506 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



The general shape of the magnet in question is shown in Figure 1. The 

 outside dimensions of the frame proper are about 101cm. X 80cm. X 40cm. 

 The base is of cast iron and of rectangular cross-section (20 cm. X 40 cm.), 

 the cylindrical arms are of soft wrought iron 25 cm. in diameter, the rec- 

 tangular pole pieces are 4.5 cm. thick, and the area of each of the opposed 

 faces is about 580 square centimeters. The four coils, which were com- 

 monly used in series, have together 2823 turns and a resistance at 20° C. 

 of about 12.4 ohms ; the magnet weighs about 1500 kilograms. 



The electromagnetic induction within so large a solid core as that of 

 this magnet practically attains its final value, as is well known, only 

 after an appreciable length of time. This time, for a given value of 

 the electromotive force in the coil circuit, depends upon the amount 



Figure 1. 



of non-inductive resistance in the circuit outside the magnet, and, for 

 a given value of the final exciting current in the coil, depends upon the 

 applied electromotive force ; under favorable circumstances it may be 200 

 seconds. At the outset Mr. J. Coulson and I obtained a large number 

 of hysteresis curves for the core under given conditions, but for steady 

 currents in the coil. The curves in Figure 2 are four of a series for 

 different widths of the air gap, each obtained after the magnet had been 

 put a number of times in succession through a Ewing's cycle with 6 

 amperes as the maximum current. The ordinates show the flux density 

 at the centre of the gap in thousands of units, and the abscissas the 

 current in amperes for the positive descending quarter of the cycle ; the 

 rest of the figures are omitted to avoid confusion. The flux was measured 

 by pulling a small thin coil of known dimensions, attached to a calibrated 



