232 INDUCTIVE AFFINITY. 



(3) If a bar of soft iron bent into the form of a horse shoe, 

 with a copper wire twisted spirally round it, be applied like 

 a lifter to the poles of a permanent magnet, in the instant 

 of the iron becoming a magnet by induction, the molecules 

 of the spiral wire become chemico-polar, and when contact is 

 broken with the permanent magnet, and the soft iron ceases 

 to be a magnet, the wire exhibits a polarity the reverse of 

 the former. By a proper arrangement, electric sparks and 

 shocks may be obtained from the wire, while the soft iron 

 included within it is being made and unmade a magnet. The 

 magneto-electric machine, is a contrivance for this purpose, and 

 is now coming to supersede the old electric machine, as a 

 source of what is termed electricity of tension. Magnetic and 

 electric eifects are thus reciprocally produced from each other. 



(4) When the pole of a magnetic needle is placed near the 

 conducting wire, the former neither approaches nor recedes 

 from the latter, but exhibits a disposition to revolve round it. 

 The extraordinary and beautiful phenomena of electrical rota- 

 tion are exhibited in an endless variety of contrivances and 

 experiments. As the magnetic needle is generally supported 

 upon a pivot, it is free to move only in a horizontal plane, 

 and consequently when the conducting wire is held over or 

 under it (the needle being supposed in the magnetic meridian), 

 the poles in beginning to describe circles in opposite directions 

 round the wire, proceed to move to the right and left of it, 

 and thus deviate from the true meridian. The amount of 

 deviation in degrees is proportional to the quantity of cir- 

 culating induction ; and may be taken to represent it, as is 

 done in a useful instrument, the galvanometer, to be after- 

 wards described. It was in the form of these deflections, 

 that the phenomena exhibited by a magnet, under the influence 

 of a conducting wire, first presented themselves to Oersted 

 in 1819. 



FIG. 22. 12. Thermo-electrical phenomena are pro- 



duced from the effect of unequal temperature 

 upon metals in contact. If heat be applied to 

 the point c, (Fig. 22.) at which two bars of 

 bismuth and antimony b and a are soldered 

 together, on connecting the free extremities 

 by a wire, the whole is found to form a weak 

 voltaic circle, with the induction from b 



