ARMATURE LAMINATIONS 17 



While each manufacturer will find it expedient to draw up 

 specifications adapted to comply with his own special require- 

 ments, it may be useful to those to whom the subject presents an 

 aspect of novelty, to set forth a specification which, while ensuring 

 fairly good material when complied with, cannot be said to impose 

 any unreasonable condition upon the contractor for the material, 

 and hence the material should be supplied at current market 

 prices. The specification is as follows: 



" Samples shall be cut from various parts of several sheets 

 selected at random from the shipment. When tested on a Ewing 

 hysteresis tester by comparison with the standardised samples 

 supplied with the instrument, the hysteresis loss at a density of 

 4000 C.G.S. lines per sq. cm., and at a periodicity of 50 complete 

 cycles per second, shall not exceed 0-5 watts per kg." 



If the Ewing hysteresis tester is not originally provided with 

 standardised samples, then there should be obtained a number 

 of samples (at least three) of widely different quality, whose 

 hysteresis loss is known ; and prior to and after the measurements 

 on the samples to be tested, readings should be taken on all these 

 standardised samples, and the constant of the instrument thus 

 obtained for each particular occasion. Should the ratios of the 

 readings obtained on the various standardised samples vary 

 materially from the ratios obtained on prior occasions, it is an 

 indication that, due to some cause, one or other of the samples has 

 altered. The greater the number of standardised samples em- 

 ployed, the greater will be the certainty with which the defective 

 sample can be identified and eliminated. Suppose that we have 

 four standardised samples, I., II., III., and IV., whose hysteresis loss 

 is known and stated, then we need first to standardise the Ewing 

 tester that is, to determine its " constant," or the factor by which 

 the instrument readings (" residual deflections ") must be multi- 

 plied to obtain the hysteresis in watts per kg. 



Table III. is drawn up to allow of the entry of the necessary 

 readings to determine this "constant." In column 2 should be 

 entered the hysteresis loss corresponding to each of the samples 

 I., II., III., and IV. Each sample should now in turn be put into 

 the instrument, and four readings taken on each. These readings 

 should be entered up in column 3. The average reading (" residual 

 deflection") for each sample should then be calculated, and entered 

 in column 4. Dividing this average deflection into the known 

 hysteresis loss of the standard sample in column 2, we obtain the 

 watts per kg. per degree " residual deflection " of the instrument. 



2 



