Properties , Specific Resistance, and Hardness of Steel. 343 



to correspond with the holes a a. This arrangement is fas- 

 tened vertically in a suitable iron stand (not shown in the 

 figure). The wire to be hardened is introduced into the tube 

 and fastened below to a brass rod b h fitting tightly in the per- 

 foration baabj and above to the spring d K. For the purpose 

 of fastening the lower end, it was found sufficient, after having 

 previously wound it round the rod b h so as to form a coil 

 which could easily be made to slide off, to push the rod 

 through baab and the coil, the latter having been introduced 

 into the tube from the top. The spring d K, around the lower 

 half of which the other end of the wire was wound, the upper 

 half being provided with a clamp-screw g g, was fastened to a 

 second arm of the stand (also omitted in the figure) . By pro- 

 perly adjusting the rod b h and the arms of the stand, the wire 

 could be brought into coincidence with the axis of the tube 

 and stretched as far as was necessary. 



A powerful galvanic current heated the wire to the degree 

 of redness desired. The former entered at gg and passed 

 back to the battery through b h, as shown in the figure. To 

 prevent the oxidation of the wire during the process of heat- 

 ing, a current of dry C0 2 gas * was passed through the tube, 

 entering by means of the hose C attached to the lower end. 



After the wire had attained a steady uniform glow, the hose 

 C was closed by the fingers, its connexion with the carbonic- 

 acid apparatus disadjusted, the open end being connected with 

 a neighbouring hydrant instead ; hereupon the faucet of the 

 latter was quickly opened, the galvanic current being at the 

 same time interrupted : the water dashing up the tube with 

 great velocity, imparts to the wire the hardness desired. 

 Before each experiment the parts of the apparatus were well 

 dried in a warm current of air. 



The apparatus described presents the following advan- 

 tages : — (1) By employing currents of different intensity, 

 thus heating the wire to different degrees of redness, we are 

 able to obtain corresponding degrees of hardness, which, 

 though scarcely distinguishable mechanically (all appearing 

 equally hard and brittle), have very different effects on the 

 magnetic and electrical properties of steel j. (2) From the 



* Having accidentally employed moist carbonic-acid gas, a small flame 

 was observed at the top of the tube. This is probably due to the com- 

 bustion of both H 2 and CO, the former being generated by the decomposi- 

 tion of aqueous vapour by the hot steel, the latter by the action of the 

 nascent H produced on the Co 2 . 



t See IX. The coercive force of steel being a minimum at a point in 

 incipient redness, it is possible that the apparatus might be used in ob- 

 taining intense circularly or longitudinally magnetic wires, In the first 



2A2 



