532 



PROFESSOR KNOTT ON THE STRAINS PRODUCED IN 



In tbc following table the various symmetrical combinations are represented by 

 units and noughts — the nought meaning that the particular coil occupying the corre- 

 sponding position was thrown out of circuit : — 



Combination 

 of Coils. 



111111 



011110 

 101101 

 110011 

 001100 



Field at centre 

 due to one Ampere. 



173 



168 

 150 



27-4 

 145 



The most important of these is the first one — all the coils being included in the 

 circuit, so that any inclosed tube would be subjected to as uniform a field as possible. 

 Even in this case, however, the value 173 does not persist throughout, but diminishes 

 slightly as we pass either way along the axis towards the end coils. It was found by 

 experiment that the average values of the fields across the section of the coils at 

 distances 0, 10*8, and 21*9 centimetres measured along the axis from the centre were as 

 the numbers 1, "971, and *935. These numbers are proportional to the induced currents 

 obtained in the secondary coil when it was placed at the distances named, no metal core 

 being within the coils. When one of the tubes was inserted, the induced currents were 

 of course greatly increased. At the same time, the average induction across any section 

 diminished very markedly as the distance from the centre increased. In other words, 

 although the tube (which lay well within the coils) was set in a field differing but 

 slightly from uniformity, the magnetisation in the tube varied very greatly as we passed 

 from centre to end. 



The following numbers, obtained experimentally when Iron No. 5 was inserted in the 

 coil, show how the total induction across the different sections varies with the position of 

 the section and with the strength of field originally applied. The values of the field are 

 the values at the middle point. 



Relative Inductions across Sections of Coil with Tube inclosed, at different 

 Positions and in different Fields. 







For Field 





Distance of 









Section from 









Centre of Coil. 



463 



238 



97-5 







1310 



1116 



700 



10-8 



1250 



1060 



630 



21-9 



724 



532 



305 



These numbers show at once how very far from a condition of even approximately 

 uniform magnetisation the iron tube is, and afford a good illustration of the manner in 

 which the lines of force leak out of the magnet as we approach the ends. They also show 

 hat there is greater relative leakage in the smaller field. Thus in field 97 '5, fully half 

 the lines of force have left the iron at distance 22 centimetres from the middle point ; 

 whereas in field 463 there is a leakage of not quite half. 



In the face of such large variations of magnetisation from section to section of the 



