishes what small amount of sound the current alone may be 

 capable of producing ; reaching a minimum, the sound again 

 begins to increase, and continues doing so until the position 

 of contact is arrived at. In this case the permanent magnet 

 is opposed (that is, lies- in the opposite direction) to the mag- 

 netization produced in the soft iron by the intermittent cur- 

 rent. The position of minimum sound evidently is that in 

 which the average permanent magnetization resulting from 

 the intermittent current is just neutralized by the permanent 

 magnet. For, as we have seen already, a small change in the 

 magnetizing force applied to an electromagnet should pro- 

 duce least mechanical force on the armature under such 

 circumstances. 



A few rough experiments bearing directly on this subject 

 were also made, an account of which may perhaps not be out 

 of place here. A long thin cylinder of soft iron was attached 

 to one arm of a balance, and hung down with its lower end 

 dipping into a coaxial solenoid consisting of two separate 

 layers of wire, through each of which independent currents 

 could be sent. 



One coil (A) was used to produce various intensities of field 

 by the passage of various currents. The other (B) was for 

 effecting a small increment in this magnetization by the 

 passage of a small current of constant amount through it. 

 In each case equilibrium was restored to the balance by the 

 addition of the necessary weights. It required *03 gram to 

 balance the small current in B when acting alone. It required 

 in all *15 g. for equilibrium when a current was running in A, 

 which acting alone was balanced by *09 g. Again, when 

 this last was increased so as to require '52 g., the two coils 

 acting together required '72 g. Here the same provocation 

 produced an effect of 6 and of 20 when under these two 

 circumstances. The numbers themselves agree with theory 

 as well as could be expected in the case of such a crude 

 experiment. We deduce from the considerations given pre- 

 viously, 



SF/SF=x/F/n/F. 



Putting in the numbers, we have respectively 20/6 or 3*3, 

 and n/52/n/9 or 2'4. 



