DETECTION OF NEEDLES. 



resting upon a steel point, so that the smallest possible amount 

 of resistance is offered to its free play. 



Fig. 18. 



When a part containing magnetic steel is brought near the 

 needle, it may be either attracted or repelled; it may move 

 upwards or downwards ; or it may exhibit disquietude according 

 to the position in which the new magnet is held. We may 

 detect the position of the foreign body, when it is of any size, by 

 ascertaining where its north and south poles lie ; and these are 

 determined by their repelling and attracting the opposite poles 

 of the magnetic needle. The disquietude, or motion upwards 

 and downwards, merely indicate magnetism, but not the direction 

 of the magnet. Mr. Smee has had more than one opportunity 

 of proving the practicability and efficiency of the plan. In one 

 instance, he succeeded in detecting a piece of needle impacted 

 in the finger of a young woman, although, upon removal, it 

 weighed but the seventh part of a grain. It gave such marked 

 indications, that he was enabled to ascertain tolerably well the 

 positions of its north and south poles, and consequently its exact 

 situation. 



In performing these experiments, it is necessary to be careful 

 to continue the voltaic current in the same direction ; for if it be 

 reversed for one instant, it would tend to demagnetise the pre- 

 viously magnetised steel. 



