254 R. S. Broiigh — "Prof. Oraliam BelPs Telej^lione. [Dec. 



and receiving stations are precisely similar. We have simply to connect 

 the one end o£ the coil of wire in each station to the line v^ire, and the 

 other end to the return wire or to earth. 



The currents are produced magneto-electrically at the sending end of 

 the line, and are received electro-magnetically at the distant end. 



Now returning to the figure we see that we have a soft iron induced 

 magnet A surrounded by a coil of wire C, and oj)posite the end of the soft 

 iron core A we have the soft iron diaphragm B. 



Lines of magnetic force radiate away from the core A, some towards 

 the disc jB, others away from the disc towards the distant end /S' of the 

 permanent magnet. These lines of force penetrate through the coil of 

 wire G. 



So long as the disc B remains at rest, the lines of force emanating 

 from A remain stationary ; but if the disc B be moved in towards or out 

 from A^ the lines of force will increase or decrease in number and will 

 change in direction. 



When the lines of force move, they cut the convolutions of the coil of 

 wire G at right angles. Now we know that if we move a conductor across 

 the lines of force in a fixed magnetic field, or, what is the equivalent, as in 

 this case, move the lines of force across a fixed conductor, an electromotive 

 force is produced in the conductor. 



Hence in this case, motions of the disc B will produce electromotive 

 forces in the wire of the coil G. 



In fact, if we attach the ends of the wire of the coil G to the terminals 

 of a sensitive galvanometer, and press the disc B in with our finger, we 

 shall see a throw of the needle in one direction, indicating a transient 

 current through the galvanometer. Holding the disc B pressed in until 

 the index of the galvanometer comes to rest, and then releasing it, we shall 

 see a throw of the needle in the opposite direction, indicating a transient 

 current through the galvanometer in the reverse direction to the first. 

 (Mr. Brough showed this experimentally with a Thomson's Reflecting 

 Galvanometer.) 



The relative direction of the current is in each case, according to 

 Lenz's Law, which is only a statement of a joarticular case of the general 

 Law of the Conservation of Energy, such that the magnetic field it produ- 

 ces tends to resist the motion impressed on the diaphragm B. Knowing 

 the polarity of the permanent magnet N S, we can at once infer the abso- 

 lute direction of each current from Ampere's Rule. 



When we press the diaphragm in, we have to do work. Part of the 

 work thus done takes the form of the potential energy of the bent dia- 

 phragm, while the other part takes the kinetic form of the transmitted 

 electrical current. When we release the diaphragm, it returns to its initial 



