302 BELL SYSTEM TECHNICAL JOURNAL 



7 = — .84 for wt = — 60°. Calculation by equation (12) gives for 

 the vertical spread of the curve at this point 2.3 per cent of the double 

 height of the curve, of which 1 per cent is contributed by the time 

 constant factors and 1.3 per cent by the condenser conductance. 

 It is seen that of the errors discussed in this and the preceding section, 

 those caused by the conductance of the condensers are the larger. 



3. Demagnetization by Current in the Search Coil. 



The voltage induced in the search coil by the change of flux in the 

 sample creates a current in the search coil circuit. The magnetic 

 field set up by this circuit in the search coil opposes changes in the 

 field due to the current in the magnetizing winding and so makes 

 the hysteresis loop appear wider than it actually is. 



At low fields the effect may be calculated with some degree of 

 accuracy. Let the field induced by the magnetizing coil be Hi cos ut, 

 and let the search coil contain a sample which has the initial permea- 

 bility ju and the cross section 5. The field set up by the search coil 

 current is then 



_^NNSdB 

 10 / R dt^^^ 



and the total field is, very nearly, 



// = III I COS co/ + — 10 ** . ;Uco sm co/ 1 . 



The induction is 



B ^ ixll ^ tilli (cos CO/ + i^ X 10-« ^ M" sin CO/ \ , (14) 



while the magnetizing field which the tube indicates due to the influence 

 of the deflector coils is Hi cos co/. The spot therefore traces an 

 ellipse in the counter-clockwise direction, so as to add to the effect of 

 hysteresis in the material. 



Assuming the values N = 30,000, / = 30, i? = 100,000, /x = 10,000 

 (permalloy), 5 = .005, co = 100, the width of the ellipse along the 

 B axis is about 6 per cent of its total height. This is a considerable 

 error, so that when working with so high a permeability a high value 

 of amplification may have to be used while the factor N^S/IR is 

 decreased. 



When the magnetizing field is such as to saturate the sample the 

 calculation of the effect of search coil current is more uncertain. 



