264: Mr. E. Taylor Jones on 



is low the permeability of iron is great, and the tubes of 

 induction pass more readily through the iron; but when the 

 induction is high and the permeability small the absence of 

 coil- windings near the surface of contact causes a greater pro- 

 portion of tubes to pass out into the air ; in other words, 

 causes a greater spreading of the tubes. The effect of the 

 absence of a given number of turnings near the plane of 

 section might be calculated [see Neumann : " Ueber die Mag- 

 netisirung eines Drehungsellipsoids/' Crelle f Bd. xxxvii. 

 (1848)], but it was deemed better first to diminish the gap 

 between the coils as much as possible. With this object the 

 coils were somewhat altered, the end-plates being altogether 

 removed and the small guiding ring being let into one of the 

 coils. The gap-width could then be reduced to zero, but a 

 width of about 1*5 millim. was necessary in order to examine 

 the contact of the pole-faces. The effect of this was to 

 increase the Tractive Force by several hundred grammes at all 

 inductions, the increase being greatest at high inductions. 

 The magnetization-curve was carefully re-determined and the 

 results calculated out as before. The attraction of the coils 

 alone was also remeasured, but found not to have been 

 appreciably increased by the shortening of the gap. 



Final Results. 



Tbe results now agreed with Maxwell's theory to one-half 

 per cent, for inductions up to 19,000 units, but between 19,000 

 and 20,000 units the square roots of the observed weights are 

 rather more than 1 per cent, below the calculated values. 



This difference at high inductions might well be due to the 

 fact that there was still a gap of 1*5 millim. between the coils, 

 which would have a spreading effect at high inductions ; to 

 the effect of the stress on the magnetization ; or to tempera- 

 ture effects, the coils being considerably heated by a current 

 of 10 amperes. A better agreement with the theory was 

 therefore hardly to be expected. 



No readings could be taken at inductions below 6000, since 

 the weight of the lower coil and half-core was sufficient to 

 overcome the stress at this induction. The following table 

 gives the values of the intensity of magnetization, induction, 

 and the square roots of the theoretical and observed Tractive 

 Forces measured in grammes weight; and the diagram (fig. 2) 

 shows the values of the Induction and the square root of the 

 observed Tractive Force, the theoretical values being repre- 

 sented by the straight line whose inclination is determined by 

 the factor y— r 



V $7R/ 



