716 



TEST OF IRON BY MAGNETISM. 



proceedings of the convention are a portion of 

 the history of 1868. 



TEST OF IRON BY MAGNETISM. A 

 sample is presented by F. A. Paget by which 

 internal flaws and solutions of continuity in 

 constructive details can be easily detected, 

 as discovered by Mr. S. M. Saxby, R. N. The 

 principle upon which the method is founded 

 is the wejl-known fact that when a bar or any 

 mass of soft iron is placed in the position of 

 the dipping-needle it is at once sensibly mag- 

 netic, the lower extremity being a north pole 

 in our latitudes, and the upper extremity a 

 south pole. In the southern hemisphere the 

 poles are of course reversed. The same action, 

 only weakened, takes place in a bar hanging in 

 a vertical or any other position, only the effect 

 is weaker the more the position of the longi- 

 tudinal axis of, for instance, a long bar, departs 

 from that of the magnetic dipping-needle. 



Fig. 1 shows, in a rough manner, the posi- 

 tions, with regard to each other, of the mag- 

 netic dip of our latitudes with its equatorial 

 magnetic plane. A sphere of soft iron, homo- 



FlG. 1. 



geneous in its internal structure, and without 

 any solution of continuity within its mass, 

 would, in section, have south polarity where 

 marked S, a neutral state in its centre where 

 left white, and an opposite north polarity 

 at N. 



Fig. 2 illustrates, as far as can be done by 

 hatchings, the magnetic condition of a bar of 

 very homogeneous soft iron, lying east and 

 west, or better, in the exact equatorial mag- 

 netic plane. 



FIG. 2. 



Fig. 3 shows the magnetic condition of a 

 similar bar lying in the dip plane. 



When, therefore, as in Fig. 4, a small com- 

 pass-needle is slowly passed in front of a bar 

 of very good iron, placed in an east and west 

 direction, the needle will not be disturbed from 



its proper direction, which is of course at 

 right angles to this, or north and south. 

 FIG. 3. FIG. 4. 



All this refers to regularly homogeneous bars 

 of best quality to bars without any mechan- 

 ical solutions of continuity. With internal 

 flaws or interruptions of continuity the bar is 

 no longer regularly magnetic. It has long 

 been known, that a good compass-needle, or a 

 good permanent magnet, must be homogeneous 

 and without flaws, in order to take and retain 

 its maximum amount of magnetism. In a 

 word, any mechanical solution of continuity is 

 accompanied with a polar solution of continu- 

 ity, and the given bar or mass with flaws 

 whether permanently magnetized or tempora- 

 rily so by the inductive action of the earth is 

 no longer one regular magnet, but several dif- 

 ferent magnets, with the different magnetisms 

 separated from each other. The delicately- 

 poised magnet of a compass can thus be made 

 to tell the presence of such solutions of con- 

 tinuity. The annexed cuts, Fig. 5, showing 



FIG. 5. 



the actual results of the test with a | in. bar, 

 12 in. long, will illustrate the manner in which 

 the compass-magnet is affected by the presence 

 of cracks, of solutions of continuity, in the 

 bar, which is supposed to be lying-in the equa- 

 torial magnetic plane, or east and west. The 

 magnet was traversed in a parallel line with 

 the centre of the bar. As long as the iron was 

 perfectly sound the needle stood at right an- 

 gles, in its proper position, in fact, without 

 being disturbed. Such a position is shown at 

 B. Slight deviations had previously been 

 caused, getting repeated toward the middle 



