256 Hlectro-Magnetic Apparatus and Experiments. 
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, 
atid 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 dus attention from these 
who may be interested. 
The following striking experiments afford still further illustra- 
tion of the action of closed secondary circuits. 
Experiment 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 asa closed circuit to prolong the magnetism of the spiral, 
and thus prevent a sudden and entire thfinence 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 a of 
the large. 
Experiment 2d.—The reciprocal action of the sieeid snout 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 sae understood from, 
