and Retractions of Rods of Iron and Steel. 181 



As the result of numerous determinations made on various 

 parts of the screw, I found that r J^ of a rotation of the screw- 

 equalled 1737 division of the telescope-scale, which gives 

 000011 inch as the value of one division of the scale. This 

 value, however, applies only to the experiments on the rods of 

 iron Nos. 1 to 6 inclusive. Before commencing the experiments 

 on the rods of steel, the distance of the scale from the mirror 

 was changed; and in this new position I found that one division 

 of the scale corresponded to 0000146 of an inch. 



Description of the helix, and measures of the resistances of its 



wires. 



The helix was a compound one, formed of four layers of cop- 

 per wire. The two inner layers formed 1069 turns of one length 

 of 303 feet of wire '087 inch in diameter. The two outer layers 

 were formed of another length of 330 feet of * 112-inch wire 

 wrapped in 850 turns. These two helices could be used sepa- 

 rately, or joined into one helix of 633 feet having 1919 turns. 



The resistance of the inner helix was *44 ohm ; the outer had 

 a resistance of *3l ohm, together giving a resistance of *75 ohm. 

 The latter resistance, added to that of the wires leading from the 

 battery through a Gaugain galvanometer to the helix and back 

 to the battery, brought up the resistance to nearly one ohm. 



A battery of 25 cells of Bunsen was used in the determination 

 of the coefficients of elongation and retraction ; and the above in- 

 terpolar resistance showed that the maximum effect of magneti- 

 zation would be given by connecting the 25 cells, 5 in couple and 

 5 in series. Whenever in this research we speak of the effect 

 of 25 cells, it is to be understood that they are connected as just 

 described. 



The iron and steel rods used in the experiments were prepared 

 for me with the well-known skill and fidelity of Mr. Wallace, of 

 Ansonia, Ct. He carefully selected the materials, and annealed 

 the iron rods by packing them, with iron scales from a rolling- 

 mill, in a wrought-iron covered box, and exposing the box to a 

 red heat for three days ; the box was then allowed to cool very 

 slowly. The steel rods were tempered as uniformly as possible 

 throughout their lengths. 



Arrangement of the apparatus, and general description of the 

 phenomena which take place on the magnetization and demagne- 

 tization of the rods of iron. 



The beam supporting the apparatus was so placed that the 

 axis of the helix was in the magnetic meridian. Each rod 

 before it was introduced into the helix was tested as to its mag- 

 netic condition by placing its length at right angles to the 



