136 



PEOCEEDINGS OF THE AMERICAN ACADEMY. 



of iron as are used in good transformers, a fair approximation to the 

 form which the current curve will have under any given circumstances 

 can be made if one has an accurate statical hysteresis diagram of the 

 core for the range required, and if the core is made of very fine var- 

 nished wire, as in the case of loading coils for long telephone circuits, 

 a' hysteresis diagram obtained either from a long " step-by-step series " 

 of measurements or from one or more oscillograms, enables one to pre- 

 dict with accuracy what the form of a current curve will be for any 

 practical case. These last statements are based on experiments such 

 as those recorded below. 



As a result of a long series of measurements, it appears that when 

 the core of the magnet Q has been well demagnetized and a series of 

 steady currents each a little stronger than the preceding one are estab- 

 lished in the exciting coil, the magnetic flux through the core in 

 thousands of maxwells follows fairly accurately the course indicated 

 in the following table : 



TABLE I. 



Figure 31 reproduces the table graphically in the full curve : the 

 vertical unit is a thousand maxwells, and the horizontal unit is 139.4 

 ampere-turns, to suit the case when the particular exciting coil used 

 has 1394 turns. The ordinates of the dotted curve represent twice the 

 corresponding values of the slope (X) of the other. A template of the 

 curve B was made as accurately as possible from a large piece of sheet 



