SHUDDEMAGEN. — DEMAGNETIZING FACTORS FOR IRON RODS. 247 



well enough for the lower intensities of H' but would be an exceedingly 

 insensitive method to use when the field H' is very high, since then 

 the induction is nearly constant along the bar except at the very ends, 

 so that the experimental error might easily be even greater than the 

 actual difference in the magnetic induction between the central part 

 of the rod and any other part. The best method seems to be to read 

 the reversal method ballistic throw from a coil wound directly over the 

 middle of the rod, and then, connecting any other coil, wound around 

 the rod nearer the end, in series with the central coil but in opposition 

 to it, observe the ballistic throw due to the difference in the flux 



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Figure 22. 



Diagram showing arrangement of secondary coil and switch-board used in the 

 work on the distribution of magnetic induction along an iron rod. 



through the two coils. This was the idea adopted. Figure 22 shows 

 diagrammatically the arrangement of the coils in one of the four different 

 cases which were tried ; the others were similarly arranged. The po- 

 sitions of all the secondary coils are shown in the diagrams drawn to 

 scale and marked with the distances between the centres of the coils. 



All the ends of the coils were led into small mercury cups in a small 

 switchboard. The extremities B 2 , C 2 , D 2 , E 2 , and one terminal of the 

 ballistic galvanometer were all dipped into cup F. If now the copper 

 connector is placed in the position A C as shown, then the ballistic 

 throw observed on reversing the current in the primary solenoid is that 



