PROFESSOR BARLOWS EXPERIMENT. 99 



longitudinally; so that the terrestrial currents 

 from east to west, and the currents about the 

 under side of every compass needle, are finally 

 brought to move in similar directions. 



The phenomenon of terrestrial magnetism is 

 admirably illustrated by Professor Barlow's in- 

 vention of winding numerous circuits of insulated 



Fig. 22. 



conducting wire about a little globe, with a com- 

 pass needle mounted on top of it, as shown in 

 Fig. 22. By employing intense battery currents 

 from a voltaic apparatus, to overpower the ter- 

 restrial currents, all the phenomena of terrestrial 

 magnetism are exhibited. 



Until the connection of the conducting wire is 

 made with the voltaic battery, the compass needle 

 placed on top of the artificial globe takes its usual 

 north and south direction, pointing to the poles of 

 the earth ; but as soon as the overpowering battery 

 current is transmitted through the conducting 

 wire wound in numerous spiral coils about the 

 little globe, the compass needle turns and points 



