PEIRCE. — MANNER OF GROWTH OF A CURRENT. 



509 



even when the gap is closed, and, for a given width of gap, the ratio of the 

 induction through a circumference of, say, 48 cm. diameter in the plane 

 of the gap with its centre (O) at the gap-centre, to the flux density at O, 

 depends slightly upon the intensity of magnetization of the iron. With 



Figure 4. 



a gap-width of 28.3 mm. this ratio increased by about 3 per cent as the 

 strength of the current in the coils rose from a small value to 6 amperes, 

 and was sensibly the same for ascending and descending branches of a 

 hysteresis cycle. 



The ordiuates of each of the curves in Figure 3 show, in millions of 



