720 Experiments on the communication of CSept. 



tial principle so successfully applied by Hauy to the discovery of 

 magnetism in minerals containing traces of iron, can be had recourse 

 to here so as to enhance still further the delicacy of these beautiful 

 instruments. 



Having thus sufficiently exposed the construction and mode of action 

 of the galvanometer, I must reserve for another place, the results of my 

 experiments in testing the value of the different methods described. 



§ 4. — Henry's Magnetic Telegraph. 



I have still however to notice another proposal which has attracted 

 great attention, and is said, on good authority, to be in course of prac- 

 tical application in the United States. 



Professor Henry proposes to employ the sudden development of 

 magnetism, occasioned in a horse shoe bar of soft iron while surrounded 

 by a spiral of insulated wire, the extremities of which are in contact 

 with a voltaic couple. The magnet thus created attracts a light piece 

 of iron which carries an arm. The arm when attracted marks dots on 

 a revolving cylinder, or may strike a bell. The arrangement is shewn 

 in the following figure. The spiral wire in the centre is a spring to 

 lift up the arm on the cessation of each stroke. 



Eleven miles of wire were employed in one of Henry's experiments; 

 but the wire was coiled spirally round a drum, a circumstance 

 which considerably invalidates the results. This will seem sufficiently 

 intelligible by reference to the construction of the " coil electro- magnetic 

 machine," described in a subsequent page. 



§ 5. — Experiments by the Author. 

 I have now given an adequate sketch of the several methods of 

 communication hitherto proposed, and I proceed to the description of 

 the experiments I have carried on, in the view of testing the compar- 

 ative merits of the preceding plans and of another, which I have 

 myself devised. 



