208 MB. S. B1DWELL ON CHANGES PRODUCED BY MAGNETISATION IN 



Ring No. I. This was made from a piece of the same rod as the last. The joint 

 was welded and the whole ring was filed smooth, but it was not turned. The ring 

 was covered with a boxwood case of circular section (something like a WINTER'S 

 ring), which was formed of two rings of semicircular section, having grooves of semi- 

 circular section cut upon their flat sides. The grooves in the two portions of the 

 boxwood ring enclosed a space slightly larger than was necessary for the reception 

 of the iron ring, allowing a play of about 1 mm. Five layers of copper wire were 

 wound over the boxwood ring. 



External diameter of iron ring 6'7 cm. 



Mean diameter 6 cm. 



Diameter of section 0'7 cm. 



Diameter of section of boxwood ring . . . . 1 '8 cm. 



Diameter of copper wire 0'07 cm. 



Total number of turns 644 



Field due to coil 42'9 X C. 



Brass rods were screwed into small holes drilled at opposite ends of a diameter of 

 the iron ring and passed freely through the boxwood casing and the coils, pieces 

 of india-rubber tubing being inserted between the rods and the wires of the coils. 



Ring No. 2 was a welded ring made from a different specimen of iron. It was, like 

 No. 0, turned to a circular section, and was remarkable for the extreme facility with 

 which it could be cut in the lathe. Rods of iron (instead of brass) were screwed into 

 two opposite holes, and care was taken to make the screws fit very tightly, with the 

 object of securing nearly perfect magnetic uniformity throughout the ring. This ring 

 was, like No. 1, covered with a wooden case of circular section, but the holes made in 

 the wood to admit the rods were furnished with short spouts of glass tubing, having 

 ;m external diameter of 0'57 cm. and projecting about 2'5 cm. ; the rods were 

 turned down to such a size as to pass freely through the tubes. The wire with which 

 the wooden ring was covered was wound in six layers closely up to the tubes, and the 

 break in the uniformity of the coil was very slight. The tubes also served to shield 

 the rods from the heat radiated by the coil when a current was passing. 



External diameter of iron ring 670 cm. 



Mean diameter ' , . .. . . . G'03 cm. 



Diameter of section - G7 cm. 



Diameter of section of wood casing , . . . 1 '3 cm. 



Diameter of copper wire 0'07 cm. 



Total number of turns 869 



Field due to coil 57'G X C 



Some experiments were also made with two iron rods. 



