176 Dr. Tyndall on Biamagiietisni and Magneanjstallic Action^ 



Each of the helices which surrounded the cores was composed 

 of two isolated wires ; the four ends of these could be so com- 

 bined that the current could pass through both at the same time, 

 as if they were a single wire, or it could be caused to traverse 

 one ^vire after the other. The first arrangement was advan- 

 tageous when a small exterior resistance was an object to be 

 secured, the second when the force of the battery was such as to 

 render exterior resistance to a certain extent a matter of indif- 

 ference. In the above experiments the first arrangement was 

 adopted. Before commencing, however, I had taken fresh acid 

 and freshly amalgamated zinc cylinders, so that the battery was 

 in good condition. Tlie second arrangement was adopted, that 

 is, the current was allowed to traverse one wire after the other, 

 and the following repulsions were observed ; the numbers refer 

 to the positions already indicated : — 



1st position 57 



2nd position 51 



3rd position 53 



4th position 48 



These experiments furnish the direct proof that calcareous 

 spar is repelled most strongly in the direction of the optic axis. 

 That Mr. Faraday has not succeeded in establishing a difierence 

 here is explained by reference to his mode of experiment. He 

 observ'ed the distance to which the spar was repelled, and found 

 this the same for all positions of the crystal. The magnetic 

 force at this distance is too weak to show a difference. In the 

 above experiments, on the contrary, the ciystal was forced back 

 into a portion of the magnetic field where the excitement was 

 intense, and here for the first time the difi"erence rises to a mea- 

 sui'able quantity. 



Carbonate of iron is a crystal of exactly the same form as cal- 

 careous spar, the iron filling up, so to speak, the exact space 

 vacated by the calcium. This crj^stal is strongly magnetic; 

 suspended in the magnetic field, that line which in calcareous 

 spar sets equatorial, sets here axial, but with an energy far sur- 

 passing the spar ; a greater diiFerential action may therefore be 

 anticipated. 



A pair of spheres were fonned from this crj^stal, but their 

 attraction was so strong, that to separate them from the magnet 

 would strain the wire beyond its limits of elasticity ; one sphere 

 only could therefore be used, the other being used as a balance- 

 weight merely. The core opposite to the latter was removed, and 

 the current sent round that helix only which surrounded the 

 former. A piece of Bristol board was placed against the end of 

 the core, and the torsion-head was so turned that when the index 

 above pointed to zero the little sphere was ou the verge of con- 



