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PROCEEDINGS OF THE AMERICAN ACADEMY. 



mum exciting field. After this 22 simultaneous determinations of H 

 and B were made in each half-cycle, and both cycles were carefully 

 plotted on such a scale that each was about 40 centimeters long. It ap- 

 peared that the maximum values of the induction were almost identical, 

 and that at no point were the plotted curves so much as 1 millimeter 

 apart. In filling the same glass tube a number of times in succession 

 from the same lot of filings, it was of course impossible to pack exactly 

 the same mass into the same space twice ; but the hysteresis diagrams. 



H 



Figure 2. 



many of which were obtained by Mr. J. Coulson and myself, were 

 always of the same shape, and the intensity of magnetization due to 

 a given exciting field seemed to be strictly proportional to the mass. 

 The density of the untreated filings was only about four tenths of that 

 of massive cast iron. 



The demagnetizing effect of the ends in a rod of solid iron only fifty 

 diameters long would of course be very serious, but in the present 

 case, due to the relatively small value of / for a given value of //, 

 it is far less important. In order to determine from my observations 

 the permeability at the centre of an endless column of material like 



