PEIRCE. — MANNER OP GROWTH OF A CURRENT. 525 



The "apparent inductance" thus defined is affected by Foucault cur- 

 rents ; its values for several points of the curve are given in Table X. 



L plotted against i yields a curve that is fairly straight. 



The theoretical curve ( Q) is very like some 14 of the actual curves which 

 have been obtained from magnets with finely divided cores, whereas the 

 actual building-up curve represented in the diagram is similar to those 15 

 which almost any electromagnet with solid core can be made to yield. 

 The eddy currents, the changes in effective permeability, and the other 

 disturbing influences taken together, do not in such a case cause the curve 

 to deviate very widely in shape from that which one could get from a 

 simple circuit with fixed inductances and no eddy currents : the resem- 

 blance can almost always be made close by proper choice of the electro- 

 motive force of the exciting battery. 



After I had determined hysteresis diagrams (some of which are shown 

 in Figure 3) for the magnet for a large number of gap-widths up to 

 28 mm., it seemed likely that for an air gap about 35 mm. wide the 

 hysteresis diagram would not be very different from a single straight line. 

 For this gap-width the induction flux through the coil should be practi- 

 cally proportional to the strength of the current and amount to about 

 6.45 X 10 6 for a current of one absolute unit so that the inductance of the 

 coil circuit should be about (28.23) (0.645) henries for a wide range of 

 currents. The form of the building-up curve of a current in the coil of an 

 electromagnet generally depends very much upon the magnetic state of 

 the iron at the outset. If a steady current which has been running 

 through the coil for some time be interrupted, and if then after a little 

 the circuit be closed again, the manner of growth of the new current is 



14 See, for instance, the fine diagram given by Dr. Thornton, Phil. Mag., 1904, 

 p. 625. 



15 J. Hopkinson and Wilson, Phil. Trans., 1895, pp. 275, 280. 



