282 Peirce — Permeabilities and Reluctivities for Steel. 



ranges, all but one of which could be shunted at pleasure out 

 of the circuit. The currents passed through a rack of three 

 rheostats specially made for the purpose by the Simplex 

 Electric Company : these had a range of about 8000 ohms. 



The ballistic galvanometer was calibrated at short intervals 

 by means of a coil (X) of 13*6 ohms resistance always in its 

 circuit. This coil, X, consisted of 534 turns of fine insulated wire 

 wound in a single layer upon a very accurately cylindrical 

 wooden core which was hung within a uniformly wound ver- 

 tical solenoid 176*2 centimeters long consisting of 5526 turns. 



Fig. 6. 

























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Fig. 6. The ordinates of the upper curve show the values of the reluc- 

 tivity of the annealed Norway iron for values of H between and 10. The 

 ordinates of the lower curve represent, on a scale one-tenth as large, the 

 reluctivities of this iron for excitations between H = 10 and H= 100. 



The effective area of the turns of X was 22,720 square centi- 

 meters and a current of one ampere sent through the solenoid 

 caused a flux of 895,400 maxwells through X. 



The remarkable solenoid, nearly five meters long, in which 

 the permeabilities were determined for values of II below 410, 

 was constructed by Mr. Thompson and his assistants in a lathe 

 the bed of which had been temporarily lengthened. The core 

 was a thick-walled solid-drawn brass tube of about one 



