256 



Remark. — The effect produced by hammering iron, occurs also in 

 steel, but with this difference : one blow is generally sufficient to 

 change the poles of iron, that has been rendered magnetic by 

 hammering, or to deprive it of magnetism after it has acquired 

 polarity ; but in steel, it frequently requires two or three or more 

 strokes of the hammer, before the effect be fully produced. 



11. An electrical discharge, made to pass through a bar of 

 iron, void of magnetism, when nearly in the position of the 

 magnetic axis, renders the bar magnetic ; the upper end be- 

 coming a south pole, and the lower end a north pole ; but the 

 discharge does not produce any polarity, if the iron be placed 

 in the plane of the magnetic equator. The effects appear to 

 be the same, whether the discharge be made on the lower or 

 upper end of the bar, or whether it is passed longitudinally or 

 transversely through the iron. 



Exp. 1. — Bar, No. 1. freed from magnetism, and placed in the posi- 

 tion of the magnetic axis, received the shock of a Ley den jar, on 

 its upper end from the positive electricity, by which it was render- 

 ed magnetic, the lower end being found to repel the north pole 

 of the compass needle of the magnetimeter, about 3°. This 

 experiment varied by giving the shock to the lower end of the 

 bar, and also by passing the discharge transversely, first through 

 the upper end, and then through the lower end, still gave similar 

 results, the lower end of the bar in each case repelling the north 

 pole of the needle '4° or 5«. 



Exp. 2. With the friendly and valuable assistance of Dr Traill, 



(with whose excellent apparatus all the electrical experiments were 

 made,) several discharges of a battery of sixteen jars were passed 

 through the iron-bar, when in the same position and circumstances 

 as in the last experiment. (1.) The lower end of the bar being 

 connected with the outside of the battery, and the discharge from 

 . the inside of the jars being made on the upper end, the magnetism 

 acquired was such, that the lower end of the bar repelled the 



needle. 



