OF ARTS AND SCIENCES. 387 



No. IV. — ON THE TIME OF DEMAGNETIZATION OF SOFT 



IRON. 



Bv W. C. HODGKINS AND J. II. JeNNINGS, 



Tns following experiments were undertaken to determine the length 

 of time that the core of an electro-mairnet remained mai^netic after tiie 

 cessation of tlie magnetizing current. A chronograph, provided with 

 two pens, was used to measure the intervals of time. The method 

 adopted to obtain these results on paj^er was as follows: The lower 

 pen of the chronograph, which was movable by a lever, worked by 

 hand, was connected with the electro-magnet and with the battery 

 which served to excite the electro-magnet. By pressing the lever 

 down, tlie circuit of the electro-magnet was broken, and at the same 

 instant the lower pen was moved upwards. The upper pen formed 

 the armature of a small electro-magnet, and moved downward upon 

 the passage of a current through the coils of this magnet. One end 

 of the wire of this small electro-magnet was connected with the posi- 

 tive pole of a Bunsen cell, the negative pole of which was joined to a 

 brass plate, which was placed immediately beneath the large electro- 

 masfnet which was to be tested. The second wire from the small 

 electi'o-magnet, which worked the upper pen of the chronograph, was 

 connected with a small piece of soft iron which formed the armature 

 of the large electro-magnet. 



It will be seen that when no current was passing through the large 

 electro-magnet, its armature would rest upon the brass plate imme- 

 diatelv beneath it, and the circuit of the upper pen of the chronograph 

 would be completed. On the other hand, upon the passage of a cur- 

 rent, the armature would be raised, thus breaking the circuit of the 

 upper pen. 



The method adopted was to pass the current through the coil of the 

 larn^e electro-magnet, start the chronograph, and then, by means of the 

 lever on the lower pen, repeatedly break and make the circuit; thus 

 making and breaking the circuit of the upper pen, and moving both 

 pens at intervals, which represented the required time of demagnetiz- 

 ation. The interval required to demagnetize the small electro-magnet, 

 ia the circuit of the upper pen, did not enter into the results, since this 

 pen was used merely to denote the instant when its circuit was made. 

 The time of demagnetization was determined in this manner, with a 

 coil 220 mm. in diameter, and with cores successively 54 mm., 41 mm., 

 and 29 mm. in diameter. The battery power varied from four cells 



