196 



Transactions. — Miscellaneous . 



■with the maximum current, and also depended on whether 

 the iron in the circuit was soHd or finely divided. 



To illustrate the difference between the curves of rise for 

 different maximum currents curve 8 is appended. 



In curve 8 (A) the maximum magnetizing force is' 132-6 

 C.G.S. units. After the steep part of the magnetizing curve 

 is passed the current rises extremely rapidly, as is evident 

 from the almost vertical line. Time of rise = O'OOlTSsec. 



Curve 8 (B) : Maximum magnetizing- force, 38-7 units. 

 None of the changes are so sudden as in the first curve. Time 

 of rise = 0-00192sec. 



Curve 8 (C) : Magnetizing force, 15 units, which is just 

 sufficient to ascend the steep part of the hysteresis curve. 

 The current rises very gradually, and there are no sudden 

 changes in the curve. 



The times taken by the currents to rise in the three, 

 cases are nearly equal, notwithstanding the fact that the 

 resistance in one case is nearly nine times that of the 

 others. 



In the above curves the iron was finely laminated, but when 

 the iron is solid the current rises very rapidly for the first few 

 ten-thousandths of a second, and then increases very slowly to 

 its final value. This is due to the fact that only the surface- 

 layers of the iron are magnetized at first, and the induction 

 penetrates but slowly into the mass of the metal, due to the 

 screening effect of induced currents. 



With large solid electro -magnets the current takes in 

 many cases over a second to rise to its maximum, and after 



