Magnetisation in the Length of Metal Rods. 119 



I made a systematic course of observations with three iron rods of 

 different thicknesses prepared in the manner described above. All 

 were of ordinary commercial iron annealed in the usual manner. 

 Their respective diameters were 2*65, 3*65, and 6*25 mm., xand 

 the length of each was 100 mm. The strength of the successive 

 magnetising currents in amperes and the corresponding temporary 

 elongations in scale divisions are given in Table II. The currents 

 can be approximately expressed as magnetising forces by multiplying 

 by 91*8, and the scale divisions can be reduced to ten-millionths of 

 the length of the rod by multiplying them by 4. 



The results are also shown graphically in the first three curves of 

 fig. 2, in which the abscissae represent the magnetising currents and 

 the ordinates the elongations. 



Fio. 2. 



The elongations are those due to temporary magnetisation only. In 

 order to avoid the uncertainty attached to the elongating effects 

 of permanent magnetism, I always at the beginning of an experiment, 

 and before making an observation, passed through the coil the 

 strongest current at my disposal, thus permanently magnetising the 

 rod to saturation. There appear to be two kinds of residual mag- 

 netism. When a strong current has been passed through the coil, 

 the magnetism which remains after the current has ceased to flow is 

 for the most part of a sub-permanent nature. If the rod is undis- 



