SHUDDEMAGEN. — DEMAGNETIZING FACTORS FOR IRON RODS. 233 



above was divided into a large number of equal intercepts, each of 

 which represented exactly 0.0010 of iV, the demagnetizing factor, for 

 the particular B corresponding to the line. The larger of these inter- 

 cepts were further subdivided into tenths by means of short dashes, and 

 each horizontal line was numbered for every 0.0010, beginning from 

 zero on the left. Thus the tracing cloth was simply a large transparent 

 scale through which the N corresponding to every Hi could be imme- 

 diately read off. The error in the inaccurate spacing of the divisions 

 of the scale was about 1 part in 200. 



Now suppose we arbitrarily say for the moment that the N for the 

 curve m = 200, all along the curve, shall be 0.0016, or the value of X 

 for the corresponding ellipsoid of revolution. By placing the tracing 

 cloth so that any desired line coincides with its corresponding B below, 

 and the magnetization curve for in = 200 crosses at N ' = 16 units, we 

 can read off the number of units for each of the other curves. After 

 doing this for all of the horizontal lines of our scale, we have a table of 

 values similar to that given for the rod of February 21, only the column 

 for m — 20(> will consist wholly of numbers 16. 



This table is thus our first approximation. We may now put away 

 our magnetization curve sheet with the scale, and proceed to get a 

 better approximation by merely studying the table. It will be noticed 

 that all the other columns will have values less than for the corres- 

 ponding ellipsoids. The only logical thing to do is to decrease the 16's 

 somewhat, at the same time decreasing every other number in the same 

 row by the same amount, so as to give a table consistent as a whole 

 when compared with the table for ellipsoids; and this gives us something 

 similar to the table given. At the best approximation, the values for 

 tn = 200 will still be a unit or two in doubt, but this will make but a 

 small error in the rods 30 to 50 diameters long. Of course individual 

 values of N in the table are subject to errors in the drawing of the 

 curve as well as observational errors, but when all the values of iV 

 for a certain length of rod are considered, a smooth curve could easily 

 be drawn throughout the range of B in the experiment. We have, 

 however, preferred to leave the tables as given directly from the last 

 approximation. 



Should any one not be cpuite satisfied with the values as tabulated 

 for any one series of experiments, he may easily change the whole table 

 to suit himself, but he must do this subject to the condition of adding 

 or subtracting the same number for any one row as it is given here. 



