of Sound in the Bell Telephone. 131 



steel plates with the pole of a powerful magnet ; and the pre- 

 sent author also found analogous effects to be produced by 

 passing currents through steel disks. It therefore became a 

 matter of some interest to determine the character of the mag- 

 netization of the telephone-disk. 



The figures exhibited two unsuspected features : — first, that 

 when the diaphragm is larger than the end-face of the magnet, 

 and even when it does not touch it, the distribution of the 

 magnetism induced in the diaphragm is partially lamellar in 

 character, partially radial. The central portion is magne- 

 tized almost entirely normally to its plane ; the exterior por- 

 tion is radially magnetized — a narrow annular region lying 

 between these, in which the character of the magnetization 

 is mixed. It was further observed that this neutral zone is 

 of greater diameter when a larger magnet is employed, and 

 that it enlarges also as the distance separating the magnet 

 and the diaphragm is increased. It is more strongly marked 

 as a region of separation between the central lamellar portion 

 and the outer radially magnetized portion in the diaphragms 

 of thin material than in those of thick. The position of this 

 neutral annular zone is marked in fig. 1 (p. 132), which is 

 a sectional diagram compiled from the figures produced by 

 filings, by a point of flexure in one of the " lines of force " pro- 

 ceeding from the pole towards the diaphragm. The second 

 feature noticed was that some of the outermost lines of force 

 ran round to the front of the disk, entering it very near its 

 circumference. 



The next effect to be studied was that produced by the 

 magnetic inductive action of a current traversing a coil of 

 wire about the pole. For the convenience of obtaining the 

 filing-figures upon glass plates but one turn of wire was em- 

 ployed, passing through holes drilled in the glass, and situated 

 as is the coil in the Bell telephone over the pole of the mag- 

 net, the position arbitrarily found the most efficient in the 

 construction of that instrument. In almost all modern ele- 

 mentary treatises on electromagnetics it is shown that the field 

 of a plane closed circuit is equivalent to that of a lamellar 

 magnetic shell of equal strength, or one which has an equal 

 number of lines of force passing through the area it occupies. 

 The result of passing the current around the pole of the mag- 

 net will therefore be, so far as the field in the plane of the coil 

 is concerned, to increase or diminish respectively the number 

 of lines of force due to the magnet by the number of lines of 

 force due to the closed circuit, according as the direction of its 

 field coincides with, or opposes that of the magnet. But the 

 action is not so simple on the regions of the field outside the 



