282 Mr. W. H. Preece on some Physical Points 



rents whose existence, though suspected, have hitherto eluded 

 the grasp of the electrician ; in fact its very delicacy has proved 

 the greatest obstacle to its general adoption. 



I. The Telephone as a Source of Electricity. 



Faraday showed that, when a closed conductor is moved 

 across the lines of force in a magnetic field, a current of elec- 

 tricity is generated within that conductor whose strength is 

 dependent upon the velocity of motion of the conductor and 

 upon the intensity of the magnetic field. It is, in fact, pro- 

 portional to the number of lines of force cut through per unit 

 of time. And also, when lines of force are projected through 

 a closed conductor, a current of electricity is generated in that 

 conductor, whose strength depends upon the magnetic intensity 

 of those lines of force, or upon their number per unit area . 

 The direction of the current in each case is found by Lenz's 

 law, viz. that the current produced tends to resist the motion 

 producing it. The new principle that has been developed by 

 Professor Graham Bell is that the form and duration of that 

 current is dependent upon the rate and duration of the motion 

 of the moving body or of those lines of force. 



Let N S, fig. 1, be a perma- 

 nent magnet, and ab a fixed, 

 closed, conducting ring of cop- 

 per around one pole of that 

 magnet. Let c be a movable 

 iron armature. Now, if we 

 regard any two lines of force 1 

 radiating from the pole N, and 

 nearly cutting the ring a b, then, 

 as we make c approach or recede 

 from N, those lines of magnetic 

 force will change their direction, 

 taking up position 2 ; and with each change of direction they 

 will cut the ring a b, and currents of electricity in different 

 directions will circulate through a b according to the direction 

 of motion of the lines of force ; and the rate of increase and 

 decrease of magnetic intensity (or of the increment and decre- 

 ment of the current) will vary directly with the rate of motion 

 of the armature c to or from the pole N. Thus, if c be a disk 

 of iron vibrating under the influence of sound, the excursions 

 to and fro of any point of the disk, though very small (in fact 

 they are so small that they can scarcely be detected by the 

 most delicate means — so small that they have led Graham Bell 

 to imagine that the vibrations are molecular), are nevertheless 

 sufficient to produce that motion of the lines of force which 

 results in currents. It is, however, a fundamental principle 



