to Reversals of Magnetization in Soft Iron. 215 



joined in series with that of the iron ring in such a way that 

 the induced currents due simply to the magnetizing field (»£)) 

 in the two rings will neutralize each other in causing the 

 ballistic effect. This was of course a mere matter of con- 

 venience, as the irreversible part of the cycle of magnetization, 



I 



-h 



depends solely upon 3 and not upon <§. 



The ballistic galvanometer was one of Sir William Thom- 

 son's design, used some years ago for investigating dynamical 

 qualities of metals. The period of its swing was made longer by 

 loading the aluminium frame of the needle. The proportion- 

 ality of the throw was tested by slipping off suddenly some 

 temporarily formed loops of moderately fine copper wire 

 from a permanent bar-magnet. This showed that the throw 

 varied according to the number of these loops throughout the 

 extent of the scale as far as the accuracy of the experiment 

 could allow. 



The magnetizing field, that is the field due to the primary 

 coils of the rings, was varied by small sudden steps by in- 

 creasing the conductivity of the circuit. This was accom- 

 plished by putting a number of Edison lamps in the circuit, 

 arranged as in the so-called Mho-box. The strength of the 

 current was measured after each step by a deci-ampere balance 

 in its circuit. 



The value of the change of magnetization of the iron wire 

 corresponding to one division of the ballistic throw was de- 

 termined by putting an earth induction-coil in the gal- 

 vanometer circuit and observing the throw when the coil was 

 quickly turned. The coil was wound upon a circular wooden 

 frame provided with proper axis : its dimensions were as 

 follows : — 



Diameter of the frame 30*5 centim. 



Diameter of the coil after winding the 



wire . . 32*4 centim. 



Number of turns of wire in the coil 



(8 layers of 27 turns each) ... 216 



Sum of areas of all the turns . . . 167640 sq. centim. 

 Resistance of the coil (at 11° C, room's 



temperature) 4*306 ohms. 



Resistance of the galvanometer . . '634 ohm. 



Resistance of leading wires . . . *229 ohm. 



So that, when the experiment with 

 the earth induction was made, the 

 circuit had the total resistance of . 5*169 ohms. 



