S42 Dr. Tyndall on the Polanty of Bismuth, 



express the thing popularly, annul each other by a species of 

 interferenee before they reach tJie body ; or does one pole induce 

 in the body a certain condition upon which the second pole acts 

 in a sense contrary to the former, both poles thus exactly de- 

 stroying each other ? If the former, then I grant you that the 

 magnetic field is rendered weaker, nay deprived of all force if 

 you will, by the introduction of the second pole ; but if the 

 latter, then we must regard the field as possessing two systems 

 of forces ; and it is to the peculiar inductive property of the 

 body, in virtue of which one system neutralizes the other, that 

 we must attribute the absence of attraction or repulsion. Once 

 grant this, however, and the question of diamagnetic polarity, so 

 far as you are concerned, is settled in the affinnative.'' 



23. Our hypothetical friend mentions it as ' an experimental 

 fact/ that if dissimilar poles of equal strengths operate upon a 

 mass of bismuth there is no repulsion. This was Reich's result — 

 a result which I have carefully tested and corroborated. I shall 

 now proceed to show the grounds which the believer in diamag- 

 netic polarity might urge in support of his last assertion. A 

 twelve-pound copper helix was removed from the limb of an 

 electro-magnet and set upright. A magnetized sewing-needle 

 being suspended from one end, the other end was caused to dip 

 into the hollow of the spiral, and to rest against its interior 

 surface. When a cun*ent was sent through the helix in a certain 

 direction, the needle was repelled towards the axis of the coil ; 

 the same end of the needle, when suspended at half an inch 

 distance from the exterior surface of the coil, was drawn strongly 

 up against it. AVhen the current was reversed, the end of the 

 needle was attracted to the interior surface of the coil, but re- 

 pelled from its exterior surface. If we suppose a little mannikin 

 swimming along in the direction of the current, with his face 

 towards the axis of the helix, the exterior surface of that end 

 towards which his left arm would point repels the north pole of 

 a magnetic needle, while the interior surface of the same end 

 attracts the north pole of a magnetic needle. The complemen- 

 tary phsenomcna were exhibited at the other end of the helix. 

 Thus if we imagine two observers placed, the one within and 

 the other without the coil, the same end thereof would be a north 

 pole to the one and a south pole to the other. 



24. If we apply these facts to the case of the helix within the 

 magnetic field, we see that each pole of the magnet had two 

 contrary poles of the helix in contact with it ; and we moreover 

 find that the quadrants which we have denominated the strongest 

 are those in which the poles of magnet and helix were in conjunc- 

 tion; while the quadrants which we have called weakest are 

 those in which the poles of magnet and helix were in opposition. 



25. ^' Which will you choose ?'' demands our hypothetical 



