94 Mr. S. Bidwell. On the Effect of Magnetisation [May 31, 



produces a difference of potential of 38 volts between its surface and 

 centre, a globe of a kilometre diameter, electrified to the same electric 

 density, reckoned according to the total electricity in any small volume 

 (electricity of air and of spherules of water, if there are any in it), 

 would produce a difference of potential of 38 million volts between 

 its surface and centre. In a thunderstorm, flashes of lightning show 

 us differences of potentials of millions of volts, but not perhaps of 

 many times 38 million volts, between places of the atmosphere distant 

 from one another by half a kilometre. 



II. " On the Effect of Magnetisation upon the Dimensions of 

 Iron Rings in Directions perpendicular to the Magnetisa- 

 tion, and upon the Volume of the Rings." By SHELFORD 

 BIDWELL, M.A., LL.B., F.R.S. Received March 2, 1894. 



A recent communication* to the Society contained an account of 

 some experiments relating to the effects of magnetisation upon the 

 dimensions of two iron rings, one of which was annealed and the 

 other hardened. The rings had the form of short cylinders about 

 6 cm. in diameter, 3 cm. in height, and 0'4 cm. in thickness. The 

 experiments in question were concerned with the circumferential 

 variations which took place along the lines of magnetisation ; those 

 to be here described deal with the concomitant variations in the 

 height of the cylinders (width of the rings) transversely to the mag- 

 netisation. On the assumption that variations similar to the latter 

 occur at the same time in the thickness of the metal, it is possible to 

 deduce the changes in the volume of the ring which attend magnet- 

 isation. 



Fig. 1, from a photograph, shows how the rings were prepared for 

 the experiments. Four brass rods were hard-soldered to the iron, 

 two of them being in a line with a diameter, while the other two were 

 attached to the edges, opposite to one another, and parallel to the axis 

 of the ring. The ring was inserted in a wooden case, also shown, 

 through holes in which the four brass rods projected. Insulated 

 wire for carrying the magnetising current was wound over the wooden 

 jacket. 



For the new experiments the ring was placed in a horizontal posi- 

 tion, one of the edge rods resting upon a brass socket on the adjust- 

 able base of the instrument, and the other, which had a chisel-shaped 

 end, actuating the lever.f To counterbalance the weight of the ring 

 a horizontal arm, carrying a sliding weight, was fixed to the lower 

 rod. 



* ' Eoy. Soc. Proc.,' vol. 55, p. 228. 



* The chisel-shaped terminal piece was removable and is not shown in fig. 1. 



