INDUCED ELECTROMOTIVE FORCE. 137 



Such a mass of iron is said to be laminated. The iron parts of 

 dynamo armatures and of transformers are always laminated. 



Examples of eddy currents. A suspended magnet which is 

 set oscillating about its axis of suspension is quickly brought to 

 rest if it is surrounded by a massive ring of copper, because the 

 eddy currents induced in the copper by the moving magnet act 

 upon the magnet with a force which is at each instant opposed to 

 the motion (Lenz's Law). 



A sheet of copper which .is suddenly thrust between the 

 poles of a strong electromagnet behaves as if it were moving 

 in a viscid liquid. Eddy currents are induced in the copper 

 and, because of these eddy currents, the magnet exerts a force 

 upon the copper which is always opposed to the motion (Lenz's 

 Law). 



An interesting effect of eddy currents is their action in prevent- 

 ing the sudden magnetization or demagnetization of a solid iron 

 rod. Thus, a bundle of iron wires surrounded by a winding of 

 wire is magnetized say in one second when the winding is con- 

 nected to a given battery, and demagnetized in a much shorter time 

 when the battery is disconnected. A solid iron rod of the same 

 size would require perhaps nine or ten seconds to be magnetized 

 by the same coil and battery, and the solid rod would lose its 

 magnetism very slowly when the battery is disconnected. The 

 eddy currents in the solid rod oppose the magnetization while the 

 rod is being magnetized, and they tend to keep up the magnetization 

 while the rod is being demagnetized (Lenz's Law). Another in- 

 teresting effect of eddy currents is that which is exemplified in the 

 ordinary "medical" induction coil, in which the "power" of the 

 coil is adjusted by moving a brass or copper tube which surrounds 

 the iron core of the coil. When the tube surrounds the entire 

 core a sudden break in the primary circuit results in a slow de- 

 magnetization of the core because of the eddy currents in the tube 

 which tends to keep up the magnetization, but when the tube is 

 withdrawn the core is demagnetized very quickly when the 

 primary circuit is broken. 



