161 



The method of obtaining data is as follows: First demagnetize the core 

 specimen to be tested by rotating the rotary commutator, thereby causing 

 an alternating current to flow through the solenoid. When the specimen is 

 demagnetized it will exert no pull on the lug (L). Next pass a very slight 

 current through the solenoid, place the lug in contact with the core and turn 

 the crank until the lug and core are separated. The number of turns can be 

 read directly from the slide index (D) and the disk (E). The current strength 

 is read from the ammeter. The current is then increased and the force meas- 

 ured which is required to separate the lug and core. This process is con- 

 tinued, noting in each instance the current strength and the pulling force of 

 the spring, until the pulling force ceases to inci'ease with an increase of cur- 

 rent, indicating that the core is saturated. The current is now decreased 

 step by step and the pulling force and the current strength noted in each 

 case. When the current reaches zero it is reversed and the process indicated 

 above is repeated. When the current is again brought back to zero it is 

 reversed the second time and increased to the point of saturation. Thus 

 data for the complete hysteresis loop may be taken. 



The equations for transforming the results from a permeameter into the 

 B and H values for plotting the hysteresis loop are as follows. (See Swing's 

 work page 248.) 



(B — H)'-XS(sq.cm.) 



Pull in lbs. = 



11183000 



Pull in lbs. 

 or B = 3344 h H 



\ S(sci. cm.) 



Pull in lbs. 

 or B = 1317 / h H 



V S(sq. cm.) 



The value of H may be determined by the following equation, in which 



N is the number of turns, I the current strength in amperes and 1 is the length 



of the solenoid in cm. 



„ 1 26 N I 

 H = 



I 



J 1—101!) 



