l86 ON PROFESSOR BELL S ARTICULATING TELEPHONE. 



faithfully transmitted by means of the articulating electric telephone. 



I now pass to certain electrical facts, and their bearing upon 

 the telephone. The intimate relation existing between electricity 

 and magnetism is probably well known to every one present. If 

 a copper wire be laid near and parallel to a magnetic needle which 

 is free to move, and a current of electricity is sent along the wire, 

 the needle is deflected to one side or the other according to the 

 direction of the current. This is the principle upon which galvano- 

 meters are constructed, and which formed the basis of the 

 early electric-telegraph instruments. The converse of this is also 

 true, viz., that if a bar magnet be brought near to and across a 

 copper wire, a current of electricity is originated in the wire. 



Through the kindness of my friend, Mr. S. P. Thompson, I am 

 enabled to show you this experimentally. I have here a hollow 

 coil of wire, the ends of which are connected with a delicate 

 galvanometer, whose needle carries a small mirror, and matters 

 are so arranged that the movement of the needle will be made 

 evident to you by the traversing of the spot of light reflected from the 

 mirror along a scale. When a bar magnet is inserted into the coil of 

 wire, a current of electricity is set up in the wire, as you will see 

 [The experiment was here performed.] The current is of very 

 short duration, and when the magnet is withdrawn from the coil 

 another current traverses the wire, but in an opposite direction to 

 the lirst. 



Now, if the bar magnet is allowed to remain in the coil of wire, 

 and any substance which affects, or is affected by, the magnetism 

 of the magnet, is brought within the range of the magnetic field, 

 a current of electricity is also set up in the coil of wire. Such an 

 alteration in the magnetic field is readily produced by bringing a 

 piece of iron or steel near to one pole of the magnet. The current 

 thus set up is weaker than the one first described ; but I hope to 

 make it visible to you, [The experiment was here performed.] 



We are now, I think, in a position to understand the present 

 form of the Articulating Telephone, a pair of which I have here. 



