1880.] On Magnetic Circuits in Electric Machines. 289 



turn an electric current in the helix, which tends to strengthen the 

 magnetization ; and also that a magnet is absolutely stronger under 

 tension than when at rest. 



On the other hand, pressure on the armature, either continuous or 

 sudden and momentary (a blow for example), causes an electric 

 current in the helices in the opposite direction to original magnetiza- 

 tion, or in other words, against magnetization ; tending thereby to 

 weaken the power of the magnet. 



The 58 lb. magnet in closed circuit was hung by its armature, and 

 on afterwards connecting its helices with the galvanometer no current 

 could be detected, but on lowering it until it rested with its whole 

 weight on the ground a current in the direction of demagnetization 

 was produced, giving a deflection of 15°. In the same way a current 

 in the direction of magnetization was obtained, giving a deflection of 

 15°, by the application of sufficient strain to lift the magnet off the 

 ground, and this result was invariable. The degree of swing, however, 

 depended upon the rapidity with which the magnet was either raised 

 or lowered. 



It may be remarked that whereas any very slight application of 

 force by pulling on the armature was sufficient to cause a current in 

 the helices giving a deflection of 5° to 10° of the galvanometer 

 needle, a great amount of pressure is necessary to produce a similar 

 deflection. A slight pull with the finger and thumb in the one case 

 was equal to the pressure of a hundredweight in the other. 



By the momentary removal of the armature, the closed magnetic 

 circuit is broken, and though by its immediate restoration a new closed 

 circuit is formed, nevertheless the tension on the molecules of iron by 

 the magnetic stress is very greatly reduced. Under these conditions a 

 very slight pressure upon the armature produces a great swing of the 

 needle, whilst a pull produces scarcely any effect at all until actual 

 movement of the armature takes place. 



If the pressure on the armature is great and continuous, a point is 

 soon reached at which a slight pressure is no longer effective. 



The effects produced are somewhat different if pressure is applied 

 unequally. For instance :— A weight of 7 lbs. placed on the armature 

 over the north pole of the 58 lb. magnet caused a current in the helices 

 giving a deflection of 20° at the galvanometer. The same weight on 

 the south pole gave the same deflection in the opposite direction, 

 Pressure with the hand produced like swings of the needle pro- 

 portionate to the force used, and the amount of swing can be easily 

 controlled, and the needle brought to rest by judicious pressure on 

 either pole of the magnet. 



If a lateral pressure be applied to one side of the armature between 

 the poles, and the needle swings say 5° : on removal of the pressure, a 

 current is produced in the opposite direction, and the reverse swing 



