Crook — Electromagnetic Alternating Currents. 139 



The tube was 50^"^ long and had an inside diameter of 2-5*^™. 

 Inside of the tube was a core of wood with a copper wire 

 running along its cylindrical axis. This system is practically 

 equivalent to sixteen concentric iron tubes •018^''' thick, with 

 their ends so connected that a current of equal current density 

 flows through each alternate one in the opposite direction or 

 through all of them in multiple, according as the experimenter 

 desires. The magnetizing coil was of the same length as the 

 iron cylinder and wound with 300 turns ]^o. 16 copper wire. 

 The test coil used for measuring the magnetic flux was wound 

 on a wooden spool made to slip over the magnetizing coil and 

 suflSciently large and heavy to fall freely and uniformly when 

 liberated. The cylinder was mounted inside the magnetizing 

 coil in a vertical position, about ^sf^ feet from the floor, and 

 so arranged that the test coil would fall from the center of the 

 cylinder to about two feet from its lower end. In this way it 

 was possible to measure the total magnetic flux in the iron. 

 The other apparatus was the same as that used in the first 

 method. 



It was found that the amount of residual magnetism remain- 

 ing in the cylinder after applying a strong magnetizing force, 

 was very small, and all attempts to show the demagnetizing 

 effects on the hysteresis cycle were unsatisfactory. However, 

 the reduction of the residual magnetism by the alternating 

 current was quite satisfactorily shown and the results are as 

 given below. 



The residual magnetism left in the iron was stronger when 

 an alternating current was present during magnetization. 

 Because of this, the alternating current was thrown off each 

 time the magnetizing force was applied, and readings were 

 taken to find the residual induction before again applying the 

 alternating current. These all gave almost constant results. 

 The readings were all reduced to the mean value. Headings 

 taken while the alternating current was flowing and those 

 taken after it was thrown off were nearly always the same. 

 However, in curve 2 the values of the residual magnetism 

 after the alternating current was thrown off were different and 

 are represented by the dotted portion of the curve. 



The alternating current was applied in four different ways 

 and the effects studied. First, it was passed through the iron 

 plates in series, so as to get the effect of the magnetically 

 compensated current. Second, it was passed through the 

 plates in multiple so as to get the combined effect of the cur- 

 rent and circular magnetism. Third, it was passed through 

 the iron plates in multiple and back through the copper rod. 

 Fourth, it was passed directly through the copper rod. The 



