INSTRUMENT FOR MEASURING MAGNETIC ATTRACTIONS, &C. 253 



S. An iron-bar, with permanent polarity, when placed any- 

 where in the plane of the magnetic equator, may be deprived 

 of its magnetism by a blow. 



Exp. — Many experiments were made on the effect of blows on iron 

 in this position : the result was always to diminish the polarity, 

 and generally wholly to destroy it. It commonly happens, that a 

 single blow with a hammer is sufficient for destroying the magne- 

 tism of the bar ; but if it have been strongly touched with a mag- 

 net, it may require two or three blows at each end. 



Remark. — For freeing iron of magnetism, it is generally recommend- 

 ed to heat it to redness, and allow it to cool in an east and west 

 position ; but as this process oxidizes the metal, and is attended 

 with loss of time, the action of the stroke of a hammer is certainly 

 preferable, and is much more effectual. Where there is no proper 

 instrument for ascertaining the elevation of the magnetic equator, 

 along its north and south axis, the effect of blows on the magne- 

 tism of iron, may be shewn with tolerable precision by a common 

 pocket compass, or other small magnetic needle. Place the com- 

 pass on a table, whose surface is pretty nearly horizontal, with the 

 needle corresponding to the direction of the north and south line 

 of the graduated plate, where there is no card ; and then place the 

 iron, the magnetism of which is to be determined, on the table, at 

 right angles to the north extremity of the needle, and an inch or 

 two distant from it, in an east and west position, marking its si- 

 tuation, either by a line dravra on the table, or by two brass pins. 

 The iron being now in the plane of the magnetic equator, will 

 produce no derangement in the needle, if it be free from perma- 

 nent magnetism ; but after being hammered in the magnetic axis, 

 on being brought to the same position again, with regard to the 

 compass, the end of the bar that was upward during the hammer- 

 ing, will be found to attract the north end of the compass-needle : 

 If hammered again, when laid horizontally, and pointing east and 

 west, its magnetism will be annihilated. 



I i 2 6. Iron 



