68 DETECTION OF NEEDLES. 



city around it or in its immediate vicinity, it evinces magnetic 

 properties. Now one of the most important properties of a 

 magnet is, it will be remembered, that its north pole will repel 

 the north pole of another magnet brought near it, but attracts 

 its south pole, the converse being the case with regard to the 

 south pole. 



These facts being known, the principle on which Mr. Smee 

 acts is readily understood. The first point is to render the 

 suspected piece of steel magnetic. The mode of effecting this 

 may be best described in the words of Mr. Smee : " When you 

 suspect the presence of a piece of needle, or other steel instru- 

 ment, you must subject the suspected part to a treatment calcu- 

 lated to render the needle magnetic ; and this is best done by 

 electro-magnetism. I have tried many forms of instruments, 

 but should prefer that represented in 

 the accompanying diagram, which is 

 made of a simple bar of soft iron 

 wound round with wire. The iron 

 has a plate of brass, B, fixed in both 

 ends to retain the wire (w) in situ ; 

 and the two ends of the wires are 

 attached to binding screws (s). 



When this instrument is connected with the poles of a voltaic 

 combination (and for this purpose any battery possessing suffi- 

 cient power may be employed, as, for example, a single cell of 

 Grove's or Smee's already described) it is to be kept in close 

 approximation with the suspected part for some little time, and 

 if there be a portion of steel conveyed therein, it will be endued 

 with magnetic properties. 



To test the existence of the magnet within the body is the 

 next point necessary, for which purpose a common sewing needle 

 previously rendered magnetic, and suspended by a portion of 

 silkworm's silk, may be employed. But it will be found that 

 the most convenient apparatus for the purpose is such as is 

 represented in the diagram. (Fig. 17.) It consists of a delicate 

 needle, about six inches long, centred upon a small agate cup 



