356 PROCEEDINGS OF THE AMERICAN ACADEMY. 



constant when strong currents should be sent through the solenoid. 

 The steel was first demagnetized by means of a long series of currents 

 in the solenoid, alternating in direction and steadily decreasing in in- 

 tensity, and then a series of steady direct currents of carefully measured 

 intensities, each a little stronger than the last, were sent through the 

 solenoid and reversed many times at each stage to determine the cor- 

 responding value of B in the steel. In this manner it was possible to 

 get a satisfactory curve of ascending reversals for the steel up to 

 H = 400 and B = 20,500, nearly. The length of the rod was, rel- 

 ative to its diameter, so great that the demagnetizing factor was very 

 small and the correction for the ends very easily made. The rod was 

 then demagnetized again, and the process described was repeated two or 

 three times until the resulting table of B versus H values seemed to be 

 well determined. After this, short pieces of various lengths, cut from 

 the rod which had been tested, were used in the yoke and were mounted 

 in different ways in the hope of discovering some satisfactory method 

 of studying the permeability of the steel by experiments upon these 

 pieces, which should give the same results up to an induction of about 

 20,000 as those already obtained by the work with the long solenoid. 

 After long trial, a length of cylinder was found which seemed, in this 

 particular yoke, to make the values of H at the centre of the length of 

 the specimen practically the same as the value in the air just outside 

 the metal. Two different materials were used in stout rod form in the 

 long solenoid, Bessemer Steel and "Compressed Steel," an extremely 

 homogeneous kind of steel prepared for us by the Boston Compressed 

 Shafting Company. 



In all the cases tried specimens of the size and shape described above 

 seemed to give the same permeability up to values of the induction as 

 great as 20,000 as the long solenoid did, and, for somewhat higher 

 values of B, to yield results which agreed with those obtained, where it 

 first becomes trustworthy, by the " Isthmus Method." 



After the central portion of each of these specimens had been covered 

 with an extremely thin coat of varnish, the diameter was determined 

 under the microscopes of a Zeiss Comparator, reading to the nearest 

 thousandth of a millimeter directly. Then two test coils, each of 

 twenty turns of very fine, well-insulated wire, were wound side by side 

 in a single layer over the varnished metal and extended over perhaps 

 a centimeter at the middle of the rod. These coils were tested against 

 each other when the specimen was in the yoke, to see if they were alike, 

 and if they were, both, in series, formed the inner test coil (L) to be 

 used in the measurements. The second testing coil (M) was wound on 

 a very carefully made spool of boxwood which had been seasoning for 



