CONSTRUCTION OF ELECTRO-MAGNETS. 105 



the other end of the spiral coil, about which the 

 same current appears to circulate in an opposite 

 direction to the movements of the hands of a watch, 

 the properties of the north pole are excited. But 

 these apparent directions depend on the position 

 of the observer facing one, or the other, end of the 

 spiral coil. It is a remarkable fact, that the pres- 

 ence of about one per cent of molecules of car- 

 bon in iron (constituting steel], imparts to it the 

 property of retaining the continuous circulation 

 of electric currents, and renders the compound 

 substance of steel permanently magnetic. The 

 wonderful facility with which electric currents 

 are excited about a steel needle, has rendered this 

 simple contrivance a very sensitive test of electric 

 excitation. By connecting the ends of the con- 

 ducting wire with the metallic roof of a large 

 building in Washington, Dr. Page found that a 

 flash of lightning, twenty miles distant, rendered 

 the needle permanently magnetic. Knives and 

 steel implements are often rendered magnets in 

 houses struck by lightning ; and even the direction 

 of the currents can be ascertained by examining 

 which end of the steel has the properties of a 

 south pole : see Fig. 26. 



If a piece of iron is substituted for a steel needle, 

 it is electro-magnetic only while the current cir- 

 culates around the coil. The conversion of pieces 

 of iron into powerful electro-magnets is effected 

 by placing them within spiral coils of insulated . 



