14 



ARMATURE CONSTRUCTION 



ment has a magnet a, pivoted on a knife-edge supported by an 

 agate bearing. The magnet carries a pointer which reads over 

 the scale c. The sample strips, 3 inches (76 mm.) long and inch 

 (15*9 mm.) wide, which are cut from the laminations of sheet iron, 

 are secured by clamps to the carrier d, and are rotated between 

 the poles of the magnet a by means of the friction wheel e and 

 the driving wheel/, thus producing reversals in the magnetisation 

 of the sample strips. Owing to the hysteresis loss in the iron, 

 a couple is exerted on the magnet a, thereby causing a deflection 

 of the pointer. Owing to windage and to hysteresis in the 

 permanent magnet, a certain deflection would be obtained even if 

 the sample had no hysteresis loss. With the scale generally 

 supplied with the instrument, this deflection is about 30. If, 

 however, 30 is subtracted from the reading obtained with a given 

 sample, the residual deflection will be approximately proportional 

 to the hysteresis loss in the sample. It is thus convenient to add 

 to the scale as provided, a series of figures say, in red ink with 

 corresponding to 30 of the original scale, assigning to the other 

 portions of the scale values less by 30 than the values of the 

 original scale. There are thus two "zeros "to the right ami left 

 of the centre of the scale, and an ungraduated portion between 

 them. Let us designate this as the scale of " residual deflections.'' 

 This proposition will be better understood by considering an 

 actual case, where measurements of three samples of previously 

 ascertained hysteresis loss gave the results set forth in Table II. 



TABLE II. 



DETERMINATION OF CONSTANT FOR EWING HYSTERESIS TESTKK. 



Hence to determine the absolute value of the loss, the 

 " residual deflection " produced by the test sample is compared 



