274 AMEUICAN Er.F.CTPJC-TRr.EOnAPII 



ihe wire between the extremities of the battery, generator, the magnetism 

 which operates the telegraph, in a manner presently to he explained. 

 I'his form of battery, called Grove's battery, will operate without much 

 diminution of power for several days together. 



hi 1819, Professor Oersted of Copenhagen discovered that an elec- 

 tical current acted upon a delicately suspended magnet, causing it to 

 assume a position at right angles to the current. 



Not long after this discovery. Ampere noticed that, when two electric 

 currents passed in the same direction through two adjacent and parallel 

 wires, these wires attracted cacli other. This proved that electric con- 

 ductors are magnetic or have the property of magnetism, whilst trans- 

 mitting a current. 



From this discovery very naturally resulted that of Arago and Davy. 

 These gentlemen, about the same time and independently of each other, 

 discovered that permanent magnetism is communicated to a rod of steel 

 by an electric current flowing at right angles to it. If the conducting 

 wire be covered with silk, cotton thread, varnish, or other non-conduct- 

 ing or insulating substance, so as to prevent contact, and wound from 

 one end to the other on a rod of any hard substance, so as to form a hol- 

 low cylinder when the rod is witlidrawn, and if the rod of steel be now 

 placed within, it is found to have received amagnetism increased in pro- 

 portion to the number of turns of the wire; for this arrangement is e- 

 •juivalcnt to causing as many electric currents, as there are turns of the 

 wire, to flow at right angles to the rod. If a rod of soft iron be substitu- 

 ted for the steel, it is found to acquire a much stronger magnetism, 

 ichilsl under ihe influence of the electric current, but to lose it as soon as 

 it is withdrawn from the hollow cylinder or ihe current ceases to Jloio. 

 The soft iron whilst magnetic is called an electro-magnet, and the tem- 

 porary property acquired electro-magnetism. 



Previously to this, it was known that a steel magnet, when bent in 

 the form of a JJ and furnished with a soft iron armature, was capable 

 of exerting a greater attractive influence, than when in the form of a 

 straight bar. Following out these hints Professor Moll formed a \J of 

 soft iron, and wound it with insulated copper wiie. He found that the 

 soft iron possessed the most surprising magnetic properties whilst under 

 the influence of the electric current. 



The first successful attempt to measure the magnetic power, thus 

 capable of being developed, was made by Professor Henry, then of Al- 

 bany, N. y ., and now of Nassau Hall College. With a bar of soft iron, 

 two inches in diameter, bent in the form of a [J and wrapped with 180 

 yards o[ well insulated copper wire. No. 18, he obtained a power equal 



