150 CORRELATION OF THE PHYSICAL FORCES. 



more, if I take an iron bar, and put it inside 

 the coil, so long as there is no electric current 

 circulating round, it has no attraction, as you 

 will observe if I bring a little iron filings or 

 nails near the iron. But now if I make contact 

 with the battery they are attracted at once. It 

 becomes at once a powerful magnet, so much so 

 that I should not wonder if these magnetic 

 needles on different parts of the table pointed to 

 it. And I will show you by another experiment 

 what an attraction it has. This piece and that 

 piece of iron and many other pieces are now 

 strongly attracted (fig. 52), but as soon as I break 



Fig. 52. 



contact the power is all gone and they fall. 

 What then can be a better or a stronger proof 



