PEIRCE. — MAGNETIC BEHAVIOR OF HARDENED CAST IRON. 355 



(G) were first turned out in the lathe and then finished by a reamer 

 made and ground by the machinery afterwards used to cut the tapers 

 on the ends of the test pieces. Each test piece of the hard cast iron 

 had first to be ground to the form of a true cylinder in a universal 

 grinding machine and then to be tapered off at the ends with the help 

 of a centre grinder, mounted motor and all, in the tool post of an 

 engine lathe. All the work was done by Mr. G. W. Thompson, the 

 mechanician of the Jefferson Physical Laboratory, in the most skil- 

 ful manner, and the reluctance of the joints must have been relatively 



Figure 2. 



very small. When a specimen of this shape was in position between 

 the pole pieces of the yoke, and a steady current of at least two or 

 three amperes was passing through the exciting coil, it was assumed 

 that the value of H within the small cylinder (C) near the middle of 

 its length was the same as the value of H in the air just outside the 

 metal. The ground of this assumption was a series of experiments 

 carried out some months ago. A piece of homogeneous steel rod about 

 half an inch in diameter and about three hundred and fifty diameters 

 long was placed within a solenoid consisting of 20,904 turns of 

 thoroughly insulated wire wound on a straight piece of stout brass tube 

 about an inch in inside diameter and rather more than sixteen feet 

 long. Near the middle of the steel rod a test coil of fine insulated 

 wire was wound closely on it, and then, with its leads, made thoroughly 

 waterproof, so that a current of tap water could be kept running around 

 the rod in the brass tube to hold the temperature of the steel nearly 



