642 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The rod was at the outset in a nearly neutral condition. A small 

 current (t) was at first applied to the solenoid, and then, with the help 

 of a set of high-resistance rheostats made by the Simplex Electric 

 Company, the rod was put many times through a hysteresis cycle with 

 this current, positively or negatively directed, to mark the limits. 

 After this the same current was slowly applied again and gradually 

 removed, and the remanent induction through the central cross section 

 of the rod was measured by means of a small test coil of very fine insu- 

 lated copper wire, so mounted that it could be quickly slipped off the 

 rod and removed from the solenoid while the rod itself remained in 

 situ. This flux was found for both directions of the magnetizing cur- 

 rent. Then the current was applied gradually as before and the cir- 

 cuit was broken by a sudden blow upon a simple switch, and the rema- 

 nant flux was determined for both directions of the exciting field. The 

 ballistic galvanometer used was a low-resistance d'Arsonval instrument 

 made for the purpose by Mr. Coulson, who worked with me in making 

 all the observations recorded here. The scale distance was about five 

 meters, and the telescope was focussed upon a real image of the scale 

 formed about two meters in front of the object glass, by a lens used as 

 the cover glass of the galvanometer mirror. 



The process just described was repeated for each of a series of cur- 

 rents of increasing intensities, and in this manner material for the 

 curves shown in Figure 3 was obtained. 



It is, of course, possible to use a magnetometric method in testing 

 the residual magnetism of short rods, but although we have made a 

 large number of determinations in this way, we have found it incon- 

 venient for several reasons. It will appear later that the lines of mag- 

 netization in the case of an anomalously magnetized rod are so folded 

 together that the external effect of the remanent magnetism is usually 

 small, and if a conveniently large magnetometer deflection is to be ob- 

 tained, the rod must be very near to the needle. It is not safe to 

 remove the iron fi:om the solenoid in order to test it outside, for a 

 slight blow might seriously alter the moment, and if the magnetism is 

 to be measured while the specimen is in its place within the solenoid, 

 the magnetometer must be set up near one end of the solenoid, where 

 it will be violently disturbed by the exciting currents and the fields 

 incident to the process of forcing the iron so many times through the 

 hysteresis cycles by which it is prepared for the tests. If a stout speci- 

 men of soft iron is placed with its axis horizontal and perpendicular 

 to the meridian, a moment large compared with the residual moment 

 to be measured is induced in it by the earth's field, and it is practi- 

 cally difficult to prevent this transverse magnetization from masking 

 the eff"ect to be measured. 



