DIMENSIONS OP RINGS AND BODS OP IRON AND OTHER METALS. 209 



Rod No. 1. Commercial iron win : 



Length 10cm. 



Diameter , 0'27 cm. 



Rod No. 2. Soft charcoal iron : 



Length ...'.. 10 cm. 



Diameter 0'33 cm. 



Each iron rod had rods of brass, 5'5 cm. in length soldered to the ends ; the 

 total length of the compound rod thus formed being 21 cm. 

 The magnetising solenoid used with the rods was as follows : 



Length 11 '6 cm. 



Diameter of wire 0'12 cm. 



Number of turns 876 



Field at centre 92 X C. nearly. 



The rods were fixed axially in the solenoid by means of corks at the two ends. 



Fig. 2. 



IX 



The nature of the measuring instrument employed is indicated in the diagram, fig. 2. 

 A detailed description of it, with a drawing, is given in the former paper. The ring S, 

 round which is wound the wire, whose ends are shown at W, W, is placed in a vertical 

 position, the lower end of one of its rods R resting in a conical recess in a brass base 

 plate E. The upper end of the other rod R', which is chisel-shaped, acts at B upon a 

 brass lever, one end of which abuts upon a fixed knife edge A, and the other upon a 

 short arm fixed perpendicularly upon the back of a small circular mirror M, which 

 turns on knife edges D about its horizontal diameter. Shallow obtuse-angled notches 

 are cut in the lever at A and B, and in the mirror arm at C. By means of a lantern, 



MDCCCLXXXVin. A. 2 E 



