256 Electro- Magnetic Apparatus and Experimefits. 



hollow magnet was entire, the insertion of an iron rod or bundle 

 of wires produced no effect. From these experiments I think 

 the existence of secondary currents flowing in the body of the 

 magnet may be very plausibly inferred. If actually determined, 

 the fact would prove important, and is well worth pursuing. I 

 soldered two wires to the edges of the enclosed sheet of iron, 

 and connected them with a galvanoscope, but could not perceive 

 any effect upon the needle. But as the instrument was by no 

 means delicate, the experiment may be regarded as valueless. 

 Having no opportunities at present of pursuing the investigation, 

 I hope that the subject may receive due attention from those 

 who may be interested. 



The following striking experiments afford still further illustra- 

 tion of the action of closed secondary circuits. 



Expei^iment 1st. — Place a straight electro-magnet upon a large 

 flat spiral of copper, in the direction of a radius of the spiral. 

 When the spiral is connected with the battery, the magnet be- 

 comes charged, and a secondary current in its wires is the conse- 

 quence. Break the battery connexion with the spiral, and ex- 

 amine by the common tests the power of the secondary from the 

 magnet. Again, break the circuit from mercury covered with oil, 

 and the secondary from the magnet will now be found stronger 

 than in the first case. When the circuit is broken over clean 

 mercury, the secondary flowing through the heated vapor and air, 

 acts as a closed circuit to prolong the magnetism of the spiral, 

 and thus prevent a sudden and entire influence upon the magnet. 

 When the mercury is covered with oil the secondary is arrested, 

 and the magnetism suddenly ceasing, exerts its whole influence 

 upon the magnet, or rather the magnetism of the bar ceases with 

 that of the spiral. The same phenomenon is well illustrated by 

 the electro-magnet alone, where the fine wire is independent of 

 the large. 



Expei^immit 2d. — The reciprocal action of the closed circuit of 

 the magnet itself upon the secondary of the spiral is more re- 

 markable. Break the battery connexion with the spiral over 

 clean mercury, when the ends of the wire on the magnet are 

 disjoined, and observe the spark ; join now the ends of the mag- 

 net wire, and on breaking the battery circuit the spark from the 

 spiral will be diminished. The manner in which the closed cir- 

 cuit operates here, will be more easily understood from, 



