﻿94 Prof. R. Threlfall on an Approximate Method 



(3) By using induction-coils of different diameters 1 

 assured myself that I was really measuring the operative 

 inductions. 



(4) The state of the surface of the bars is an important 

 matter. If the ends are rough, of course the contact is at 

 points. This leads to a concentration of induction at these 

 places, and is a very constant source of error. My best bars, 

 however, gave just as anomalous results as my roughest ones, 

 so that the deviation could not be attributed to this cause. 



(5) Finally I tried, without much confidence, the effect of 

 a small indirectness of pull. This was done by winding two 

 wide solenoids and leaving sufficient space between their ends 

 to see what was going on at the point of junction ; a gas- 

 flame was put on the side of the junction, remote from the 

 observer. 



It was then found that one side of the bar which was being 

 pulled off invariably remained in contact after the other side 

 had slightly separated — when this was prevented by slightly 

 guiding the spring-balance by hand, the agreement was as 

 good as at higher inductions. The explanation is now obvious. 

 If the bars separate slightly just at one side, two things happen. 

 (1) The total reluctance of the circuit increases. (2) The 

 induction concentrates at the parts in contact. At low induc- 

 tions the effect of (2) overpowers that of (1). At high induc- 

 tions, when the permeability of the iron becomes less, the 

 induction is less free to distribute itself, and also the traction 

 of the bars being greater, the phenomenon does not begin to 

 manifest itself till rupture is just about to be produced, or it 

 produces a veiy much smaller percentage error. Of course 

 all this might have been foreseen, but one's experience with 

 strong magnets — in which case it is notorious that it is much 

 more difficult to pull off an armature straight than sideways — 

 misled me. 



On examining other bars which had given similar results 

 at low inductions, I found I could similarly diminish or increase 

 the apparent traction by varying very slightly the direction 

 of pull. Thus with a pair of flat-ended bars and an induction 

 of about 3000 C.G.S. the calculated pull was one pound 

 seven ounces ; but the observed pull was always over two 

 pounds, and in some experiments about three pounds. On 

 guiding the bars so that no wedge-shaped gap appeared, the 

 traction could be got down to about one pound eight ounces. 

 I do not think it is possible to get much closer than this, for 

 if proper arrangements are made to absolutely ensure a really 

 true and rigid separation, friction would inevitably come iu 



